An N-methylated residue at the n -3 position of the chain was used to reduce the maximum number of H-bonds realizable by some D,L-alternating oligopeptides ing4 '-, TtB5 '-and Tl/rs 6-helices and thus increase for the oligopeptides, the relative stability of largerb -helices. With D,L-alternating oligoleucines of the series Boc-Leu,-OMe, however, this approach did not produce the helices expected. Although tl/r"-helices with only one free NH per strand would theoretically be possible, the N-methylated oligoleucines formed instead flawed g4 4-helices having three free NH's in CHCI, as well as in other solvents of low polarity. These observations confirm that the stability ofg-helices does not depend only on the number of intra-or interstrand H-bonds, and corroborate the idea thatg -helices with large cavities are inherently unstable.

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The conformational behavior of members of the series Boc‐(L‐Nle)m‐(D‐Nle‐L‐Nle)(n‐m)/2‐OMe (m= 0 or 1; n= total number of residues) with n≤ 12, and of analogs of comparable chain length having a NMe‐group on the (n ‐ 3)th residue has been investigated. The study has shown that D,L‐alternating oligonorleucines behave very differently from stereo‐co‐oligopeptides of D‐alloisoleucine and L‐isoleucine, D‐ and L‐valine, or D‐ and L‐leucine. In particular, it has been found that oligonorleucines do not form β‐helices as do the other oligopeptides. Instead, they form aggregates (very likely of the α‐pleated sheet type), which are insoluble in common organic solvents even at moderate chain lengths. In marked contrast with this behavior, N‐methylated analogs such as those studied, with n from 9 to 15, cannot generate very stable aggregates owing to the N‐methyl group, and they prefer to form β‐helices. These β‐helices have been found by solution 1H NMR techniques to be almost exclusively of the types β4,4 (single‐stranded with about 4.4 residues per turn) and ↓↑5,6 (double‐stranded, antiparallel, with about 5.6 residues per turn).

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