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