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