Conformational energy analysis of melanostatin

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Abstract
Conformational energy calculations were carried out on the hypothalamic hormone melanostatin, a tripeptide with the primary structure H‐L‐Pro‐L‐Leu‐Gly‐NH2. The calculated lowest energy conformation was a type II β bend, very similar to that reported in an X‐ray crystal study. This conformation, however, was only one of 109 low‐energy structures (≤ 3 kcal/mol above the global minimum), indicating that the molecule in solution exists as an ensemble of conformations and is very flexible, in agreement with relaxation data from n.m.r. measurements. A statistical analysis yielded an average end‐to‐end distance of 6.8 Å and a bend probability of 0.62, suggesting that, in nonpolar solvents, bend structures predominate within the statistical ensemble. The statistical analysis, however, also yielded a probability of only 0.11 for the occurrence of a 4 → 1 hydrogen bond. Hence, the calculations show that, although bend conformations predominate, bends would be difficult to observe in solution if the experiments were designed only to detect 4 → 1 hydrogen bonds.

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