Conformationally constrained D,L‐alternating oligopeptides

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Abstract
The possibility of selectively reducing the number of β‐helical structures theoretically possible for a D,L‐alternating peptide by using a N‐methyl group as conformational constraint is considered. Some 1H‐nmr data regarding Boc(L‐Nle‐D‐Nle)3‐L‐Nle‐ D‐MeNle‐L‐Nle‐D‐Nle‐L‐Nle‐OMe (I), its formyl analogue (II), and the pentadecapeptide Boc(D‐Leu‐L‐Leu)5‐ D‐MeLeu‐(L‐Leu‐D‐Leu)2‐OMe (III) are presented. It is shown that these alternating stereocooligopeptides with a N‐methyl group in the (n − 3) (I and II) or (n − 4) position (III) differ drastically in their behavior from the corresponding nonmethylated compounds. In chloroform, I and II form predominantly ↑↓ β7.2‐helices and III forms almost exclusively ↑↓ β5.6 or ↑↓ β7.2‐helices. The helices are in every case those having the maximum possible number of interchain H bonds.