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Journal articles on the topic 'Ring formation'

Author: Grafiati

Published: 4 June 2021

Last updated: 9 June 2025

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1

Pellissier, Hélène. "The Use of Domino Reactions for the Synthesis of Chiral Rings." Synthesis 52, no. 24 (July 22, 2020): 3837–54. http://dx.doi.org/10.1055/s-0040-1707905.

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This short review highlights the recent developments reported in the last four years on the asymmetric construction of chiral rings based on enantioselective domino reactions promoted by chiral metal catalysts.1 Introduction2 Formation of One Ring Containing One Nitrogen Atom3 Formation of One Ring Containing One Oxygen/Sulfur Atom4 Formation of One Ring Containing Several Heterocyclic Atoms5 Formation of One Carbon Ring6 Formation of Two Rings7 Conclusion

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2

Tuma, Rabiya S. "Contractile ring formation." Journal of Cell Biology 174, no. 3 (July 24, 2006): 319b. http://dx.doi.org/10.1083/jcb.1743iti5.

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3

Guosheng Wang, Guosheng Wang, and Siyu Han and Ronghui Xu Siyu Han and Ronghui Xu. "The Ring Formation Mechanism in Cyclization of Berberine." Journal of the chemical society of pakistan 43, no. 3 (2021): 308. http://dx.doi.org/10.52568/000578/jcsp/43.03.2021.

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Berberine hydrochloride is a natural alkaloid with significant antitumor activities against many types of cancer cells, can be synthesized by cyclic reaction with hydrochloride condensate and glyoxal as raw materials and copper chloride as catalyst. In this study, the transition and energy change for the each reaction step was calculated by the density functional theory program Dmol3 in Materials Studio 2017. and the results testified that there are two ring formation in the cycliztion process, and according to the result we proposed the mechanism of this cyclization reaction. We also use infrared and ultraviolet spectroscopy to monitor the reaction process in real time and prove the ring formation process. The reaction mechanism was firstly proposed at the basic results of above.

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4

Guosheng Wang, Guosheng Wang, and Siyu Han and Ronghui Xu Siyu Han and Ronghui Xu. "The Ring Formation Mechanism in Cyclization of Berberine." Journal of the chemical society of pakistan 43, no. 3 (2021): 308. http://dx.doi.org/10.52568/000578.

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Berberine hydrochloride is a natural alkaloid with significant antitumor activities against many types of cancer cells, can be synthesized by cyclic reaction with hydrochloride condensate and glyoxal as raw materials and copper chloride as catalyst. In this study, the transition and energy change for the each reaction step was calculated by the density functional theory program Dmol3 in Materials Studio 2017. and the results testified that there are two ring formation in the cycliztion process, and according to the result we proposed the mechanism of this cyclization reaction. We also use infrared and ultraviolet spectroscopy to monitor the reaction process in real time and prove the ring formation process. The reaction mechanism was firstly proposed at the basic results of above.

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5

Combes, Françoise. "Ring and Lens Formation." International Astronomical Union Colloquium 157 (1996): 286–98. http://dx.doi.org/10.1017/s0252921100049927.

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AbstractThe dynamical mechanism to form rings at Lindblad resonances in a barred galaxy is now well-known: due to its dissipative character, the gas is forced in a spiral structure, and experiences torques from the bar potential. Angular momentum is transferred until gas accumulates in the resonant rings. Some problems remain however to account for all observations, such as the very different time-scales for nuclear, inner and outer ring formation, while the three are frequently observed in the same galaxy; the shapes, orientations and thickness of the rings, etc... The adequacy of the present gas dynamical modelizations is discussed.Lenses are secondary components of barred galaxies that could originate from bar evolution. No model until now has met the observational constraints, in particular the sharp edge of the lenses, their strong velocity anisotropy, and their small thickness. We propose here that lenses are the result of partial bar destruction, a necessary step in a feedback cycle of bar formation-destruction, a cycle driven by gas accretion.

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6

Kim, Woong-Tae, Woo-Young Seo, and Yonghwi Kim. "Formation of nuclear rings of barred galaxies and star formation therein." Proceedings of the International Astronomical Union 9, S303 (October 2013): 43–53. http://dx.doi.org/10.1017/s174392131400012x.

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AbstractBarred galaxies contain substructures such as a pair of dust lanes and nuclear rings, with the latter being sites of intense star formation. We study the substructure formation as well as star formation in nuclear rings using numerical simulations. We find that nuclear rings form not by the Lindblad resonances, as previously thought, but by the centrifugal barrier that inflowing gas along dust lanes cannot overcome. This predicts a smaller ring in a more strongly barred galaxy, consistent with observations. Star formation rate (SFR) in a nuclear ring is determined primarily by the mass inflow rate to the ring. In our models, the SFR typically shows a short strong burst associated with the rapid gas infall and stays very small for the rest of the evolution. When the SFR is low, ages of young star clusters exhibit an azimuthal gradient along the ring since star formation takes place mostly near the contact points between the dust lanes and the nuclear ring. When the SFR is large, on the other hand, star formation is widely distributed throughout the whole length of the ring, with no apparent age gradient of star clusters. Since observed ring star formation appears long-lived with episodic bursts, our results suggest that the bar region should be replenished continually with fresh gas from outside.

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7

Wu, Yu-Ting, and Ing-Guey Jiang. "On the formation of ring galaxies." Proceedings of the International Astronomical Union 6, S271 (June 2010): 102–9. http://dx.doi.org/10.1017/s1743921311017509.

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AbstractThe formation scenario of ring galaxies is addressed in this paper. We focus on the P-type ring galaxies presented in Madore, Nelson & Petrillo (2009), particularly on the axis-symmetric ones. Our simulations show that a ring can form through the collision of disc and dwarf galaxies, and the locations, widths, and density contrasts of the ring are well determined. We find that a ring galaxy such as AM 2302-322 can be produced by this collision scenario.

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8

Pearce, F. R., and P. A. Thomas. "Ring formation in triaxial potentials." Monthly Notices of the Royal Astronomical Society 248, no. 4 (February 15, 1991): 688–700. http://dx.doi.org/10.1093/mnras/248.4.688.

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9

Sammes, Peter G., and David J. Weller. "Steric Promotion of Ring Formation." Synthesis 1995, no. 10 (October 1995): 1205–22. http://dx.doi.org/10.1055/s-1995-4099.

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10

Dutta, Aloke K., and Jared A. Butcher. "Macrocyclic ring formation in micelles." Tetrahedron Letters 27, no. 29 (January 1986): 3343–44. http://dx.doi.org/10.1016/s0040-4039(00)84791-7.

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11

Spiridonova, Sofya. "Formation dynamics in geostationary ring." Celestial Mechanics and Dynamical Astronomy 125, no. 4 (May 10, 2016): 485–500. http://dx.doi.org/10.1007/s10569-016-9693-0.

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12

Bournaud, F., and F. Combes. "Formation of polar ring galaxies." Astronomy & Astrophysics 401, no. 3 (April 2003): 817–33. http://dx.doi.org/10.1051/0004-6361:20030150.

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13

Bonanomi, G., G. Incerti, A. Stinca, F. Cartenì, F. Giannino, and S. Mazzoleni. "Ring formation in clonal plants." Community Ecology 15, no. 1 (June 2014): 77–86. http://dx.doi.org/10.1556/comec.15.2014.1.8.

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14

Henisch, H. K. "Liesegang ring formation in gels." Journal of Crystal Growth 76, no. 2 (August 1986): 279–89. http://dx.doi.org/10.1016/0022-0248(86)90372-6.

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15

Güth, Werner, and Bezalel Peleg. "On ring formation in auctions." Mathematical Social Sciences 32, no. 1 (August 1996): 1–37. http://dx.doi.org/10.1016/0165-4896(96)00808-6.

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16

Tang, Jay, Josef Käs, Jagesh Shah, and Paul Janmey. "Counterion-induced actin ring formation." European Biophysics Journal 30, no. 7 (December 1, 2001): 477–84. http://dx.doi.org/10.1007/s002490100178.

