" one of R1, R3, R4 and R6 is a straight chain or branched, cyclic, alkyl, alkenyl, alkynyl or aryl group; and any one or more of R1, R3, R4 and R6 may optionally be covalently linked with each other, with Y or with L when L is C or CH to form a cyclic moiety; Z is selected from the group consisting of amine, spermiyl, carboxyspermiyl, guanidyl, spermidinyl, putricinyl, diaminoalkyl, pyridyl, piper" . . . . . . .