Mechanism of adsorption of hard and soft metal ions to Saccharomyces cerevisiae and influence of hard and soft anions

The applicability of the hard-and-soft principle of acids and bases in predicting metal adsorption characteristics in a biological context was investigated for metabolism-independent uptake of the metal ions Sr2+, Mn2+, Zn2+, Cu2+, Cd2+, and Tl+ by Saccharomyces cerevisiae. Metal adsorption increased with external metal concentration (5 to 50 micromolar), although some saturation of uptake of the harder ions examined, Sr2+, Mn2+, and Zn2+, was evident at the higher metal concentrations. Cation displacement experiments indicated that, with the exception of Tl+, relative covalent bonding (H+ displacement) of the metals was greater at low metal concentrations, while weaker electrostatic interactions (Mg2+ plus Ca2+ displacement) became increasingly important at higher concentrations. These results were correlated with curved Scatchard and reciprocal Langmuir plots of metal uptake data. Saturation of covalent binding sites was most marked for the hard metals, and consequently, although no relationship between metal hardness and ionic/covalent bonding ratios was evident at 10 micromolar metal, at 50 micromolar the ratio was generally higher for harder metals. Increasing inhibition of metal uptake at increasing external anion concentrations was partially attributed to the formation of metal-anion complexes. Inhibitory effects of the hard anion SO4(-2) were most marked for uptake of the hard metals Sr2+ and Mn2+, whereas greater relative effects on adsorption of the softer cations Cu2+ and Cd2+ were correlated with complexation by the soft anion S2O3(-2). Inhibition of uptake of the borderline metal Zn2+ by SO4(-2) and that by S2O3(-2) were approximately equal. The results showed good agreement with the hard-and-soft principle with respect to both the nature of bonding and preferred ligand binding of the metals examined, and the implications for biological systems are discussed