The substructure R represents one of a halogen, —{HEAD ~ SH}, —SZ, —S—S M L C2, wherein Z represents a thiol protecting {TAIL ~ group} 
The substructure R represents one of a halogen , — { HEAD ~ SH } , — SZ , — S — S M L C2 , wherein Z represents a thiol protecting { TAIL ~ group }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL