George, Luke, and Animesh have recently had a paper published in The Journal of Physical Chemistry Letters (DOI: 10.1021/acs.jpclett.7b00829) in collaboration with Patrick Rowe and Alessandro Troisi (both formerly Warwick Chemistry, now UCL and Liverpool respectively). In this latest work they explore the relationship between structure and energy transport on the nanoscale. In particular, they look at 50,000 different ways of arranging 6 bacteriochlorophyll molecules between a fixed input and output molecule. One such arrangement is the naturally occurring one found in the famous Fenna-Matthews-Olsen (FMO) light-harvesting complex — a prototypical component of photosynthesis. Whether the FMO has been adapted to support very efficient transport of a quantum of energy from where it is absorbed (elsewhere in the organism, eventually arriving at the 'input') to the reaction centre (the ‘output’, where it is stored as chemical energy) is a long-standing question. By looking at many alternative structures, one can gain some insight into this puzzle, and also try to identify which structural features of a general structure are important for optimising energy transport. Such insight would likely be very useful in designing artificial energy transport structures such as those found in solar cells.

By modelling the positions and orientations of the molecules within the FMO complex in physical space, George and co. were able to generate FMO-like structures and select those which are physically plausible. By generating large libraries and then calculating the intermolecular couplings associated with a given structure, they were able to look at the energy transport efficiency of each, and discover how specialised the FMO complex is with respect to alternatives. Among these samples, the 'size’ of the structure and the orientation of the molecules was found to be particularly important, showing significant correlation with transport efficiency. The molecule closest to input and output tended to be of particularly high importance. Furthermore, the FMO molecule itself was in the top 1% for energy transport efficiency, partly because of its relatively compact size. This seems to suggest nature has optimised the structure to some extent. There is some scope for improvement, however, and thanks to these new results we have some idea of how to improve on natural selection.