Heteronuclear Vicinal Coupling Constants and Site-Specific Isotopic Substitution in the Investigation of Rotational Isomerism in Leucine

Vicinal coupling constants about the Cα-Cβ bond between combinations of 1H, 13C, and 15N nuclei were measured in three isotopic isomers of leucine. The couplings found were used to calculate populations of staggered rotamers. These values, which are overdetermined by the several coupling constants, are found to be self-consistent for both anionic and cationic forms. The necessity for simplification of the spin system by substitution of deuterium for hydrogen is discussed. A statistical method of using the observed couplings to make stereochemical assignments is introduced.