Unfolding of an α-helix in peptide crystals by solvation: conformational fragility in a heptapeptide

Karle, Isabella L. ; Flippen-Anderson, Judith L. ; Uma, K. ; Balaram, P. (1993) Unfolding of an α-helix in peptide crystals by solvation: conformational fragility in a heptapeptide Biopolymers, 33 (5). pp. 827-837. ISSN 0006-3525

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Official URL: http://www3.interscience.wiley.com/journal/1075896...

Related URL: http://dx.doi.org/10.1002/bip.360330511

Abstract

The structure of the peptide Boc-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe has been determined in crystals obtained from a dimethylsulfoxide-isopropanol mixture. Crystal parameters are as follows: C38H69N7O10 · H2O · 2C3H7OH, space group P21, a = 10.350 (2) Å, b = 26.084 (4) Å, c = 10.395(2) Å,β = 96.87(12), Z = 2, R = 8.7% for 2686 reflections observed > 3.0 σ (F). A single 5 →1 hydrogen bond is observed at the N-terminus, while two 4 →1 hydrogen bonds characteristic of a 310-helix are seen in the central segment. The C-terminus residues, Ala(6) and Leu(7) are expended, while Val(5) is considerably distorted from a helical conformation. Two isopropanol molecules make hydrogen bonds to the C-terminal segment, while a water molecule interacts with the N-terminus. The structure is in contrast to that obtained for the same peptide in crystals from methanol-water [ I. L. Karle, J. L. Flippen-Anderson, K. Uma, and P. Balaram (1990) Proteins: Structure, Function and Genetics, Vol. 7, pp. 62-73] in which two independent molecules reveal an almost perfect α-helix and a helix penetrated by a water molecule. A comparison of the three structures provides a snapshot of the progressive effects of solvation leading to helix unwinding. The fragility of the heptapeptide helix in solution is demonstrated by nmr studies in CDC13 and (CD3)2SO. A helical conformation is supported in the apolar solvent CDCl3, whereas almost complete unfolding is observed in the strongly solvating medium (CD3)2SO.

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