"A compound of the formula ##STR19## wherein n is an integer from 0 to 3,m is an integer from 4 to 9, Z is a C2 -C6 alkylene group, A is H, --SH, --SPO3 H2, --N(CH2)q --SH, --N(CH2)q --SPO3 H2, wherein q is an integer 2 to 4, and B1 and B2 are each independently H, --(CH2)q --SH, or --(CH2)q --SPO3 H2,with the proviso that at least one of A, B1 and B2 is other than H. 2." . . . .