{HEAD ~ R6} and {TAIL ~ R9} are each, independently, hydrogen, alkyl of 1 6 carbon atoms, or aralkyl of 7 10 carbon atoms 
{ HEAD ~ R6 } and { TAIL ~ R9 } are each , independently , hydrogen , alkyl of 1 6 carbon atoms , or aralkyl of 7 10 carbon atoms
HEAD HEAD HEAD HEAD HEAD OTH 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