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
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The conformation of methyl N-(2,2,5,5-tetramethyl-1-oxypyrrolinyl-3-carbonyl)-L-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 3-fluoro- and 2-fluorotryptophan 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 hyperfme 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 (1 ~-63) with a near perpendicular (2 ~(+105) orientation of the indole ring. The second conformer was characterized by ~1 ~-95 and 2 ~-105, 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. 1990, American Chemical Society. All rights reserved.
