Structure and Conformation of Spin-Labeled Methyl L-Phenylalanate and L-Phenylalanine Determined by Electron Nuclear Double Resonance Spectroscopy

Article abstract of DOI:10.1021/j100176a021

The conformation of L-phenylalanine and of methyl L-phenylalanate acylated at the amino nitrogen position with the nitroxyl spin-label 2,2,5,5-tetramethyl-1-oxypyrroline-3-carboxylic acid has been determined by electron nuclear double resonance (ENDOR) spectroscopy and molecular modeling.ENDOR resonance absorptions of spin-labeled derivatives in frozen glassy solutions were well resolved for each class of protons of the amino acid moiety under conditions of low modulation depth of the radio-frequency field and were assigned by selective deuteration.From the maximum and minimum ENDOR shifts that correspond to the principal hyperfine coupling components, the dipolar hyperfine contributions were calculated to estimate electron-nucleus separations.Spin-labeled derivatives of L-phenylalanine with fluorine substituted at the C^δ1- and C^ζ-positions of the side chain were employed for assigning the conformation of the aromatic side chain.Conformational analysis on the basis of torsional angle search calculations constrained by the ENDOR determined electron-nucleus separations showed that the predominant conformer of spin-labeled derivatives of the C^δ1-fluorinated derivatives of L-phenylalanine and of its methyl ester analogue belongs to the classical g^- rotamer in which the α-C^β-C^γ-C^δ1(-F)> torsion angle has a value of -83 deg corresponding to an antiperpendicular orientation of the o-fluorobenzyl side chain.The results are explained on the basis of dipolar interactions between the peptide bond, the carboxylate group, and the o-fluorobenzyl side chain.This conformation of the side chain was independent of solvent polarity.The results also indicate that in the preferred conformer the carboxylate group acquires a conformation similar to that of a fully extended polypeptide in a β-strand.