Single-molecule excitonic dynamics for robust functionality of diatom light-harvesting complexes

Tjaart Krügera, Maxime Alexandreb, Pavel Malýb,c, Claudia Bücheld, Tomáš Mancalc, Rienk van Grondelleb
aUniversity of Pretoria, Private bag X20, Hatfield, 0028, South Africa; bVU University, De Boelelaan 1081, 1081 HV Amsterdam, the Netherlands; cCharles University in Prague, Ke Karlovu 3, 121 16 Prague 2, Czech Republic; dUniversity of Frankfurt, Max von Laue Str. 9, 60438 Frankfurt, Germany

We present the first single-molecule spectroscopy investigation of FCPa/b complexes from the diatom Cyclotella meneghiniana and contrast the results to those obtained from the homologous plant LHCII complex to explore the relationship between protein structure, dynamics and functionality. FCP and LHCII complexes showed a strong correlation between the frequency of intensity and spectral fluctuations, suggesting fluorescence blinking to be a sensitive marker of conformational dynamics in these complexes. FCP exhibited considerably more static disorder than LHCII and the protein conformational flexibility was found to be directly related to the protein’s ability for photoprotective functional tuning, i.e., more flexibility allows more control. The protein rigidity was regulated in different ways for FCPa and LHCII: through replacement of one subunit with a more rigid one and by protonation, respectively. The single molecule results showed excellent agreement with results from previous in vitro bulk studies, and for FCP the single-molecule conditions appear to be more specific and reliable for studies of intrinsically quenched states.

Despite the absence of a crystal structure, we were able to reproduce a rich variety of experimental single-molecule spectra of FCP using excitonic modelling. Of interest is a quasi-stable state near 680 nm, which makes FCP behave spectro­scopically like LHCII and more robust to static disorder. This new state, which is stabilised by particular environmental conditions such as low temperatures, most likely corresponds to a particular protein conformation inducing a configurational change of Chl a611, one of the terminal emitter pigments. The results indicate that slow conformational dynamics in the terminal emitter domain dramatically modulate the excitonic coupling in this domain and consequently also the transfer efficiency of excitation energy to neighbouring complexes.

Figure 1: Dynamic functional switching of FCP complexes and the proposed molecular mechanism underlying each state, based on single molecule spectroscopy and excitonic modelling.