These compounds comprise a moiety which provides {HEAD ~ a detectable species} when released from {TAIL ~ the compound} in an environment of pH 9 or less  
when released from
These compounds comprise a moiety which provides { HEAD ~ a detectable species } when released from { TAIL ~ the compound } in an environment of pH 9 or less
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH