# Molecular structure of Boc-Aib-Phe-Aib-Met-$NH_2$.DMSO. A fragment of a biologically active enkephalin analog

Prasad, Venkataram BV and Sudha, Subrahmanyam T and Balaram, Padmanabhan (1983) Molecular structure of Boc-Aib-Phe-Aib-Met-$NH_2$.DMSO. A fragment of a biologically active enkephalin analog. In: Journal of the Chemical Society: Perkin Transactions 1: Organic and Bio-Organic Chemistry (2). pp. 417-421.

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The tetra peptide t- butyloxycarbonyl- a-aminoisobutyryl- a-aminoisobutyryl- L-phenylalanyl- L-methionyl amide crystallizes in the orthorhombic space group P21212, with a = 9.096, b = 18.067, c = 21.701 A and Z = 4. The crystals contain one molecule of dimethyl sulphoxide (DMSO) associated with each peptide. The structure has been solved by direct methods and refined to an R value of 0.103 for 2 672 observed reflections. The peptide adopts a distorted 31 helical structure stabilized by two intramolecular 4 $\rightarrow$ 1 hydrogen bonds between the Boc CO and Aib(1) CO groups and the NH groups of Phe(3) and Met(4), respectively. A long hydrogen bond (N 0 = 3.35 A) is also observed between Aib(2) CO and one of the terminal amide hydrogens. The DMSO molecule is strongly hydrogen bonded to the Aib(1) NH group. The solid-state conformation agrees well with proposals made on the basis of n.m.r. studies in solution.