Sulfhydryl groups in hemoglobin. A new molecular probe at the alpha1 beta 1 interface studied by Fourier transform infrared spectroscopy

Abstract

Infrared absorption bands due to sulfhydryl groups (VSH) of alpha-104(G11) and beta-112 (G14) cysteine residues of human carboxyhemoglobin (HbCO) have been observed near 2560 cm- minus 1 by use of Fourier transform infrared (FTIR) spectroscopy. The beta-93 cysteine SH groups absorb infrared radiation so weakly that they are not distinguished from background. Only single SH absorption bands due to the alpha-104 cysteines of pig and horse hemoglobin are observed. The SH absorption bands from human HbCO disappear in alkali, are broadened by detergent or guanidinium chloride, and show a complex titration curve, and an isotopic frequency shift (VSD/VSH equals to 0.7267) virtually identical with that reported for methanethiol. The integrated absorption coefficient (epsilonmM(area)) for 0.1 M ethanetiol increased with H-bond acceptor solvents in the order: CCl4 (0.07), water (0.21), acetone (0.43), and N,N-dimethyl-acetamide (1.35 mM- minus 1 cm- minus 2). Comparison of the integrated absorption coefficients for the alpha-104 cysteine SH (2.43), and the beta-112 SH (0.80), of human HbCO with those of ethanetiol solutions suggested specifically H-bonded structures with peptide carbonyl groups 4 (or 3) residues back in the G helices. This was found to agree with a molecular model of the alpha-chain G helix of horse HbO2 built to coordinates from M. F. Perutz.