Dihedral-angle dependence of the vicinal 15N,13C spin-coupling constants. A new NMR parameter for the conformational analysis of amino acids and peptides

The first successful observation of the vicinal ¹⁵N,¹³C spin-coupling constants in a series of amino acids, comprising Val, Ile, Leu and Thr, in which the α-nitrogens are fully replaced with ¹⁵N, is described. A Karplus-type dihedral-angle dependence was noted for the coupling constants between the α-nitrogen and γ-carbon nuclei which may, in turn, be applied to the conformational analysis of other amino acids and small peptides. The potential usefulness and limitation of this new parameter in such applications is briefly discussed.