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