Nuclear magnetic resonance study of side-chain conformation of tyrosyl residue in [Met5]-enkephalin. Solvent and temperature dependence.

[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.