Homonuclear internuclear double resonance spectroscopy as a basis for determination of amino acid conformation.

INDOR (Internuclear Double Resonance) spectroscopy is shown to be superior to conventional (spectra obtained not by sweeping, but by maintaining constant the decoupling frequency) nuclear single- or double-resonance techniques for conformational studies of amino acids and amino acid residues in the following ways: (a) INDOR spectra of amino acids are inherently simpler than conventional proton magnetic resonance spectra of amino acids, and INDOR spectra of individual amino acid residues are slightly, if at all, complicated by overlap with either solvent peaks or the transitions of nuclei in other residues. (b) For each amino acid, the side-chain and C(alpha) proton belong to a particular class of spin system characterized by unique INDOR spectra, the pattern of which aids in the proper assignment of spectral lines. (c) For an amino acid with a first-order spin system, INDOR spectra directly reveal hidden chemical shifts and coupling constants.