each Z is independently selected from the group consisting of —OH, —COOH, —CNHR, —{TAIL ~ NHCR} and {HEAD ~ —NHR}, wherein each R is independently chosen, and defined as above; and 
each Z is independently selected from the group consisting of — OH , — COOH , — CNHR , — { TAIL ~ NHCR } and { HEAD ~ — NHR } , wherein each R is independently chosen , and defined as above ; and
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH