STRUCTURE AND CONFORMATION OF SPIN-LABELED AMINO-ACIDS IN FROZEN-SOLUTIONS DETERMINED BY ELECTRON NUCLEAR DOUBLE-RESONANCE .1. METHYL N-(2,2,5,5-TETRAMETHYL-1-OXYPYRROLINYL-3-CARBONYL)-L-ALANATE, A MOLECULE WITH A SINGLE PREFERRED CONFORMATION
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The conformation of methyl L-alanate acylated at the amino nitrogen position with the nitroxyl spin-label 2,2,5,5-tetramethyl-1-oxopyrroline-3-carboxylic acid has been determined by electron nuclear double resonance (ENDOR) spectroscopy and computer-based molecular modeling. ENDOR resonance absorptions for each class of protons of the amino acid moiety were well resolved under conditions of very low modulation depth (6-8 kHz) of the radio frequency field and were assigned on the basis of selective deuteriation. From analysis of the dependence of the ENDOR spectra on H0, we have identified the maximum and minimum ENDOR shifts of each class of protons, which correspond to their principal hyperfine coupling (hfc) components. Under the point-dipole approximation, the dipolar hfc components yielded estimates of the electron-nucleus separations with less than a 4% error based on the ENDOR line widths. Computer-based torsion angle search calculations were carried out in order to determine the conformational space compatible with hard-sphere nonbonded constraints and with the ENDOR-determined electron-nucleus separations. The N-acyl peptide linkage of the spin-label with the alanyl moiety is characterized by a planar trans conformation and is rigidly constrained. Three conformations of the methyl carboxylate ester group were identified as compatible with ENDOR-determined distance constraints and were equivalent to conformations of peptides in right-handed -helical, polyproline II, and 27 ribbon structures. Potential energy calculations indicated that at least 90% of the molecules in solution at the freezing temperature of methanol have a methyl carboxylate conformation similar to that of a right-handed -helix. 1993, IEEE. All rights reserved. 1990, American Chemical Society. All rights reserved.