N.m.r. studies on the conformation of cyclo(‐l‐Trp‐l‐His) in solutions

Abstract

N.m.r. studies on the conformations of cyclo(‐l‐Trp‐l‐His) in acidic and basic D2O and DMSO solutions are presented here. The detailed conformational analysis of the cyclodipeptide molecule is based on 1H chemical shifts, spin‐spin coupling constants and NOE measurements. The conformational analysis consists of the following steps: 1) Calculations of the allowed {χn} states of the peptide molecule, (an allowed {χn} state is characterized by defining the two torsional angles χ1 and χ2 for each amino acid residue, which does not violate the excluded volume criteria). 2) Calculations of the induced chemical shifts of the His H5 and Hβ resonances arising from the indole and imidazole ring current effects, in the various allowed {χn} states and the comparison of the calculated and measured chemical shifts of the above resonances. 3) The conformation of the DKP ring and the distribution of the relative populations among the three rotational isomers of each of the side chains were estimated from the measured coupling constants. Finally, the NOE measurements enabled us to determine the various pairs of protons which are spatially in close proximity to each other.