Determination of α-Amino Group pK by Cyanate Reaction

The measurement of the kinetics of the reaction of cyanate with terminal α-amino groups was used to determine the pK of the amino groups and the pH-independent second order rate constants. The technique was applied to two peptides, glycylglycine and l-valyl-l-leucyl-l-seryl-l-glutamylglycine, with good agreement with the known pK values. The technique was also applied to the main component ferrimyoglobins of sperm whale (Physeter catodon), California grey whale (Eschritius gibbosus), pilot whale (Globicephala melaena), Dall porpoise (Phocoenoides dalli dalli), and harbor or common seal (Phoca vitulina). The pK values for the α-amino groups of the ferrimyoblobins, listed in order as above, were 7.96, 7.74, 7.43, 7.22, and 7.66, respectively. The range of rate constants for the ferrimyoglobins was 5 to 17 m-1 s-1. For the dipeptide and pentapeptide the rate constants were 28 and 36 m-1 s-1, respectively. Structural factors presumably control the values for the rate constants for the proteins.