FRAG is a shiftable detectable species which provides {HEAD ~ a detectable species} when released from {TAIL ~ the reducible compound}, and 
when released from
FRAG is a shiftable detectable species which provides { HEAD ~ a detectable species } when released from { TAIL ~ the reducible compound } , and
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH