CONFORMATIONAL-ANALYSIS OF 4 BETA-METHYLPHENYLALANINE STEREOISOMERS IN A BIOACTIVE PEPTIDE BY Z-FILTERED RELAY NMR-SPECTROSCOPY

The rotamer populations around the C-alpha-C-beta bond of amino acid r esidues with one beta-proton can be calculated from homonuclear and he teronuclear vicinal coupling constants. The measurement of long-range heteronuclear coupling constants, however, suffers from the inherent l ow sensitivity of the heteronuclear multiple bond experiments. In this paper it is demonstrated that z-filtered homonuclear and heteronuclea r relay spectroscopy provides a sensitive and accurate means for the e valuation of conformationally important vicinal coupling constants. Si de-chain conformations of the four stereoisomers of beta-methylphenyla lanine residues in synthetic octapeptide analogues of CCK-8 were deriv ed from the measured coupling constants. This information is not easil y available from conformational analysis based on simple steric consid erations.