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17

Akhmetov, D. G. "Model of vortex ring formation." Journal of Applied Mechanics and Technical Physics 49, no. 6 (November 2008): 909–18. http://dx.doi.org/10.1007/s10808-008-0113-4.

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18

Sommer, Andrei P., and Noemi Rozlosnik. "Formation of Crystalline Ring Patterns on Extremely Hydrophobic Supersmooth Substrates: Extension of Ring Formation Paradigms." Crystal Growth & Design 5, no. 2 (March 2005): 551–57. http://dx.doi.org/10.1021/cg0496989.

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19

Wang, Kai, Chenghao Jiang, Zhenming Zhang, Chunyu Han, Xuewei Wang, Yaping Li, Kaiting Chen, and Junfeng Zhao. "Cut and sew: benzofuran-ring-opening enabled cyclopentenone ring formation." Chemical Communications 56, no. 84 (2020): 12817–20. http://dx.doi.org/10.1039/d0cc05271j.

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20

Berta, René, and Ladislav Lukáč. "Ash ring formation in lime rotary kilns." Advances in Thermal Processes and Energy Transformation 1, no. 1 (2018): 01–04. http://dx.doi.org/10.54570/atpet2018/01/01/0001.

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This paper describes in different sections all know factors influencing ash ring formation in lime rotary kilns. Ash ring is mostly influence by fuel and lime properties and its impurities and process conditions. The paper describes different types of ash rings that might formed in rotary kilns. Factors were analyzed from feed batch, fuel to construction parts of rotary kilns. Papers describes all know factors how to prevent ash ring formation and how decrease ash ring formation and its influence on operational conditions of lime rotary kilns.

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21

Tebbs, Irene R., and Thomas D. Pollard. "Separate roles of IQGAP Rng2p in forming and constricting the Schizosaccharomyces pombe cytokinetic contractile ring." Molecular Biology of the Cell 24, no. 12 (June 15, 2013): 1904–17. http://dx.doi.org/10.1091/mbc.e12-10-0775.

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Eukaryotic cells require IQGAP family multidomain adapter proteins for cytokinesis, but many questions remain about how IQGAPs contribute to the process. Here we show that fission yeast IQGAP Rng2p is required for both the normal process of contractile ring formation from precursor nodes and an alternative mechanism by which rings form from strands of actin filaments. Our work adds to previous studies suggesting a role for Rng2p in node and ring formation. We demonstrate that Rng2p is also required for normal ring constriction and septum formation. Systematic analysis of domain-deletion mutants established how the four domains of Rng2p contribute to cytokinesis. Contrary to a previous report, the actin-binding calponin homology domain of Rng2p is not required for viability, ring formation, or ring constriction. The IQ motifs are not required for ring formation but are important for ring constriction and septum formation. The GTPase-activating protein (GAP)–related domain is required for node-based ring formation. The Rng2p C-terminal domain is the only domain essential for viability. Our studies identified several distinct functions of Rng2 at multiple stages of cytokinesis.

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22

Mizuno, T., K. Yoshioka, Y. Sato, H. Yokoi, M. Takita, and S. Nagano. "Multicellular Ring Formation in Dictyostelium discoieum." Seibutsu Butsuri 39, supplement (1999): S40. http://dx.doi.org/10.2142/biophys.39.s40_1.

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23

Hubber, D. A., and A. P. Whitworth. "Binary star formation from ring fragmentation." Astronomy & Astrophysics 437, no. 1 (June 10, 2005): 113–25. http://dx.doi.org/10.1051/0004-6361:20042428.

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24

Rolando, Richard J., and Christopher W. Macosko. "Ring formation in linear stepwise polymerization." Macromolecules 20, no. 11 (November 1987): 2707–13. http://dx.doi.org/10.1021/ma00177a013.

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25

Delcourt, D. C., J. A. Sauvaud, and T. E. Moore. "Cleft contribution to ring current formation." Journal of Geophysical Research 95, A12 (1990): 20937. http://dx.doi.org/10.1029/ja095ia12p20937.

