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 Ag_n 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 lysine2+/hemes of c-cytochromes to reduce Ag⁺ to AgNPs was studied in an enzymatic experiment and with living bacteria Geobacter sulfurreducens (Gs).