Models of interacting photosynthetic light harvesting complexes constructed from modified tobacco mosaic virus capsid proteins

Energy transfer in photosynthesis is orchestrated via dynamic networks of pigment arrays embedded within protein complexes that often span a lipid membrane. This process has been imitated in models that seek both to elucidate the structural characteristics leading to efficient energy transfer in photosynthetic organisms and to use these principles to design more efficient or "greener" solar technology. Pigment conjugates of assemblies of the tobacco mosaic virus capsid protein (TMVCP) have been extensively used as model light harvesting systems due to their synthetic tractability and structural similarity to many photosynthetic light harvesting complexes. This dissertation demonstrates how changes to theTMVCP sequence, including permutation and point mutations, affect the assembly state structure of TMVCP. In doing so, the utility of charge detection mass spectrometry for the characterization of heterogeneous assemblies of large protein complexes is demonstrated. In addition, several methods for dual protein functionalization are developed, which expand the TMVCP light harvesting model to capture interactions between adjacent light harvesting complexes. A site-selective, asymmetric, protein–protein conjugation using two expressed non-canonical amino acids and an oxidative coupling reaction is used to covalently link an engineered TMVCP assembly containing donor pigment arrays to an engineered TMVCP assembly containing acceptor pigment arrays. This model demonstrates directional energy transfer from the donor to acceptor complexes with 21% efficiency, measured by a decrease in donor fluorescence lifetime when donor complexes are in the presence of acceptor complexes versus donor complexes alone. Separately, an engineered TMVCP assembly is site-selectively labeled with a donor pigment, followed by labeling with a non-expressed His tag at an engineered cysteine residue catalyzed by the enzyme tyrosinase. This His tag is associated with a supported lipid bilayer, and the TMVCP light harvesting complexes associated with bilayers demonstrate lateral mobility, imitating the movement of natural photosynthetic light harvesting complexes within phospholipid bilayers. This is the first example of a photosynthetic membrane mimic using entirely synthetic protein, lipid, and pigment components