Conformation factors affecting

Conformational analysis is the study of how conformational factors affect the structure of a molecule and its physical chemical and biological properties... 

Fig. 12.6. Conformational factors affecting syn and anti diastereoselectivity in Sharpless epoxidation. If substituent R4 > H, A1,3 strain favors the syn product. If R4 = H, the preferred transition structure leads to anti product. Reproduced from/. Org. Chem., 67, 1427 (2002), by permission of the American Chemical Society.

Conformational analysis is the study of how conformational factors affect the structure of a molecule and its properties. In this chapter we ll examine the conformations of various alkanes and cycloalkanes, focusing most of our attention on three of them ethane butaney and cyclohexane. You will see that even simple organic molecules can exist in many conformations. Conformational analysis will help us to visualize organic molecules in 3D and to better understand their structure and properties. 

The photochemistry of conjugated polyenes has played a central role in the development of modern molecular photochemistry, due in no small part to its ultimate relevance to the electronic excited state properties of vitamins A and D and the visual pigments, as well as to pericyclic reaction theory. The field is enormous, tremendously diverse, and still very active from both experimental and theoretical perspectives. It is also remarkably complex, primarily because file absorption spectra and excited state behavior of polyene systems are strongly dependent on conformation about the formal single bonds in the polyene chain, which has the main effect of turning on or off various pericyclic reactions whose efficiencies are most strongly affected by conformational factors. 

L. Hough and A. C. Richardson, Conformational and electronic factors affecting the nucleophilic degradation of diethylsulphonylglycopyranosylmethane derivatives, J. Chem. Soc. (1962) 1024-1032. 

Fat absorption of protein additives has been studied less extensively than water absorption and consequently the available data are meager. Although the mechanism of fat absorption has not been explained, fat absorption is attributed mainly to the physical entrapment of oil (7). Factors affecting the protein-lipid interaction include protein conformation, protein-protein interactions, and the spatial arrangement of the lipid phase resulting from the lipid-lipid interaction. Non-covalent bonds, such as hydrophobic, electrostatic, and hydrogen, are the forces involved in protein-lipid interactions no single molecular force can be attributed to protein-lipid interactions ( ). 

Directionality has also been studied for the second step. Once the tetrahedral intermediate (67) is formed, it loses Y (giving the product) or X (reverting to the starting compound). Deslongchamps has proposed that one of the factors affecting this choice is the conformation of the intermediate more specifically, the positions of the lone pairs. In this view, a leaving... 

Chapters 7 to 12 deal with factors affecting conformational equilibria and complex formation. Angyal explains the effect of calcium chloride on the anomeric equilibria of certain sugars by formation of especially stable complexes. These involve an axial-equatorial-axial sequence of three hydroxyl groups on a six-membered ring or a cis-cis sequence on a five-membered ring. Lemieux and Brewer use model compounds to study solvation effects on the orientation of the hydroxymethyl... 

Conformational factors play a role with large macromolecules. Changes in the conformation of macromolecules may affect the local environment and, thus, the retention energy of specific coordinating groups. Retention of metals at the surface or inside particles may be influenced by similar effects. 

The procedures and calculations described in this chapter provide considerable insight into the factors affecting the conformations of polypeptides and proteins. The computer programs for gramicidin-S, oxytocin, vasopressin, etc., can also be used for larger structures—of the size of ribonuclease and lysozyme—although the required computer time is considerably increased. 

Effects of Coordination Sphere Structural Factors Affecting B —>Z Conformational Changes... 

A lot of fresh material, especially the quantum mechanical study of conformational equilibria made good progress and improved enormously our insight into the steric and electronic factors affecting various conformers. Many conformational equilibria have to be reevaluated. 

In solution and apparently on the cell surface as well, integrins are not fixed in a particular conformation, but equilibrate between them (Takagi et al, 2002) (Fig. 3A-G). Whether the equilibrium favors the bent, low affinity conformation or the extended, high affinity conformation is affected by the presence of activating intracellular factors and the... 

Influence of Physical Structure. The hydrolytic behavior of cellulose is much influenced by its physical structure and lateral order [121-132]. Wood cellulose was hydrolyzed twice as fast as cotton [125]. Hydrolysis rate was significantly increased by physical or chemical pretreatment, with the effect depending on the source of cellulose. Hill and coworkers [127,128] reported that mercerization increased the hydrolysis rate of cotton (by 40%) and of ramie (7%), whereas the opposite effect was observed for linen and a-cellulose samples showing an approximately 30% reduction. Based on kinetic analysis, they concluded that the end-attach model proposed by Sharpies [121] can only be applied to the cellulose II structure and not to the cellulose I crystallite. Thus, the conformation of cellulose is also a significant factor affecting its reactivity and possibly the hydrolytic mechanism as well. 

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