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