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