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Stereochemical criteria

Venkatachalam, C.M. Stereochemical criteria for polypeptides and profeins. V. Conformation of a system of fhree linked peptide unifs. Biopolymers 6 1425-1436, 1968. [Pg.34]

Over 20 different methods have been proposed for predictions of secondary stmcture they can be categorized in two broad classes. The empirical statistical methods use parameters obtained from analyses of known sequences and tertiary stmctures. All such methods are based on the assumption that the local sequence in a short region of the polypeptide chain determines local stmcture as we have seen, this is not a universally valid assumption. The second group of methods is based on stereochemical criteria, such as compactness of form with a tightly packed hydrophobic core and a polar surface. Three frequently used methods are the empirical approaches of P.Y. Chou and G.D. Fasman and of J. Gamier, D.J. Osguthorpe and B. Robson (the GOR method), and third, the stereochemical method of V.l. him. [Pg.351]

Sasisekharan, V. 1962. Stereochemical Criteria for Polypeptide and Protein Structures, in Collagen, Ramanathan, N., ed. 39-78. Madras, India, Wiley. [Pg.157]

Ramakrishnan, C. and Ramachandran, G. N. (1965). Stereochemical criteria for polypeptide and protein chain conformations. Biophys. J. 5, 909-9CB. [Pg.171]

Bartlett and co-workers have shown that in those rare cases where 2 + 2 and 2 + 4 cycloadditions occur simultaneously between an olefin and a diene, the 2 + 2 addition, as judged by stereochemical criteria, follows the stepwise biradical path whereas the 2 + 4 is stereospecific and concerted.56... [Pg.640]

Internal diastereomers of the kind depicted in Figure 3 have two kinds of interaction between the chiral centers the total interaction and that part of the total interaction which is discriminatory. The latter we call the diastereotopic interaction, and it is only this part of the interaction that causes either Kx in Figure 3 to be displaced from unity or the activation energies of the diastereomeric transition states to be different. It follows that the object in asymmetric synthesis is to maximize the diastereotopic interaction. In the absence of a clear lock-and-key compatibility for the inducing molecule and the substrate, the stereochemical criteria for maximizing the diastereotopic interaction are not always obvious. For example, it is a commonly held view that an increase in the relative steric bulk of certain groups will increase discrimination. This may only increase the nondiscriminatory interaction unless such elaboration is directed at features of the substrate which control the discrimination. [Pg.338]

The structure (XCIIa) of this alkaloid has long been known (Volume II, p. 100) (63,64). Its UV- and IR-spectra as well as a number of chemical reactions clearly indicate that it is 10-methoxyajmalicine. Selenium dehydrogenation yields 5-methoxyalstyrine and the stereochemical criteria as already outlined confirm C/D and D/E trans junctions and a C-19 equatorial methyl. [Pg.714]

Rauvanine, C23H28O5N2.5H2O, mp 129°-135°, [a]D + 32 (CHCI3) has been isolated from JR. vomitoria (65). It contains three methoxyls and its UV-, IR-, and NMR-spectra indicate clearly that it is a 5,6-dimethoxy heteroyohimbane. Stereochemical criteria relegate it to ajmalicine and consequently its structure is XCIIb, indicating a very close relation to raumatorine which accompanies it in the plant (65,48). [Pg.714]

The loss of solubility of a protein at an interface and the observed area per residue suggested that the molecule unfolded and adopted the P conformation. This structure with intermolecular hydrogen bonds adequately accounted for the area observed and is still postulated by some workers (3). Cheesman and Davies (4) suggested other extended conformations with the orientation of the side chains largely determined by their hydrophobic or polar character. Their proposals, mainly based on early work on synthetic polypeptides, do not conform to present stereochemical criteria nor do they take account of the possibility that the a-helix or related helical conformations might be present. [Pg.339]

These are distinguishable by both kinetic and stereochemical criteria the kinetic criteria are employed for the designation of the reactions as SN1 (substitution, nucleophilic, unimolecular) and SN2 (substitution, nucleophilic, bimolecular). [Pg.126]


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See also in sourсe #XX -- [ Pg.51 ]

See also in sourсe #XX -- [ Pg.51 ]




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