Paper BI-TuP19
Nanoscale Patterning of Photosynthetic Light Harvesting Proteins

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Here we present the patterning of light harvesting 2 (LH2) complexes from the photosynthetic bacterium Rhodobacter sphaeroides. LH2 complexes consist of circular arrays of bacteriochlorophyll and carotenoid molecules, held together by a cylindrical assembly of polypeptides. As many as 100 LH2 complexes are organised in the membrane to form an interconnected energy transfer networks comprising thousands of bacteriochlorophyll molecules that absorb photons, channelling the excitation energy down an energy gradient towards the reaction centre (RC), leading to a charge separation that drives subsequent biosynthetic reactions in the cell. When removed from the photosynthetic membrane, LH2 complexes retain the ability to absorb light, and they emit the energy as fluorescence. This property has been utilised in order to gain insight into the biological functionality of the LH2 after immobilization. Alkanethiol SAMs on gold surfaces have been used in conjunction with photolithographic techniques to produce patterned assemblies of LH2. Selective exposure of alkanethiols to UV light (wavelength 244 nm) leads to their photo-oxidation to alkylsulfonates, which may be displaced by a second thiol in a solution-phase process. The adsorption of LH2 onto SAMs with a variety of functional groups has been measured in order to determine which surfaces resist non-specific adsorption. In contrast to plasma proteins, which adsorb strongly to most surfaces, simple patterns consisting of hydrophilic and hydrophobic regions may be used effectively to pattern LH2. Covalent attachment to carboxylic acid groups using carbodiimide activation methods is an effective means of immobilising LH2 at the surface. Fluorescence spectroscopy measurements of proteins immobilized by attachment to patterned SAMs have confirmed that biological function is retained, leading to the observation of absorption spectra qualitatively identical to those of complexes in solution. Nanoscale chemical patterns have been fabricated using scanning near-field photolithography (SNP), in which a scanning near-field optical microscope coupled to a UV laser is used to selectively expose regions of a SAM. Using SNP, lines of carboxylic acid functionalised thiols as small as 70 nm have been fabricated in monolayers of perfluorinated thiols, and used to form LH2 structures with a width of less than 100 nm.