{HEAD ~ SC} is {TAIL ~ a conjugated substance} that is said first member of a specific binding pair 
is
{ HEAD ~ SC } is { TAIL ~ a conjugated substance } that is said first member of a specific binding pair
HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH