The Nucleophilic Reactivity of Valine Methylamide. A Model of the N-Terminal Valine Residues of Hemoglobin.

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Abstract
Valine methylamide (1) was synthesized as a model of the N-terminal valine residues of [human] Hb and a separation system was devised for its reaction products with a few mutagenic/carcinogenic alkylating agents. The pK''a of 1 was 7.65 at 37.degree. C or about 1 pH unit higher than those previously determined for the N-terminal valine residues of liganded Hb, in line with the existence of salt bridges between these residues and negatively charged amino acids. The nucleophilicity, n, of 1 in the Swain-Scott scale was about 4.35, which in combination with a relatively low pKa-value in comparison to other aliphatic amines makes it the most reactive model of nucleophilic amino groups in proteins at physiological pH. The reactivity vs. the N-terminal valine residues of Hb of the few, relatively small, alkylating agents studied to date were slightly lower than predicted from their reactivity vs. 1 also when the Bronsted dependence of the nucleophilicity on the basicity was taken into account.

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