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