R11 is H, CH3, {TAIL ~ a carboxylic acid} or a biologically compatible salt of {HEAD ~ a carboxylic acid} 
R11 is H , CH3 , { TAIL ~ a carboxylic acid } or a biologically compatible salt of { HEAD ~ a carboxylic acid }
OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD