Conformational analysis of thyrotropin releasing factor by proton magnetic resonance spectroscopy

The proton magnetic resonance spectrum of thyrotropin releasing factor (TRF) in solution in deuterium oxide and deuterated dimethylsulfoxide (DMSO–d 6 ) has been analyzed. Two forms differing in cis – trans isomerism about the His‐Pro peptide bond are observed. From the temperature dependence of chemical shift of the amide protons, it is concluded that TRF in DMSO–d 6 does not contain intramolecular hydrogen bonds. Measurement of NHC α H coupling constant provides an estimate of the histidine dihedral angle ϕ. Structural information about the histidine side‐chain is deduced from C α HC β H coupling constants and from the nonequivalence of the two prolyl δ‐protons. In DMSO–d 6 , there is evidence for a tautomeric equilibrium corresponding to an exchange of imidazole proton between the two nitrogen atoms N‐δ and N‐ε. In water, the N‐εH tautomer is found to be the predominant tautomeric form of the imidazole ring. These results in combination with energy calculation, vibrational analysis, and carbon nmr studies allow the determination of the conformationof TRF.