Nuclear magnetic resonance analyses of side chain conformations of histidine and aromatic amino acid derivatives

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Abstract
Stereoselectively β‐deuterated species were synthesized of Ac‐His‐NHMe, Ac‐His‐OEt, Ac‐His‐OH and H‐His‐NHMe, which are useful as models of histidine residues in peptides. From the spectral comparison of 1H n.m.r., the β‐proton resonances of the normal species were unambiguously assigned. In (C2H3)2SO, C22H5O2H, C2H3O2H, and C52H5N solution and in aqueous solution, the lower‐field and higher‐field components of β‐proton resonances of the four histidine derivatives are assigned to the pro‐R and pro‐S protons, respectively. The alternative assignments apply for Ac‐His‐NHMe, Ac‐His‐OEt and Ac‐His‐OH in non‐polar solvents such as C2HCl3. Vicinal coupling constants 3JαβS and 3JαβR were obtained for calculating the fractional populations of rotamers about the Cα–Cβ bond. The rotamer populations depend little on the ionization states of the α‐amino and carboxyl groups or the imidazole ring. The rotamer populations depend significantly on the solvent polarity, similar to those of Phe, Tyr and Trp derivatives. For the two β‐proton resonances of His, Phe, Tyr, and Trp derivatives in a variety of solvents, linear relationships are found between the differences in chemical shifts and the differences in vicinal coupling constants.

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