{TAIL ~ R4} represents hydrogen or a linear or branched {HEAD ~ C1 C6 alkyl} optionally substituted by hydroxy, mercapto, C1 C3 alkythio, amino, carboxy or ureido 
{ TAIL ~ R4 } represents hydrogen or a linear or branched { HEAD ~ C1 C6 alkyl } optionally substituted by hydroxy , mercapto , C1 C3 alkythio , amino , carboxy or ureido
TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH