R16 and {HEAD ~ R17} are each, independently, hydrogen, {TAIL ~ alkyl} of 1 12 carbon atoms, or n Ar 
R16 and { HEAD ~ R17 } are each , independently , hydrogen , { TAIL ~ alkyl } of 1 12 carbon atoms , or n Ar
OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH