"According to one aspect, Y is an amino acid or polypeptide having an amino group covalently bound to an a-carbon of the amino acid and the amino group is covalently bound to a linker group R 2 for example (CH 2 ) n ; n is an integer of, 1 , 2, 3, or 4, and Ri is aliphatic, such as alkyl, straight or branched chain, saturated or unsaturated; and salts thereof." . . . .