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

Abstract

The reduction of Ag ions to Ag atoms is a highly endergonic reaction step, only the aggregation to Ag 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 Fe /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 Fe /hemes of c-cytochromes to reduce Ag to AgNPs was studied in an enzymatic experiment and with living bacteria Geobacter sulfurreducens (Gs).

A review on the biosynthesis of metal and metal salt nanoparticles by microbes

“…Furthermore, Vasylevskyi et al carried out an in-depth investigation of biomineralization process of silver ion at molecular level using Geobacter sulfurreducens. 122 Vasylevskyi and his group explained that the reduction of Ag + ion to Ag 0 is an endergonic reaction, whereas aggregation to Ag n clusters is an exergonic process which further leads to stable AgNPs synthesis. This hypothesis was based on various experiments conducted during AgNPs synthesis by photoinduced electron transfer through tetrapeptide/Ag + solution (shown in Fig.…”

“…132,133 The similar phenomenon of electron transfer via c-cytochromes was observed in another report on silver nanoparticles synthesis by Geobacter sulfurreducens. 122 During this process, electrons generated through respiration were transported by Fe 2+ /hemes of c-cytochromes (Ppc) via periplasmic space to Fe 3+/ hemes of the outer membrane cytochromes (Omc). Subsequently, electrons were transferred from protein core of Omc to the surface attached Ag + ions to synthesize AgNPs while oxidizing the cytochromes again to Fe 3+/ hemes (Fig.…”

Metal Nanoparticle–Microbe Interactions: Synthesis and Antimicrobial Effects

“…Using less effective silver ion binding amino acids instead of histidine, such as serine, aspartate, asparagine, or lysine, the intermediate nanocomposites are not observed, but one directly observes formation of AgNPs, indicating a faster nucleation and growth. [ 125 ] This again shows the potential of histidine to stabilize rather Ag + than Ag 0 .…”

“…Using less strong silver ion binding amino acids in place of histidine, e.g., serine, aspartate, asparagine, or lysine, leads to a faster formation of AgNPs, yet still the rate of the reaction is in the order of hours. [ 125 ] For such short peptides, one can conclude that strong metal ion binders, which favor Ag + , inhibit the aggregation of silver atoms and stabilize the metal cations rather than metal atoms. Longer peptides, e.g., decapeptides, induce faster AgNP formation, probably because they can bind more than one silver ion and favor reduced metal–metal ion (Ag 0 –Ag + ) interactions.…”

“…A direct proof for the participation of cytochromes in the silver ion reduction and formation of AgNPs in living bacteria was given by following the relative amounts of Fe 2+ and Fe 3+ hemes and the increase of the AgNP concentration by UV–vis spectroscopy. [ 125 ] While horse heart c‐cytochrome alone, similar to the supernatant of G. sulfurreducens , is unable to reduce the silver ions due to the endergonic first reduction steps to elemental silver (see Figure 5), bacteria continuously produce electrons during their mineral respiration process, and possess thus enough “driving force” for the generation of AgNPs. This emphasizes that the availability of excess silver ions and electrons (i.e., supplied from the metabolism) are requirements to ensure the nucleation and growth of silver clusters for a successful AgNP synthesis.…”