Single molecule studies on purple bacterial antenna complexes

Photosynthesis begins with the absorption of solar energy by antenna pigments. This absorbed energy is then rapidly and efficiently transferred to reaction centres where it is trapped and used to drive photosynthesis. Light-harvesting is therefore and essential and fundamental part of this very important biological process that ultimately provides all the food we eat, the oxygen we breathe and cleans the air by removing carbon dioxide. This proposal aims to use a combination of X-ray crystallography and single molecule spectroscopy to study both the physical and electronic structure of a group on light-harvesting antenna complexes obtained from purple photosynthetic bacteria. These bacteria have proved to be excellent model systems in which to study the fundamental reactions going on in the earliest reaction of photosynthesis. Normal spectroscopy looks at the properties of large populations of molecules, so-called ensemble spectroscopies. However because this averages over all the molecules present and because each molecule is not identical many important details are averaged out and lost. Looking at individual molecules allows these details to be observed and they are, in this case with the antenna complexes, essential if we wish to understand how antenna complexes function. In this regard single molecule spectroscopy is a uniquely important tool.

This research aims to use a combination of low temp. single molecule spectroscopy and X-ray crystallography to compare the electronic structure and the physical structure of the Bchl molecules in a range of purple bacterial antenna complexes. This data is absolutely required in order to undertsand the detailed molecular mechanisms of energy transfer that are at the heart of photosynthetic light-harvesting. The single molecule approach allows the detailed information that is typically lost in ensemble spectroscopy to be measured. This is essential information if the molecular details of light-harvesting are to be really, deeply understood. In particular we will be determining if there are gaps in the electronic structure of the LH1/RC core complexes that could correlate with physical gaps through which quinones could move at part of the cyclic electron transfer process. We shall also be using the combination of X-ray crystallography and single molecule spectroscopy to study some LH2 complexes that have unsual absorption spectra. The aim here is to be able to understand how these different spectroscopic form can be produced and to get a deeper understanding of how the pigment-binding apoproteins can modulate the photochemical properties of the Bchl molecules, in order to tailor them for optimum fuction.