X is selected from {HEAD ~ the group} consisting of {TAIL ~ O} , S , and >N R5, wherein R5 is hydrogen or an alkyl group having from 1 to 9 carbon atoms 
X is selected from { HEAD ~ the group } consisting of { TAIL ~ O } , S , and > N R5 , wherein R5 is hydrogen or an alkyl group having from 1 to 9 carbon atoms
OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH