Multistate conformational model of a single LH2 complex: Quantitative picture of time-dependent spectral fluctuations

The fluorescence (FL) spectrum of single LH2 complexes fluctuates on a time-scale of seconds in wavelength, spectral shape and intensity [D. Rutkauskas, V. Novoderezhkin, R.J. Cogdell, R. van Grondelle, Biophys. J. 88 (2005) 422]. Here we propose a model capable to explain the statistics of these time-dependent FL-fluctuations. In this model this evolution of the spectra is described in terms of slow conformational motion of the pigment-protein complex inducing random shifts of the site energies. These shifts manifest themselves as inhomogeneous broadening of the bulk spectra and can be directly observed as spectral fluctuations in single molecule experiments. Our model assumes a finite number of conformational sub-states for each pigment of the complex and allows a calculation of the conformational dynamics by introducing phenomenological transfer rates between these sub-states. The simplest version of the model with two conformations for each pigment in enough to explain the fast small-size (±10 nm) fluctuations within the 860-880 nm spectral region. Larger spectral jumps that are observed in the experiment (10-20 nm jumps to the blue and 10-40 nm to the red) can only be reproduced by including additional conformational states. The simplest model includes one more pair of states (producing big red and big blue shifts of the site energies). This four-state model enables us to reproduce quantitatively (i) the distribution of the FL peak positions, (ii) the changes of the width and asymmetry of the FL profiles as a function of the FL peak position, and (iii) distribution of the amplitudes and times of spectral jumps as a function of the initial FL peak position. We classify and describe the dynamic patterns that appear most often in the measured FL traces and in the calculated dynamics. We demonstrate a similarity between the measured and modelled patterns and show how these main types of the dynamics are related to specific changes in the conformational state of the 18 pigments of the LH2-B850 antenna.