when Z or Z??? comprises a nitrogen moiety, a hydrogen of the Z or Z??? nitrogen moiety may be replaced by the group L, wherein L is an amino acid radical, a peptide radical having up to 20 amino acid moieties, or any other small molecule that is a substrate for a nonluciferase; with the proviso that when L is an amino acid radical or a peptide radical, at least one of W1 or a W2 is not H;