No abstract

Some cyclic oligopeptides formed by an equal number of alternating D‐ and L‐amino‐acid residues have been synthesized by using the hydrochloride of the open‐chain peptide acid as precursor and the mixed‐anhydride condensation method. The cyclic oligopeptides (tetra‐, hexa‐, and octavaline, hexaleucine, and hexaphenylalanine) form very stable H‐bonded structures (IR‐amide band at 3270–3290 cm−1) which are insoluble in common organic solvents. In CF3COOH/CDCI3 (25°), they yield 1H‐NMR spectra snowing the expected equivalency of the various amino‐acid residues.

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The possibility of selectively reducing the number of beta-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 -- beta 7.2-helices and III forms almost exclusively -- beta 5.6 or -- beta 7.2-helices. The helices are in every case those having the maximum possible number of interchain H bonds.

New conformational data for members of the series BOC-(D-V~~),-(L-V~~~-V~~)(,.,),~-OM~ (rn = 0 or 1 ; n = total number of residues) are presented and used together with earlier results to formulate general conclusions regarding types and relative stabilities of 8-helices formed by D,L-alternating valine peptides. The helices, singleor double-stranded, are of the tightest geometry, the single-stranded helices having only 4,4 residues per turn (84s4-helices) and the double-stranded ones only about 5,6 residues per turn (tl p5p6-and ?t fi5s6-helices). The doublestranded helices are of the type with the highest possible number of interstrand H-bonds [2(n-l)] and the antiparallel ones are always strongly preferred. In chloroform solution right-and lefthanded @4-helices and left-handed tl /35*6-and ?? /35*6-helices are formed by the oligovalines studied with different preferences depending on the peptide chain length and on the parity of the number (odd or even) of residues. The reasons for these preferences are discussed.