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26

Araujo, N. C. S., E. Vieira Neto, and D. W. Foryta. "Formation of the G-ring arc." Monthly Notices of the Royal Astronomical Society 461, no. 2 (May 5, 2016): 1868–74. http://dx.doi.org/10.1093/mnras/stw1055.

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27

MOHSENI, KAMRAN, HONGYU RAN, and TIM COLONIUS. "Numerical experiments on vortex ring formation." Journal of Fluid Mechanics 430 (March 10, 2001): 267–82. http://dx.doi.org/10.1017/s0022112000003025.

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28

Caglayan, Humeyra, Irfan Bulu, Marko Loncar, and Ekmel Ozbay. "Cavity formation in split ring resonators." Photonics and Nanostructures - Fundamentals and Applications 6, no. 3-4 (December 2008): 200–204. http://dx.doi.org/10.1016/j.photonics.2008.09.001.

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29

Pisaroni, M., R. Sadi, and D. Lahaye. "Counteracting ring formation in rotary kilns." Journal of Mathematics in Industry 2, no. 1 (2012): 3. http://dx.doi.org/10.1186/2190-5983-2-3.

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30

Chatterjee, Tapan K. "The formation of faint ring structures." Astrophysics and Space Science 121, no. 2 (1986): 213–24. http://dx.doi.org/10.1007/bf00653694.

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31

Addinall, S. G., E. Bi, and J. Lutkenhaus. "FtsZ ring formation in fts mutants." Journal of bacteriology 178, no. 13 (1996): 3877–84. http://dx.doi.org/10.1128/jb.178.13.3877-3884.1996.

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32

Kaur, Harsimran, Suresh Kumar, Kashmir Singh, and Lalit M. Bharadwaj. "Divalent cation induced actin ring formation." International Journal of Biological Macromolecules 48, no. 5 (June 2011): 793–97. http://dx.doi.org/10.1016/j.ijbiomac.2011.03.004.

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33

Spouge, John L. "Equilibrium ring formation in polymer solutions." Journal of Statistical Physics 43, no. 1-2 (April 1986): 143–96. http://dx.doi.org/10.1007/bf01010576.

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34

Woo, J. M. Y., A. Morbidelli, S. L. Grimm, J. Stadel, and R. Brasser. "Terrestrial planet formation from a ring." Icarus 396 (May 2023): 115497. http://dx.doi.org/10.1016/j.icarus.2023.115497.

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35

Takahashi, Sanemichi Z., Eiichiro Kokubo, and Shu-ichiro Inutsuka. "Planetesimal Formation by the Gravitational Instability of Dust Ring Structures." Astrophysical Journal 945, no. 2 (March 1, 2023): 120. http://dx.doi.org/10.3847/1538-4357/ac9fd0.

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Abstract We investigate the gravitational instability (GI) of dust ring structures and the formation of planetesimals by their gravitational collapse. The normalized dispersion relation of a self-gravitating ring structure includes two parameters that are related to its width and line mass (the mass per unit length). We survey these parameters and calculate the growth rate and wavenumber. Additionally, we investigate the formation of planetesimals by growth of the GI of the ring that is formed by the growth of the secular GI of the protoplanetary disk. We adopt a massive, dust-rich disk as a disk model. We find the range of radii for fragmentation by the ring GI as a function of the width of the ring. The innermost radius for the ring GI is smaller for a smaller ring width. We also determine the range of the initial planetesimal mass resulting from the fragmentation of the ring GI. Our results indicate that the planetesimal mass can be as large as 1028 g at its birth after the fragmentation. It can be as low as about 1025 g if the ring width is 0.1% of the ring radius, and the lower limit increases with the ring width. Furthermore, we obtain approximate formulae for the upper and lower limits of the planetesimal mass. We predict that the planetesimals formed by the ring GI have prograde rotations because of the Coriolis force acting on the contracting dust. This is consistent with the fact that many trans-Neptunian binaries exhibit prograde rotation.

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36

Yokoshima, Satoshi. "Synthesis of Polycyclic Natural Products through Skeletal Rearrangement." Synlett 31, no. 20 (July 23, 2020): 1967–75. http://dx.doi.org/10.1055/s-0040-1707904.

