Thermodynamic characterizations of an intramolecularly hydrogen bonded C-5-structure across proteinogenic residue

Thermodynamic investigations of a smallest possible intramolecularly hydrogen bonded C-5-structure, across a Thr residue, in model peptides Boc-Xxx-Thr-NH2 (Xxx=Pi e, 1 or Leu, 2), indicated unusual thermal stability of the structure in non-polar medium. An analysis of van't Hoff plots, constructed from variable temperature H-1 NMR data, yielded the thermodynamic parameters of a hydrogen bonded five-membered ring. The non-significance of the spatial organizations of the preceding (CH3)-H-delta bearing hydrophobic proteinogenic residue on the thermal stability of the C-5-structure has been observed. The results revealed that the contribution of this element of secondary structure is quantifiable and the stability appeared to be roughly comparable to other intramolecularly hydrogen bonded reverse turn structures frequently observed in polypeptides and proteins. (C) 1997 Federation of European Biochemical Societies.