Scanning tunnelling spectroscopic imaging of Photosystem II

Abstract

Scanning probe microscopy is a useful alternative to crystallographic methods in the study of PS II structure. In particular, scanning tunnelling microscopy (STM) and spectroscopy (STS) allow high-resolution (2 - 4 Å) structure determination with high-resolution (< 1 meV) electronic spectroscopy on isolated single molecules of various photosynthetic proteins including PS II, LHC II, antennae complexes and chlorophyll binding proteins. The pros and cons of STM/STS are discussed and PS II structural results using STM are compared with corresponding crystallographic results. The suggested protein organization is supported by corresponding STM of CP43-depleted cores. This work is extended to two-dimensional arrays of PS II and amorphous high-density films of small photosynthetic proteins. We also observe evidence for transmolecular electron tunnelling in these 2D PS II arrays. The main electronic conduction mechanisms in single PS II molecules are determined and a simple model described which includes all these mechanisms. This model is consistent with a time-averaged theory based on the known electron transfer processes in PS II. Single-molecule electronic properties of PS II are compared with those for other organic photovoltaics thereby demonstrating the importance of PS II as a model in biomimetic applications. Finally, the potential of STM in imaging other biomolecular components in plants is demonstrated by STM of DNA with 0.5 Å resolution.