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