Electron tunneling through mutant azurins on mixed SAM gold electrodes

We are investigating interfacial electron transfer rates of P. aeruginosa azurin and its mutants using electrochem. at 1:1 CH3(CH2)nSH:HO(CH2)nSH mixed-SAM gold electrodes. We have examd. interfacial electron transfer rates of mutant azurins in which asparagine-47 was replaced by alanine, aspartic acid, lysine, arginine, leucine, threonine, serine, and glutamine. The N47D mutant on a mixed SAM exhibited a well-defined electrochem. response; N47T and N47S gave a weak signal; but the other 5 mutants showed no response. It is likely that the N47 side-chain carbonyl interacts with the mixed SAM surface, providing a very favorable electron tunneling pathway to the copper via an N47-C112 hydrogen bond. We also have examd. interfacial electron transfer rates of M121X mutant azurins. The ET rates for M121E are 2 orders of magnitude smaller; those for M121Q are almost 1 order of magnitude smaller; and those for the M121L protein are similar to those of wild-type azurin at pH 7. In addn., we have found that type zero azurins have higher ET rates than those of type 2 proteins (rates for C112D are 3 orders of magnitude smaller; those for type zero C112D/M121X (X=L and I) are 2 orders of magnitude smaller than wild-type at pH 7).