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