A compound having the structure: L-R a -B-----Z wherein Z is a protein component of the compound, which protein component comprises one or more polypeptides, wherein at least one of the one or more polypeptides comprises consecutive amino acids which (i) are identical to a stretch of consecutive amino acids present in a chain of an F c domain of an antibody; (ii) bind to an F c receptor; and (iii) have at their N-terminus a cysteine or a selenocysteine; wherein B is an organic acid residue, or a stretch of 1-50 amino acid residues linking R a and Z; wherein the dashed line between B and Z represents a peptidyl linkage between an N-terminal cysteine or selenocysteine of a polypeptide of Z and an amino acid residue or an organic acid residue of B; wherein L is selected from the group consisting of: a tetrazine and a trans-cyclooctene; and wherein R a is a bond or an organic structure consisting of a chain of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more moieties, wherein each moiety is independently selected from the group consisting of [PEG(y)]z, polyalkylene glycol, polyoxyalkylated polyol, polyvinyl alcohol, polyvinyl alkyl ether, poly(lactic acid), poly(lactic-glycolic acid), polysaccharide, a branched residue, C 1 -C 10 alkyl, C 3 -C 10 cycloalkane, C 2 -C 10 alkene, C 5 -C 10 cycloalkene, amine, sulfur, oxygen, succinimide, maleimide, glycerol, triazole, isoxazolidine, C 2 -C 5 acyl, C 2 -C 5 acylamino, C 2 -C 5 acyloxy, succinyl, malonyl, glutaryl, phthalyl, adipoyl, an amino acid, an aryl group, a heteroaryl group, a carbamate, a chemical structure containing a cyclooctane fused to a dihydropyridazine, a chemical structure containing a cyclooctene fused to a triazole, a chemical structure containing a cyclooctene fused to a isoxazolidine, a dibenzocyclooctene, a dibenzoazacyclooctene, wherein X 1 is CH or N, X 2 is CH 2 or a carbonyl group, and R 5 is an aryl or alkyl group; wherein [PEG(y)]z is: wherein y = 1-100 and z = 1-10. wherein Z is a protein component of the compound, which protein component comprises one or more polypeptides, wherein at least one of the one or more polypeptides comprises consecutive amino acids which (i) are identical to a stretch of consecutive amino acids present in a chain of an F c domain of an antibody; (ii) bind to an F c receptor; and (iii) have at their N-terminus a cysteine or a selenocysteine; wherein B is an organic acid residue, or a stretch of 1-50 amino acid residues linking R a and Z; wherein the dashed line between B and Z represents a peptidyl linkage between an N-terminal cysteine or selenocysteine of a polypeptide of Z and an amino acid residue or an organic acid residue of B; wherein L is selected from the group consisting of: a tetrazine and a trans-cyclooctene; and wherein R a is a bond or an organic structure consisting of a chain of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more moieties, wherein each moiety is independently selected from the group consisting of [PEG(y)]z, polyalkylene glycol, polyoxyalkylated polyol, polyvinyl alcohol, polyvinyl alkyl ether, poly(lactic acid), poly(lactic-glycolic acid), polysaccharide, a branched residue, C 1 -C 10 alkyl, C 3 -C 10 cycloalkane, C 2 -C 10 alkene, C 5 -C 10 cycloalkene, amine, sulfur, oxygen, succinimide, maleimide, glycerol, triazole, isoxazolidine, C 2 -C 5 acyl, C 2 -C 5 acylamino, C 2 -C 5 acyloxy, succinyl, malonyl, glutaryl, phthalyl, adipoyl, an amino acid, an aryl group, a heteroaryl group, a carbamate, a chemical structure containing a cyclooctane fused to a dihydropyridazine, a chemical structure containing a cyclooctene fused to a triazole, a chemical structure containing a cyclooctene fused to a isoxazolidine, a dibenzocyclooctene, a dibenzoazacyclooctene, wherein X 1 is CH or N, X 2 is CH 2 or a carbonyl group, and R 5 is an aryl or alkyl group; wherein [PEG(y)]z is: wherein y = 1-100 and z = 1-10. <EOS>