in which R1 is an amino acid side chain or a functionalized amino acid side chain; R3 is a functional group including, but not limited to hydrogen, alkyl, cycloalkyl, aryl and ???NHP, wherein P is selected from the group consisting of H, alkyl, cycloalkyl, aryl and a protecting group; and X??? is the conjugate base of an acid having a pKa of less than about 4.