Reaction of 3-isothiocyanatonaphthalene-1,5-disulphonate with amino acids, peptides, and proteins [proceedings].

Phenylarsenious acid and diethyltin, which are known to have a great affinity for dithiol groups (Aldridge & Cremer, 1955; Peters, 1946), would be expected to compete with triethyltin for the two cysteine components of the triethyltin-binding sites. This was observed, although the affinities obtained with phenylarsenious acid and diethyltin for untreated cat haemoglobin (2x 1 0 4 ~ ' and approx. 1 0 z ~ ' respectively) are less than with triethyltin for cat haemoglobin ( 3 . 5 ~ I O 4 ~ ' ) , and lower than would be expected from previous studies on their reactivity with dithiol compounds (Aldridge & Cremer, 1955). The measured affinities of phenylarsenious acid and diethyltin for cat haemoglobin treated with diethyl pyrocarbonate at pH6.0 were the same as untreated haemoglobin, indicating that their low affinities were not due to the postulated two-cysteine site only being formed after treatment with diethyl pyrocarbonate. Phenylarsenious acid and diethyltin are, however, known to have a lower affinity for monothiols than for dithiols and from the data of Aldridge & Cremer (1955) their affinities for monothiols may be calculated to be of the order of 5 x 1 0 3 ~ ' and 5 x 102~-' respectively. It therefore seems likely that the observed competition between phenylarsenious acid or diethyltin and triethyltin for cat haemoglobin is due to the ability of the former compounds to react with the cysteine residues at the binding sites. Triethyltin binding to cat haemoglobin erg-dimers (formed by preparing a solution of the haemoglobin in I .5~-guanidine hydrochloride/O. 1 M-phosphate buffer, pH 7.3; Kawahara et al., 1965) was also examined, and two molecules of triethyltin were found to bind to each molecule of haemoglobin. Phenylarsenious acid was again shown to compete with the triethyltin, with a similar affinity to that obtained with the tetramer. The inability of the phenylarsenious acid and diethyltin to react with the cysteine residues as a dithiol moiety suggests that the relative positions of the thiol groups within the haemoglobin molecule prevents the formation of the stable fiveor six-membered rings as observed with, e.g., 2,3-dimercaptopropanol (Peters, 1948). This might be expected if the triethyltin were pentaco-ordinate when bound, with the two cysteine residues as axial ligands.