Structure and conformation of spin-labeled amino acids in frozen solutions determined by electron nuclear double resonance. 2. Methyl N-(2,2,5,5-tetramethyl-1-oxypyrrolinyl-3-carbonyl)-L-tryptophanate, a molecule with multiple conformations

Abstract

The conformation of methyl N-(2,2,5,5-tetramethyl- l-oxypyrrolinyl-3-carbonyl)-~-tryptophanate in frozen solutions has been determined by application of electron nuclear double resonance (ENDOR) spectroscopy and computer-based molecular modeling. ENDOR spectra of methyl L-tryptophanate and of the corresponding methyl esters of 0-fluore and q24uorotryptophan acylated at the amino group with the spin-label 2,2,5,5-tetramethyl- 1 -oxypyrroline-3-carboxylic acid exhibited well-resolved resonance adsorptions from protons and fluorines of the amino acid moiety. The ENDOR shifts were shown to correspond to principal hyperfine coupling (hfc) components, from which the dipolar contributions were estimated to calculate electron-nucleus separations. The ENDOR data indicated that there are two distinct conformations of spin-labeled methyl tryptophanate, the relative populations of which were dependent on solvent polarity. Torsion angle search calculations constrained by the ENDOR data showed that the predominant conformation in methanol was similar to that of a classical g- rotamer (x, - -63') with a near perpendicular (x2 - +loso) orientation of the indole ring. The second conformer was characterized by x, - -95' and x2 - -1 05O, indicative of an antiperpendicular orientation. In chloroform/toluene only the antiperpendicular conformer was detected. The different solvent-dependent orientations of the indole ring with respect to the nitroxyl group are explained on the basis of dipolar interactions of the aromatic side chain with solvent and with the peptide bond. The structure and conformation of amino acids and peptides in solution are of considerable importance in biophysical studies since the distribution of the side-chain dihedral angles is far from random and preferred conformations of amino acid side chains are frequently observed in proteim2 In general, the conforma- tional analysis of amino acid and peptide derivatives in solution has been carried out on the basis of circular dichroism, infrared spectroscopy, nuclear magnetic resonance (NMR3), and potential energy calculations. Analysis of direct structural data obtained by NMR methods depends primarily on estimates of the vicinal, through-bond coupling constants of H with the peptide amide proton HN or on estimates of coupling constants of protons bonded to adjacent carbons along the side chain.4 Reliable estimates of