  The Fenna Mathews Olson (FMO) complex of green sulphur bacteria is an example
of a photosynthetic pigment protein complex, in which the electronic properties
of the pigments are modified by the protein environment to promote efficient
excitonic energy transfer from antenna complexes to the reaction centres. Many
of the electronic properties of the FMO complex can be extracted from knowledge
of the static crystal structure. However, the recent observation and analysis
of long lasting quantum dynamics in the FMO complex point to protein dynamics
as a key factor in protecting and generating quantum coherence under laboratory
conditions. While fast inter and intra molecular vibrations have been
investigated extensively, the slow dynamics which effectively determine the
optical inhomogeneous broadening of experimental ensembles has received less
attention. Our study employs constrained geometric dynamics to study the
flexibility in the protein network by efficiently generating the accessible
conformational states from the published crystal structure. Statistical and
principle component analysis reveal highly correlated low frequency motions
between functionally relevant elements, including strong correlations between
pigments that are excitonically coupled. Our analysis reveals a hierarchy of
structural interactions which enforce these correlated motions, from the level
of monomer monomer interfaces right down to the alpha helices, beta sheets and
pigments. In addition to inducing strong spatial correlations across the
conformational ensemble, we find that the overall rigidity of the FMO complex
is exceptionally high. We suggest that these observations support the idea of
highly correlated inhomogeneous disorder of the electronic excited states,
which is further supported by the remarkably low variance of the excitonic
couplings of the conformational ensemble.
