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