Conformational analysis of four β‐methylphenylalanine stereoisomers in a bioactive peptide by z‐filtered relay NMR spectroscopy

The rotamer populations around the C‐α—C‐β bond of amino acid residues with one β‐proton can be calculated from homonuclear and heteronuclear vicinal coupling constants. The measurement of long‐range heteronuclear coupling constants, however, suffers from the inherent low sensitivity of the heteronuclear multiple bond experiments. In this paper it is demonstrated that z‐filtered homonuclear and heteronuclear relay spectroscopy provides a sensitive and accurate means for the evaluation of conformationally important vicinal coupling constants. Side‐chain conformations of the four stereoisomers of β‐methylphenylalanine residues in synthetic octapeptide analogues of CCK‐8 were derived from the measured coupling constants. This information is not easily available from conformational analysis based on simple steric considerations.