Fluorescence Microscopy on Single Light Harvesting Complexes

Abstract

The capturing of sun light by antenna complexes of photosynthetic units in bacteria and plants has long been a subject of intense studies /1/. A wealth of information from optical spectroscopy ranging from fs transient absorption to low temperature spectral hole burning is known nowadays /2, 3/. In the recent past the successful analysis of the crystal structure of the light harvesting 2 complex (LH2) of the photosynthetic bacterium Rhodopseudomonas Acidophila /4, 5/ has promoted considerable experimental and theoretical effort to analyze the energy transfer dynamics within this complex 131. From the crystal structure it is clear that the bacteriochlorophyll (BChl) A molecules being the major chromophores responsible for capturing sun light in the complex are organized in two coplanar rings with a distance of 15 Å. Ring 1 consists out of 9 BChl A molecules with large intermolecular spacing. Ring 2 comprises 18 BChl A molecules in close proximity /4/. Both rings are ‘connected’ by carotenoids. The large intermolecular spacing and their mutual orientation leave the BChl A molecules in ring 1 uncoupled /6, 7/, whereas the BChl A molecules in ring 2 show a strong intermolecular coupling thus forming a ring shaped molecular aggregate /4, 5/. The main absorption of the chromophores in ring 1 lies around 800 nm whereas the absorption band of the aggregate of molecules in ring 2 is centered around 850 nm. From time resolved optical measurements it is clear that there is a fast energy transfer (timeconstant = 1ps) between the molecules in ring 1 and 2 /6/. Once the excitation energy has reached ring 2 it is delocalized in the ring on a timescale of roughly 100 fs /8/. The thermal equilibrium within ring 2 is reached on a ps timescale /9, 10/. Any structural change in the rings caused by phonons or slow protein motions will cause a reduction in symmetry of the ring and significantly alter the energy level scheme together with a change in the radiative properties of the complex /11, 12/. The light emitted by a ring with circular symmetry is randomly polarized in the plane of the ring. Any symmetry reduction of the ring will create elliptically or linearly polarized light.