ABSTRACT
This chapter includes the modication of hydroxyl and carboxyl functions in proteins. The commonality between these functional groups is that they represent different oxidation states of carbon.1 Serine and threonine contain hydroxyl groups, while tyrosine contains a phenolic group (dened as a hydroxyl group bound to an aromatic ring). The phenolic hydroxyl of tyrosine has a lower pKa (pKa ∼ 10) than serine or threonine (pKa ∼ 13) and is considered more reactive; the formation of O-acetyl tyrosine is more likely than the formation of O-acetyl serine with both reversed by mild base or neutral hydroxylamine.2 The pKa for the tyrosine hydroxyl group is lowered (pKa ∼ 7) on nitration with either tetranitromethane (TNM) or peroxynitrite.3 The formation of dinitrotyrosine would result in a further decrease in the pKa (pKa ∼ 4).4 While not directly relevant to the issue of tyrosine nitration, the pKa for 2,4,6-trinitrophenol is 0.71.5 Serine, threonine, and tyrosine are not considered to be ionizable groups of proteins, while the carboxyl groups of aspartic acid (pKa ∼ 3.5) and glutamic acid (pKa ∼ 4.2) are ionizable groups.6 Carboxyl groups are unique in that there are two carbon-oxygen bonds with different bond lengths.7
