Conformational aspect

Similar conformational equilibria occur in arenoquinolizidines (e.g., structures 43-45 for benzo[ ]quinolizidines and 46-48 for dibenzo[ g]quinolizidines). 

Substituents may play a crucial role in the conformation of quinolizidine systems. Thus, compound 49 shows a trans-conformation 50 with all three hydroxyl groups in equatorial positions. For its diastereomer 51, a -conformation 52 was initially proposed, but the H NMR data point at the /raor-conformation 53, with axial orientation of the hydroxy substituents and presumably stabilized by an intramolecular hydrogen bond 2004T3009 . 

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Miyazawa, T. Conformational aspects and biological functions of biomolecules. J. Mol. Struct. 126 (1985) 493-508... 

The conformational aspects of the substitution reactions with sulfuryl chloride have been summarized (27). 

Several aspects of polymer thin films have thus been investigated while many others are still unexplored. These include structural and conformational aspects where polymer thin film properties are theoretically well-treated but experimental data are generally missing. However, with further development of experimental techniques this area might become accessible in the near future. 

Ponnuswamy MN, Gromiha MM, Sony SMM, Saraboji K (2006) Conformational Aspects and Interaction Studies of Heterocyclic Drugs. 3 81-147 Prabhakar YS, Solomon VR, Gupta MK, Katti SB (2006) QSAR Studies on Thiazolidines A Biologically Privileged Scaffold. 4 161-248... 

Waluk J (2000) Conformational aspects of intra and intermolecular excited state proton transfer. In Waluk J (ed) Conformational analysis of molecules in excited State. Willey-VCH, Weinheim, pp 57-112... 

An experimental study of barbituric acid found one new polymorph where molecules in the asymmetric unit adopted two different conformations [10]. The conformational aspect was investigated through the use of ab initio calculations, which permitted the deduction that the new form found would have a lower lattice energy than would the known form. It was also found that many hypothetical structures characterized by a variety of hydrogen-bonding structures were possible, and so the combined theoretical and experimental studies indicated that a search for additional polymorphs might yield new crystal structures. 

R. Wolfenden, Conformational Aspects of Inhibitor Design Enzyme-Substrate Interactions in the Transition State , Bioorg Med. Chem. 1999, 7, 647-652... 

For example, the representation of 5 requires information on the configuration about both C-atoms as well as the statement of at least one dihedral angle, e.g., <5(Cla—C-cfcp— Cl ) = — 60°. If a polycentric molecule corresponds to the equilibrium of an ensemble of conformations, one could describe it with the dihedral angles of all participating conformations, or neglect the conformational aspect and use the configurations as the characteristic class feature of the ensemble. Except for conformation analytic problems, the latter treatment suffices for the solution of chemical problems. 

In the treatment of stereochemical aspects for many chemical problems such as synthetic design representation of tri- and tetracoordinate monocentric configurations by their parity descriptors suffices. The conformational aspect as well as higher coordinate and polycentric configurations can be neglected. Then, it is possible to reduce the CC-matrices to parity vectors Pn whose components +1,0, — 1 represent the configurational features. 

Gibb, W., Jeffery, J. Steric, chiral and conformational aspects of the 3-hydroxy- and 20-hydroxysteroid dehydrogenase activities of cortisone reductase preparations. Biochim. Biophys. Acta 268, 13—20 (1972). 

The major structural factors that influence the reactivity of aspartic acid residues are i) conformational aspects of the peptide, particularly the local flexibility of the peptide chain as dictated by primary, secondary, and tertiary structure, and ii) the amino acid sequence (i.e., the nature of the adjacent residues). Most of the available evidence concerns the influence of adjacent residues, as discussed in this section. 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

In Section lU-B we showed that the chemical shift of methyl carbons in polypropylene can be calculated considering the relative position of the adjacent substituents, expressed in configurational terms (m or r, Oor 1). An alternative interpretation, with a better physical basis, takes into account only conformational aspects. Experimental evidence from hydrocarbon spectra has established that the presence of a gauche conformation between carbon atoms separated by three bonds causes an upfield shift(7 effect) (206). For flexible molecules the magnitude of this shift depends on the fraction of conformers having gauche interactions with the observed carbons (207). 

Computational data enabled charge delocalization modes, substituent effects, and in relevant cases conformational aspects to be addressed, and comparison with experiments were found to be fairly good. Moreover, the calculated relative stabilities of the carbocations correlated reasonably well with the available biological activities. 

Carbon-13 NMR spectroscopy has also been utilized to probe the structure, isomers, and conformational aspects of 1,2,5-oxadiazole chemistry. The dc values for monocyclic furazans vary from 140 to 160 ppm depending on the substituent, and C- H couplings of ca. 200 Hz are observed for the parent furazan and monosubstituted derivatives (92KGS1 lOl >. For furoxans the most noteworthy feature of their carbon spectra is the large displacement of the C(3) resonance to lower chemical shift. While the C(4) peak remains in the range 140-160 ppm that for C(3) is shifted to 100-125 ppm. At low temperatures benzofuroxan shows two peaks at 113.7 (C(3)) and 152.2 (C(4)) which coalesce on warming. The parent furoxan has C- H couplings of 211 Hz for C(3) and 202 Hz for... 

The metal salts of peroxynitrite have also been studied using ab initio methods. We use the same nomenclature as McGrath and Powland s early paper on ONO—OH , which distinguishes different conformers by their dihedral angles. For example, the structure with a cis ONO—O arrangement and the alkali atom bonded to the terminal oxygen with a perpendicular orientation is called cis,perp Figure 1 describes the conformational aspects of the conformers studied. 

Grosu and co-workers also studied the complex configurational and conformational aspects of the unique stereochemistry of substituted spiro-l,3-oxathiane derivatives 100-102 by NMR spectroscopy <2001T8751>. 

The next three sections (Sections 7.7.1, 7.7.2, and 7.7.3) cover fluorescence spectroscopy, I15-18 infrared, and circular dichroism, three powerful approaches to characterize the structure and conformational considerations of synthetic peptides. Section 7.7.1 deals with the use of fluorophores and broad aspects of fluorescence spectroscopy to characterize conformational aspects of peptide structure. In a similar manner, Section 7.7.2 covers a broad aspect of the uses of infrared (IR) techniques to study peptide conformations 19-22 Many IR techniques are discussed, as are approaches for the study of specific peptidic structures including amyloid, p-turn, and membrane peptides. Finally, there is a section on circular dichroism (Section 7.7.3) that covers the major issues of concern for peptide synthetic chemists such as the assignments of a-helix, 310-helix, -sheets and P-turns, and polyproline helices 23-25 There is also a brief description of cyclic peptides. 

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