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