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