Nucleophilicities of amino acids and peptides

The kinetics of the reactions of amino acids with stabilized diarylcarbenium ions (Ar2CH+) have been studied photometrically in aqueous solution at variable pH. In the range of 10.5 < pH < 12, the amino acids react much faster than the competing nucleophiles hydroxide and water. Though the pK(aH) values of the amino acids vary by almost four units, the nucleophilic reactivities of all primary amino groups differ by less than a factor of 4. The secondary amino group of proline is 102 times more reactive, and the thiolate site in cysteine exceeds the reactivities of the primary amino groups by a factor of 10(4). Nucleophilicity parameters N as defined by the correlation log k(20 degrees)C = s( N + E) have been determined in order to include amino acids into the most comprehensive nucleophilicity scales presently available, which provide a direct comparison of n-, pi-, and sigma- nucleophiles.