Formation of Silver Nanoparticles by Electron Transfer in Peptides and c-Cytochromes.

The reduction of Ag+ ions to Ag0 atoms is a highly endergonic reaction step, only the aggregation to Agn clusters leads to an exergonic process. These elementary chemical reactions play a decisive role if Ag nanoparticles (AgNPs) are generated by electron transfer (ET) reactions to Ag+ ions. We studied the formation of AgNPs in peptides by photoinduced ET, and in c-cytochromes by ET from their Fe2+ /hemes. Our earlier photoinduced experiments in peptides had demonstrated that histidine prevents AgNP formation. We have now observed that AgNPs can be easily synthesized with less-efficient Ag+ -binding amino acids, and the rate increases in the order lysine<asparagine<aspartate<serine. The ability of Fe2+ /hemes of c-cytochromes to reduce Ag+ to AgNPs was studied in an enzymatic experiment and with living bacteria Geobacter sulfurreducens (Gs).