1. Nuclear magnetic resonance analyses of side chain conformations of histidine and aromatic amino acid derivatives. Solvent and pH dependence
J Kobayashi, T Higashijima, T Miyazawa Int J Pept Protein Res. 1984 Jul;24(1):40-7. doi: 10.1111/j.1399-3011.1984.tb00925.x.
Stereoselectively beta-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 beta-proton resonances of the normal species were unambiguously assigned. In (C2H3)2SO, C2(2)H5O2H, C2H3O2H, and C5(2)H5N solution and in aqueous solution, the lower-field and higher-field components of beta-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 3J alpha beta S and 3J alpha beta R were obtained for calculating the fractional populations of rotamers about the C alpha-C beta bond. The rotamer populations depend little on the ionization states of the alpha-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 beta-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.
2. Nuclear magnetic resonance study of side-chain conformation of tyrosyl residue in [Met5]-enkephalin. Solvent and temperature dependence
J Kobayashi, T Higashijima, U Nagai, T Miyazawa Biochim Biophys Acta. 1980 Feb 27;621(2):190-203. doi: 10.1016/0005-2795(80)90171-3.
[Met5]-Enkephalin and tyrosine methylamide containing (2S, 3S)-[2,3-2H]-tyrosine and [Met5]-enkephalin containing (2S, 3R)-[3-2H]tyrosine were synthesized. 270 MHz 1H-NMR spectra of the normal species and selectively deuterated species were analyzed. The lower field and higher field beta-proton signals of the Tyr1 residues of [Met5]-enkephalin (and tyrosine methylamide) were unambiguously assigned to the pro-S and pro-R protons, respectively, in organic solvents, but the alternative assignments apply in 1H2O. For [Met5]-enkephalin and tyrosine methylamide (model for N-terminal tyrosyl residue), the rotamer populations around the Calpha-Cbeta bond of tyrosyl residue were determined in a variety of solvents. Rotamer populations of the tyrosyl residue depend on solvent polarity. In aqueous solution, the rotamers I and II are predominant while in weakly polar solvents the rotamer I becomes predominant. For the tyrosyl residue of the dipolar form of [Met5]-enkephalin in (C2H3)2SO solution, anomalous temperature dependences of rotamer populations are observed. This anomaly is suggested to be due to the equilibrium of the folded and extended conformations of the main chain. In fact, the temperature dependences of rotamer populations become normal in the presence of 2 M NH4ClO4, which weakens the attraction between the N-terminal and C-terminal groups of the dipolar form and thus reduces the population of the folded form in (C2H3)2SO solution.
3. Nuclear magnetic resonance study of side-chain conformation of phenylalanine residue in [Met5]-enkephalin: solvent, pH, and temperature dependence
J Kobayashi, U Nagai, T Higashijima, T Miyazawa Biochim Biophys Acta. 1979 Mar 27;577(1):195-206. doi: 10.1016/0005-2795(79)90021-7.
[Met5]-Enkephalin and N-acetylphenylalanine methylamide containing (2S,3S)-[2,3-2H2]Phe were synthesized 270 MHz 1H NMR spectra of the normal and selectively deuterated species were analysed. The lower-field and higher-field beta-proton signals of the Phe4 residue of [Met5]-enkephalin were unambiguously assigned to the pro-S and pro-R protons, respectively. The same assignments apply to N-acetylphenylalanine methylamide in polar organic solvents and in 2H2O, but the alternative assignments apply in C2HCl3. For [Met5]-enkephalin, the vicinal spin coupling constants 3JalphabetaS and 3 JalphabetaR and the rotamer populations around the Calpha-Cbeta bond were determined in a variety of solvents. From the pH and temperature dependences of rotamer populations of [Met5]-enkephalin, the side-chain conformation of the Phe residue in 2H2O solution was found to be considerably different from that in (C2H3)2SO solution. Rotamer populations of the Phe4 residue of [Met5]-enkephalin in organic solvents depend on solvent polarity. As compared with the reference model molecule of N-acetylphenylalanine methylamide, the rotamer populations of Phe4 of [Met5]-enkephalin are affected possibly by steric repulsion with other residues; the rotamer I is primarily favored but the rotamer II is appreciably destabilized in weakly polar solvents.
