Z1 is oxygen, {HEAD ~ sulfur} or {TAIL ~ an unsubstituted or lower alkyl} substituted nitrogen; and 
Z1 is oxygen , { HEAD ~ sulfur } or { TAIL ~ an unsubstituted or lower alkyl } substituted nitrogen ; and
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH