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