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Construction of rings through reliable reactions followed by changes in the ring size or the connectivity through skeletal rearrangement provides molecules with a wide range of skeletons. In this account, our syntheses of polycyclic natural products through skeletal rearrangement are discussed.1 Introduction2 Synthesis through Changes in the Ring Size3 Synthesis by Biomimetic Strategies4 Synthesis through Metathesis5 Synthesis through Temporary Formation of a Ring6 Conclusion

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37

Gorog, J. Peter, and W. Ray Leary. "Ring removal in rotary kilns used by the pulp and paper industry." March 2016 15, no. 3 (April 1, 2016): 205–13. http://dx.doi.org/10.32964/tj15.3.205.

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Ring formation occurs in the rotary kilns used by the pulp and paper industry to regenerate the lime needed to produce white liquor. The main rings are: burning zone rings, mid-kiln rings, dust rings and/or mud balls, and mud rings. While the exact mechanisms of ring formation are not completely understood, frequent changes in the production rate and the mud solids, high soda content in the mud, and high levels of dust are believed to play a major role in ring formation. The presence of rings can restrict the movement of material through the kiln. If undetected, this can result in overheating of the refractory lining. In the worst case, the presence of rings can lead to unscheduled downtime to clean out the ring and replace damaged refractory bricks. Depending on the severity, the cost of repairs and lost production can be in excess of US$3 million for a single event. Even if the root cause of ring formation is known, from a practical standpoint, it can be difficult to eliminate rings. In mills where ring formation is a recurring problem, mechanical ring removal systems should be considered. This paper describes the use of ring removal systems as a cost effective way to mitigate the problems associated with ring formation.

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38

Proshina, I., O. Sil’chenko, and A. Moiseev. "Star formation in outer rings of S0 galaxies." Astronomy & Astrophysics 634 (February 2020): A102. http://dx.doi.org/10.1051/0004-6361/201936912.

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Aims. Although S0 galaxies are often thought to be “red and dead”, they frequently demonstrate star formation organised in ring structures. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. Here we study the moderate-luminosity nearby S0 galaxy, NGC 4513. Methods. By applying long-slit spectroscopy along the major axis of NGC 4513, we measured gas and star kinematics, Lick indices for the main body of the galaxy, and strong emission-line flux ratios in the ring. After inspecting the gas excitation in the ring using the line ratios diagnostic diagrams and showing that it is ionised by young stars, we determined the gas oxygen abundance using popular strong-line calibration methods. We estimated the star formation rate (SFR) in the outer ring using the archival Galaxy Evolution Explorer (GALEX) ultraviolet images of the galaxy. Results. The ionised gas counter-rotates the stars over the whole extension of NGC 4513 suggesting that it is being accreted from outside. The gas metallicity in the ring is slightly subsolar, [O/H] = −0.2 dex, matching the metallicity of the stellar component of the main galactic disc. However the stellar component of the ring is much more massive than can be explained by the current star formation level in the ring. We conclude that the ring of NGC 4513 is probably the result of tidal disruption of a massive gas-rich satellite, or may be the consequence of a long star-formation event provoked by gas accretion from a cosmological filament that started some 3 Gyr ago.

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39

Aijaz, Rashid, and Ahad Shazia. "FtsZ, A Tubulin Homologue Spatiotemporally Regulates the Bacterial Cell Division." International Journal of Trend in Scientific Research and Development 1, no. 5 (July 24, 2017): 1273–80. https://doi.org/10.31142/ijtsrd2410.

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Most common pattern of Bacterial division is binary fission. Bacterial division occurs by the construction of Z ring at appropriate time and at a defined site. FtsZ is the major component of Z ring formation, and the polymerization of FtsZ is regulated in space and time by negative and positive regulators of FtsZ assembly. In the bacterial cell positive and negative regulators help in attaining a critical concentration gradient, such that at mid cell the critical concentration required for Z ring formation is lowest, thus Z ring formation takes place at this site only. Aijaz Rashid | Shazia Ahad "FtsZ, A Tubulin Homologue Spatiotemporally Regulates the Bacterial Cell Division" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-1 | Issue-5 , August 2017, URL: https://www.ijtsrd.com/papers/ijtsrd2410.pdf

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40

Chen, Hsin, Audrey S. Howell, Alex Robeson, and Daniel J. Lew. "Dynamics of septin ring and collar formation in Saccharomyces cerevisiae." Biological Chemistry 392, no. 8-9 (August 1, 2011): 689–97. http://dx.doi.org/10.1515/bc.2011.075.

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Abstract Although the septin ring and collar in budding yeast were described over 20 years ago, there is still controversy regarding the organization of septin filaments within these structures and about the way in which the ring first forms and about how it converts into a collar at the mother-bud neck. Here we present quantitative analyses of the recruitment of fluorescently-tagged septins to the ring and collar through the cell cycle. Septin ring assembly began several minutes after polarity establishment and this interval was longer in daughter than in mother cells, suggesting asymmetric inheritance of septin regulators. Septins formed an initial faint and irregular ring, which became more regular as septins were recruited at a constant rate. This steady rate of septin recruitment continued for several minutes after the ring converted to a collar at bud emergence. We did not detect a stepwise change in septin fluorescence during the ring-to-collar transition. After collar formation, septins continued to accumulate at the bud neck, though at a reduced rate, until the onset of cytokinesis when the amount of neck-localized septins rapidly decreased. Implications for the mechanism of septin ring assembly are discussed.

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41

Li, Zhi‐Yun. "Ring Formation in Magnetically Subcritical Clouds and Multiple‐Star Formation." Astrophysical Journal 556, no. 2 (August 2001): 813–22. http://dx.doi.org/10.1086/321593.

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42

Morbidelli, A. "Planet formation by pebble accretion in ringed disks." Astronomy & Astrophysics 638 (May 29, 2020): A1. http://dx.doi.org/10.1051/0004-6361/202037983.

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Context. Pebble accretion is expected to be the dominant process for the formation of massive solid planets, such as the cores of giant planets and super-Earths. So far, this process has been studied under the assumption that dust coagulates and drifts throughout the full protoplanetary disk. However, observations show that many disks are structured in rings that may be due to pressure maxima, preventing the global radial drift of the dust. Aims. We aim to study how the pebble-accretion paradigm changes if the dust is confined in a ring. Methods. Our approach is mostly analytic. We derived a formula that provides an upper bound to the growth of a planet as a function of time. We also numerically implemented the analytic formulæ to compute the growth of a planet located in a typical ring observed in the DSHARP survey, as well as in a putative ring rescaled at 5 AU. Results. Planet Type I migration is stopped in a ring, but not necessarily at its center. If the entropy-driven corotation torque is desaturated, the planet is located in a region with low dust density, which severely limits its accretion rate. If the planet is instead near the ring’s center, its accretion rate can be similar to the one it would have in a classic (ringless) disk of equivalent dust density. However, the growth rate of the planet is limited by the diffusion of dust in the ring, and the final planet mass is bounded by the total ring mass. The DSHARP rings are too far from the star to allow the formation of massive planets within the disk’s lifetime. However, a similar ring rescaled to 5 AU could lead to the formation of a planet incorporating the full ring mass in less than 1/2 My. Conclusions. The existence of rings may not be an obstacle to planet formation by pebble-accretion. However, for accretion to be effective, the resting position of the planet has to be relatively near the ring’s center, and the ring needs to be not too far from the central star. The formation of planets in rings can explain the existence of giant planets with core masses smaller than the so-called pebble isolation mass.

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43

Chassignet, Eric P., and Douglas B. Boudra. "Dynamics of Agulhas Retroflection and Ring Formation in a Numerical Model. Part II. Energetics and Ring Formation." Journal of Physical Oceanography 18, no. 2 (February 1988): 304–19. http://dx.doi.org/10.1175/1520-0485(1988)018<0304:doarar>2.0.co;2.

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44

Daisaka, Hiroshi. "N-body Simulations of Planetary Rings." Symposium - International Astronomical Union 208 (2003): 387–88. http://dx.doi.org/10.1017/s0074180900207390.

