ABSTRACT
METALLOPROTEINS: EMERGING THEMES 1109 a similar unique ability to bind putative 7r-bonding ligands such as CO in an aqueous environment (cf. Sec. 1 .2 ).
The concentrations of the biologically important transition metals in seawater vary widely, as shown by the following approximate values of the negative logarithm of their total (all forms) molar concentration; from Mo as the most abundant (-7) and V (-7.5) through Ni, Zn, W, Cu, and Cr (all from -8 to -8.5), Fe (-9), and Mn (-9.5) down to Co (-11) [61]. There may, of course, be wide local variations. It has been pointed out that Cu would have been virtually unavailable to organisms in the Earth’s primitive anaerobic atmosphere (with relatively high concentrations of H2S) because of the very low solubility product of cuprous sulfide; by contrast, Fe has probably become much less accessible in the later aerobic environment because of the insolu bility of ferric hydroxide and related solids [61]. The availability of Cr and Co would probably have changed less as the atmosphere became oxidizing.
