LAUSR

In the absence of sodium dithionite (DT), addition of the large anions Br-, I- and HS- to the MoFe proteins of Azotobacter vinelandii (Av1) and Clostridium pasteurianum (Cp1) released ~1.0 H2/MoFe protein with an associated increase in the absorbance from 400 to 800 nm, indicative of protein oxidation. The reaction of I- with Cp1 released ~1.0 H2/Cp1 with 0.91 ± 0.12 I-/Mo or 1.82 I-/ Cp1. Oxo anions phosphate, molybdate and ADP also produced ~1.0 H2/ MoFe protein with similar increases in absorbance. H2 was not evolved with Cl- addition but in contrast to other anions, the absorbance decreased from 400 to 800 nm. In the presence of large anions and with excess DT both Cp1 and Av1 slowly evolve H2 through the process of DT reducing oxidized MoFe proteins and anions inducing their oxidation to form H2. The results suggest that anions expose or activate the P centers so their low potential electrons can be transferred to electron acceptors or react with H+ to form H2. Anions could function in a similar manner to the Fe protein in activating P centers to release electron during catalytic activity.