The terminal amine groups of oligonucleotide VIII can be reacted with the thiol compound II as follows: ##STR20## wherein b is an integer equal to 0 or 2 to 20; c is 1 or 2; Z is oxygen or NH2 + ; and W is hydrogen or --NHC(???O)--R' where R' is an alkyl group having 1 to 6 carbon atoms.