Single Molecule Spectroscopy of Monomeric LHCII: Experiment and Theory

Abstract

We derive approximate equations of motion for excited state dynamics of a multilevel open quantum system weakly interacting with light to describe fluorescence-detected single molecule spectra. Based on the Frenkel exciton theory, we construct a model for the chlorophyll part of the LHCII complex of higher plants and its interaction with previously proposed excitation quencher in the form of the lutein molecule Lut 1. The resulting description is valid over a broad range of timescales relevant for single molecule spectroscopy, i.e. from ps to minutes. Validity of these equations is demonstrated by comparing simulations of ensemble and single-molecule spectra of monomeric LHCII with experiments. Using a conformational change of the LHCII protein as a switching mechanism, the intensity and spectral time traces of individual LHCII complexes are simulated, and the experimental statistical distributions are reproduced. Based on our model, it is shown that with reasonable assumptions about its interaction with chlorophylls, Lut 1 can act as an efficient fluorescence quencher in LHCII.

Figure 1

Bulk (a) Absorption and (b) Fluorescence spectrum of the Qy band of LHCII monomers. The points are experimental values taken from ref. , the lines are calculated by our exciton model. The coloured lines are individual excitonic contributions, the black line is the overal spectrum.

Figure 2. Experimental (red) and calculated (green) fluorescence peak distribution.

(a) Peak position histogram, (b) FL peak position and intensity plot. Theoretical points are calculated as individual realizations of energetic disorder.

Figure 3. Role of Lut 1 parameters.

Dependence of the fluorescence yield on (a) Lut 1 S1 energy and (b) Lut 1 - Chl a612 coupling. The energy dependence is calculated with Redfield (blue) and Modified Redfield (red) theory for comparison. The dependence on the coupling strength depicts calculated points (Redfield theory) fitted with exponential dependence. Already a realistically small coupling around 12 cm⁻¹ leads to significant FL quenching.

Figure 4

Experimental (red) and calculated (green) probability distribution of dwell times in the (a) ON and (b) OFF state, logarithmic scale. While the OFF state distribution follows a power law, the ON state distribution has an exponential tail at longer times.

Figure 5. Experimental (red) and calcuated (green) intensity blinking statistics.

(a) The percentage of time the complexes dwelled on respective intensity levels. The two-state structure of the low-intensity OFF states and higher intensity ON states is apparent. (b) How often per minute the complexes accessed the respective intensity levels. The experimental frequency is higher due to switching within the ON/OFF states. (c) The intensity correlation function.