Single-molecule chiroptical spectroscopy: Fluorescence excitation of individual helicene molecules in polymer-supported thin-films.

We present results of fluorescence excitation circular dichroism studies of the chiroptical response of single (bridged triarylamine) helicene molecules immobilized at a polymer interface. We extract directly dissymmetry parameters, and corresponding probability distributions, associated with the single-molecule fluorescence excitation associated with modulation of a circular polarized excitation field for three different excitation wavelengths (405, 440, 457 nm) showing circular dichroism in bulk films. The observed single molecule chiroptical response is anomalously large in comparison with the results of time-dependent density functional calculations, and the observed defocused emission patterns seem to indicate a higher multipole nature to the transition probed. Our results provide new insights into chiroptical properties of chiral fluorophores that are hidden under the extensive averaging associated with conventional chiroptical probes.

Extracellular electron transfer via microbial nanowires.

Microbes that can transfer electrons to extracellular electron acceptors, such as Fe(iii) oxides, are important in organic matter degradation and nutrient cycling in soils and sediments. Previous investigations on electron transfer to Fe(iii) have focused on the role of outer-membrane c-type cytochromes. However, some Fe(iii) reducers lack c-cytochromes. Geobacter species, which are the predominant Fe(iii) reducers in many environments, must directly contact Fe(iii) oxides to reduce them, and produce monolateral pili that were proposed, on the basis of the role of pili in other organisms, to aid in establishing contact with the Fe(iii) oxides. Here we report that a pilus-deficient mutant of Geobacter sulfurreducens could not reduce Fe(iii) oxides but could attach to them. Conducting-probe atomic force microscopy revealed that the pili were highly conductive. These results indicate that the pili of G. sulfurreducens might serve as biological nanowires, transferring electrons from the cell surface to the surface of Fe(iii) oxides. Electron transfer through pili indicates possibilities for other unique cell-surface and cell-cell interactions, and for bioengineering of novel conductive materials.