pK values for the dissociation of the imidazolium, ammonium, and imino groups of histamine, 5-iodohistamine, 2,5-diiodohistamine, N-acetylhistamine, A-acetyl-5-iodohistamine, and N-acetyl-2,5-diiodohistamine were determined by electrometric and spectrophotometric titrations. 0 and 5°values were estimated from the temperature dependence of the dissociation constants of histamine and its mono-and diiodo derivatives. The results did not support the hypothesis of an intramolecular NH-N hydrogen bond between the side-chain amino group and an imidazole nitrogen in aqueous solutions of histamine.Niemann and Hays,1 based mainly on the histamine-like activity of ß-(2-pyridyl) ethylamine, proposed that the biological activity of histamine depends on the existence of an intramolecular H bond between the protonated amino group and the "pyridine" nitrogen of the imidazole ring. This hypothesis was strengthened by Lee and Jones'2 analysis of the structural re-
A combined resin solvation-peptide chain motion and kinetics of coupling reaction approach was applied to monitor details of the synthesis of TM-34, a 34-residue transmembrane segment of the bradykinin receptor. The dynamics of resinbound peptide fragments attached to a stable free radical amino acid were examined by EPR spectroscopy. In agreement with an abrupt decrease (from 83 to 43%) in peptide purity occurring in the 12−16 region when DMF was used, a much more strongly immobilized chain population was detected, especially at the 12-mer stage. Conversely, faster
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