Carbon-13 nuclear magnetic resonance studies of structure and function in thyrotropin-releasing factor. Determination of the tautomeric form of histidine and relation to biology activity

Determination of microscopic acid?base parameters from NMR?pH titrations

“…Other complications arise from the N 1 -H and N 3 -H tautomerism of the imidazole ring (see 5 in Table 2). By using N-alkylated model compounds, tautomeric ratios have been determined for imidazole, histidine, and histamine derivatives including peptides and proteins by 1 H NMR titrations [122,123,124], 15 N NMR titrations [125,126,127], and 13 C NMR titrations in aqueous [122,128,129,130,131,132] and solid phases [132,133]. Microconstants of individual N 1 -H and N 3 -H tautomers of histidine derivatives have been estimated using model com- pounds [122] and directly from the 14 N NMR-pH titration of glycylhistamine and sarcosylhistamine [99].…”

13C-nuclear magnetic resonance study of [85% 13C-enriched proline]thyrotropin releasing factor: 13C-13C vicinal coupling constants and conformation of the proline residue.

“…'SC NMR studies showed that the His-Pro peptide bond adopts both cis and trans configurations with the trans isomer being preponderant under all conditions (20)(21)(22)(23)(24). 'SC spin lattice relaxation studies demonstrated that there is little change in the relative mobilities of the individual residues upon titration of the imidazole ring of histidine (23 (Fig. la, b).…”

“…Conformational studies, both theoretical and experimental, are also numerous. The choices of conformation permitted by this short peptide molecule, which contains a proline residue known to introduce significant steric hindrance, are reflected in the various models proposed (12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24).…”

“…2). This (21)(22)(23), cannot be totally excluded as long as more accurate relationships between 3Jc-c and dihedral angles are lacking.…”

Determination of the preferred tautomeric form of 4‐nitrohistidine

“…The amino acid histidine is present in the active center of several enzymes and is probably involved in the recognition of peptide-hormones at the receptor level [2]. The bacitracin [5], thyroliberin [6], horse cyanoferrimyoglobin [7] and P. aeruginosa azurin [8]. The bacitracin [5], thyroliberin [6], horse cyanoferrimyoglobin [7] and P. aeruginosa azurin [8].…”