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We present the formation and evolution of a structure in particles obtained from a local N-body simulation of a dense planetary ring like Saturn's ring. Our simulations show in a particle system the spontaneous formation of a spatial structure like wakes, clumps, and a structure which could be induced by the viscous overstability. Such a formation depends on parameters characterizing a ring system: the wake is likely to form in Saturn's ring and the existence of the wake is consistent with observations. The viscous overstability would be a good candidate for the explanation of subring structures in the ring.

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45

Girvin, Zebediah C., Mary Katherine Andrews, Xinyu Liu, and Samuel H. Gellman. "Foldamer-templated catalysis of macrocycle formation." Science 366, no. 6472 (December 19, 2019): 1528–31. http://dx.doi.org/10.1126/science.aax7344.

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Macrocycles, compounds containing a ring of 12 or more atoms, find use in human medicine, fragrances, and biological ion sensing. The efficient preparation of macrocycles is a fundamental challenge in synthetic organic chemistry because the high entropic cost of large-ring closure allows undesired intermolecular reactions to compete. Here, we present a bioinspired strategy for macrocycle formation through carbon–carbon bond formation. The process relies on a catalytic oligomer containing α- and β-amino acid residues to template the ring-closing process. The α/β-peptide foldamer adopts a helical conformation that displays a catalytic primary amine–secondary amine diad in a specific three-dimensional arrangement. This catalyst promotes aldol reactions that form rings containing 14 to 22 atoms. Utility is demonstrated in the synthesis of the natural product robustol.

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46

SAU, RAJES, and KRISHNAN MAHESH. "Passive scalar mixing in vortex rings." Journal of Fluid Mechanics 582 (June 14, 2007): 449–61. http://dx.doi.org/10.1017/s0022112007006349.

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Direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharibet al. J. Fluid Mech.vol. 360, 1998, p. 121), is found to be 3.6. Simulations are performed for a range of stroke ratios (ratio of stroke length to nozzle exit diameter) encompassing the formation number, and the effect of stroke ratio on entrainment and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar-containing fluid is found to depend linearly on stroke ratio until the formation number is reached, and falls below the linear curve for stroke ratios greater than the formation number. This behaviour is explained by considering the entrainment to be a combination of that due to the leading vortex ring and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result.

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47

Kamijo, Keiju, Naoya Ohara, Mitsuhiro Abe, Takashi Uchimura, Hiroshi Hosoya, Jae-Seon Lee, and Toru Miki. "Dissecting the Role of Rho-mediated Signaling in Contractile Ring Formation." Molecular Biology of the Cell 17, no. 1 (January 2006): 43–55. http://dx.doi.org/10.1091/mbc.e05-06-0569.

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In anaphase, microtubules provide a specification signal for positioning of the contractile ring. However, the nature of the signal remains unknown. The small GTPase Rho is a potent regulator of cytokinesis, but the involvement of Rho in contractile ring formation is disputed. Here, we show that Rho serves as a microtubule-dependent signal that specifies the position of the contractile ring. We found that Rho translocates to the equatorial region before furrow ingression. The Rho-specific inhibitor C3 exoenzyme and small interfering RNA to the Rho GDP/GTP exchange factor ECT2 prevent this translocation and disrupt contractile ring formation, indicating that active Rho is required for contractile ring formation. ECT2 forms a complex with the GTPase-activating protein MgcRacGAP and the kinesinlike protein MKLP1 at the central spindle, and the localization of ECT2 at the central spindle depends on MgcRacGAP and MKLP1. In addition, we show that the bundled microtubules direct Rho-mediated signaling molecules to the furrowing site and regulate furrow formation. Our study provides strong evidence for the requirement of Rho-mediated signaling in contractile ring formation.

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48

Hess, B. Andes. "On the Concerted Ring Opening of Protonated Squalene Oxide and A-Ring Formation in the Biosynthesis of Lanosterol." Collection of Czechoslovak Chemical Communications 68, no. 1 (2003): 202–10. http://dx.doi.org/10.1135/cccc20030202.

