First Principles Modeling and Time-Resolved Circular Dichroism Spectroscopy of The Fenna-Matthews-Olson Complex

The Fenna-Matthews-Olson (FMO) complex is a photosynthetic pigment-protein complex that has been the subject of study of decades of research, both experimental and theoretical. The FMO complex is small enough that computational modeling is feasible, while the rich excitonic interactions between the pigments give rise to absorption and circulardichroism (CD) spectra with many interesting details. This makes FMO an excellent testing ground for new predictive modeling techniques. In this work we model the FMO complex from first-principles, wherein the only input is the X-ray crystal structure of the protein. We compute steady-state absorption and CD spectra of wild-type (WT) FMO as well as two mutants, Y16F and Q198V, in which amino acid residues near pigment 3 and pigment 7 are replaced respectively. CD spectra contain extra structural information and thus provide another avenue of investigation into the electronic properties of the FMO complex. We find that while there are large structural changes in the mutants, not all of the structural changes produce significant spectral changes. We conclude that the primary contributor to the spectral changes in Y16F is the breaking of a hydrogen bond between the nearby tyrosine and pigment 3. On the other hand, the spectral changes in Q198V are due to a collection of effects cancelling one another out to varying degrees, all induced by widespread structural changes as a result of the mutation. We then perform time-resolved absorption and CD spectroscopy measurements on WT, Y16F, and Q198V FMO to provide a high quality set of experimental data against which the first-principles spectra can be validated. We find that in order to accurately model the triplet energy transfer dynamics in FMO two effects must be accounted for in the modeling: (1) the Stark shift caused by the rotation of the bacteriochlorophyll’s permanent dipole moment upon entering a triplet state, and (2) decays must be modeled as Boltzmann populations rather than individual pigments