Rotational Isomerism in Leucine: Proton Magnetic Resonance Study of [γ-2H]Leucine and Thermodynamic Analysis

The 1H NMR spectrum of [γ2H]leucine has been measured and analyzed to yield values of3 J.(Ha-H3) over a range of pD and temperature. These coupling constants have been used to calculate populations of rotational isomers at a range of temperatures in the cationic, anionic, and zwitterionic states. The rotamer in which the side chain is gauche to the amino group and trans to the carboxylate group has the highest population under all conditions. The thermodynamic values for the interconversion of rotational isomers of leucine (about the Ca-Cβ bond) are derived from experimental populations. For example, the interconversion between the most stable rotamer, in which the isopropyl group is trans to the carboxylate group and gauche to the amino group, and the least stable rotamer, in which the isopropyl group is gauche to both C° substituents, is described for the leucine anion by ΔG°25 = 1000 cal/mol, ΔH° = 2200 cal/mol, and ΔS° = 4 eu. The enthalpies and entropies are discussed in terms of possible intramolecular forces and solvation effects. The free energies are compared to those observed in other amino acids. Differences in rotameric state entropies ranging from 1 to 5 eu are found. These results indicate that isotopic substitution of the y position for the purposes of spectral simplification is effective and suggests that the technique may be generally useful for other amino acids with γ protons.