# Effects of End Group and Aggregation on Helix Conformation: Crystal Structure of $Ac-(Aib-Val-Ala-Leu)_2-Aib-OMe$

Karle, IL and Anderson, Flippen JL and Uma, K and Balaram, P (1996) Effects of End Group and Aggregation on Helix Conformation: Crystal Structure of $Ac-(Aib-Val-Ala-Leu)_2-Aib-OMe$. In: Journal of Peptide Science, 2 (2). pp. 106-116.

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Official URL: http://dx.doi.org/10.1002/psc.52

## Abstract

The role of end groups in determining stereochemistry and packing in hydrophobic helical peptides has been investigated using an \alpha -aminosobutyric acid (Aib) containing model nonapeptide sequence. In contrast to the Boc-analogue, $Ac-(Aib-Val-Ala-Leu)_2-Aib-OMe$ crystallizes with two independent molecules in a triclinic cell. The cell parameters are: space group P1, a=10.100(2)A, b=15.194(4) A, c=19.948(5) A, \alpha =63.12(2), \beta =88.03(2), \gamma =88.61(2), Z=2, R=7.96% for 5140 data where |Fo|>3\sigma (F). The two independent molecules alternate in infinite columns formed by head-to-tail hydrogen bonding. The helices in the two independent molecules are quite similar to each other but one molecule is rotated $\approx 123^ \circ$ about its helix axis with respect to the other. All the helical columns pack parallel to each other in the crystal. Replacement of the bulky Boc group does not lead to any major changes in conformation. Packing characteristics are also similar to those observed for similar helical peptides.

Item Type: Journal Article Journal of Peptide Science European Peptide Society and John Wiley & Sons, Ltd. The copyright belongs to European Peptide Society and John Wiley & Sons, Ltd. all parallel helix assemblies;helix transition;helices;two conformers;water associated with non polar helices;x ray crystallography Division of Biological Sciences > Molecular Biophysics Unit 18 Jan 2006 30 Dec 2016 04:53 http://eprints.iisc.ac.in/id/eprint/4996