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Density functional calculations were performed on a model system of squalene oxide to study the mechanism of the formation of ring A in the biosynthesis of lanosterol from squalene. When (2Z)-6,7-epoxy-3,7-dimethyloct-2-ene was protonated, it was calculated to undergo a very facile ring opening of the oxirane in concert with the formation of the six-membered ring of the 4-(hydroxymethyl)-1,2,3,3-tetramethy1cyclohexyl cation. A study of the reaction pathway (IRC) indicates a very early transition structure in which the carbon- carbon double bond participates anchimerically in the ring-opening of the protonated oxirane. It is suggested that the primary role of the enzyme in this first step of the biosynthesis of lanosterol is protonation of the oxirane ring along with holding the substrate in the proper conformation for the concerted ring-closure to occur. The similarity between this mechanism and that recently proposed for concerted C-ring expansion and D-ring formation in the biosynthesis of lanosterol is discussed.

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49

El-amine, Nour, Sabrya C. Carim, Denise Wernike, and Gilles R. X. Hickson. "Rho-dependent control of the Citron kinase, Sticky, drives midbody ring maturation." Molecular Biology of the Cell 30, no. 17 (August 2019): 2185–204. http://dx.doi.org/10.1091/mbc.e19-04-0194.

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Rho-dependent proteins control assembly of the cytokinetic contractile ring, yet it remains unclear how those proteins guide ring closure and how they promote subsequent formation of a stable midbody ring. Citron kinase is one important component required for midbody ring formation but its mechanisms of action and relationship with Rho are controversial. Here, we conduct a structure–function analysis of the Drosophila Citron kinase, Sticky, in Schneider’s S2 cells. We define two separable and redundant RhoGEF/Pebble-dependent inputs into Sticky recruitment to the nascent midbody ring and show that each input is subsequently required for retention at, and for the integrity of, the mature midbody ring. The first input is via an actomyosin-independent interaction between Sticky and Anillin, a key scaffold also required for midbody ring formation. The second input requires the Rho-binding domain of Sticky, whose boundaries we have defined. Collectively, these results show how midbody ring biogenesis depends on the coordinated actions of Sticky, Anillin, and Rho.

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50

Arai, Ritsuko, and Issei Mabuchi. "F-actin ring formation and the role of F-actin cables in the fission yeastSchizosaccharomyces pombe." Journal of Cell Science 115, no. 5 (March 1, 2002): 887–98. http://dx.doi.org/10.1242/jcs.115.5.887.

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Cells of the fission yeast Schizosaccharomyces pombe divide by the contraction of the F-actin ring formed at the medial region of the cell. We investigated the process of F-actin ring formation in detail using optical sectioning and three-dimensional reconstruction fluorescence microscopy. In wild-type cells, formation of an aster-like structure composed of F-actin cables and accumulation of F-actin cables were recognized at the medial cortex of the cell during prophase to metaphase. The formation of the aster-like structure seemed to initiate from branching of the longitudinal F-actin cables at a site near the spindle pole bodies, which had been duplicated but not yet separated. A single cable extended from the aster and encircled the cell at the equator to form a primary F-actin ring during metaphase. During anaphase,the accumulated F-actin cables were linked to the primary F-actin ring, and then all of these structures seemed to be packed to form the F-actin ring. These observations suggest that formation of the aster-like structure and the accumulation of the F-actin cables at the medial region of the cell during metaphase may be required to initiate the F-actin ring formation. In the nda3 mutant, which has a mutation in ß-tubulin and has been thought to be arrested at prophase, an F-actin ring with accumulated F-actin cables similar to that of anaphase wild-type cells was formed at a restrictive temperature. Immediately after shifting to a permissive temperature, this structure changed into a tightly packed ring. This suggests that the F-actin ring formation progresses beyond prophase in the nda3 cells once the cells enter prophase. We further examined F-actin structures in both cdc12 and cdc15 early cytokinesis mutants. As a result,Cdc12 seemed to be required for the primary F-actin ring formation during prophase, whereas Cdc15 may be involved in both packing the F-actin cables to form the F-actin ring and rearrangement of the F-actin after anaphase. In spg1, cdc7 and sid2 septum initiation mutants, the F-actin ring seemed to be formed in order.

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