The terminal amine groups of oligonucleotide VIII can also be reacted with the thiol compound III as follows: ##STR21## wherein b is an integer equal to 0 or 2 to 20; d is an integer equal to 2 to 6; ?? is --SR or --C(???O)R' where R is 2-pyridyl, 3-carboxy-4-nitrophenyl, 5-nitro-2-pyridyl, or similar group and R' is an alkyl group having 1 to 6 carbon atoms; and Y is an organic group which fo