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Abstract

The compact ordered conformations of the molecule N-Acteyl-L-Prolyl-D-Alanyl-Methylamide have been studied by semiempirical energy calculations in vacuum and circular dichroism (CD) in solution. The presence of ordered structure has been observed in hydrogen bond promoting solvents like trifluoroethanol by CD studies. In hydrogen bond breaking solvents, like trifluoroacetic acid (TFA), significant fraction of the ordered conformers probably assume extended conformation without intramolecular hydrogen bonds and perhaps are in equilibrium with the fraction of compact ordered structures. The trend observed in going from nonpolar to polar solvent is also compatible with the previous NMR studies in solution. The semiempirical energy calculations have been carried out in the allowed region for β-bends. The flexibility of pyrrolidine ring has been incorporated into the calculations. Representative puckerings, namely, A-type (Cy-exo) and B-type (Cy-endo) have been considered in this study. The results show the B-type to be slightly preferred over the A-type in this tripeptide moiety. The minimum energy conformation predicted from these studies agree only minimally with that found in crystal structure. A better agreement is found after performing the calculations using the geometrical data as observed in the crystal structure of this molecule. Our studies demonstrate that solvent solute interactions are minimal in nonpolar solvents and the predicted minimum energy conformations are preserved at least in nonpolar solvents.

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