{HEAD ~ The labeling agent} is a composition, usually a conjugated chemical compound which has {TAIL ~ two properties} 
{ HEAD ~ The labeling agent } is a composition , usually a conjugated chemical compound which has { TAIL ~ two properties }
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL