Solid-state NMR applied to photosynthetic light-harvesting complexes

This short review describes how solid-state NMR has provided a mechanistic and electronic picture of pigment–protein and pigment–pigment interactions in photosynthetic antenna complexes. NMR results on purple bacterial antenna complexes show how the packing of the protein and the pigments inside the light-harvesting oligomers induces mutual conformational stress. The protein scaffold produces deformation and electrostatic polarization of the BChl macrocycles and leads to a partial electronic charge transfer between the BChls and their coordinating histidines, which can tune the light-harvesting function. In chlorosome antennae assemblies, the NMR template structure reveals how the chromophores can direct their self-assembly into higher macrostructures which, in turn, tune the light-harvesting properties of the individual molecules by controlling their disorder, structural deformation, and electronic polarization without the need for a protein scaffold. These results pave the way for addressing the next challenge, which is to resolve the functional conformational dynamics of the lhc antennae of oxygenic species that allows them to switch between light-emitting and light-energy dissipating states

Fluorescence Intermittency from the Main Plant Light-Harvesting Complex: Resolving Shifts between Intensity Levels

We present a simple method to resolve discrete intensity shifts from time-resolved single-molecule emission data. This new method uses multiples of the standard deviation of the measured intensities that are integrated into short time bins. By applying the technique to trimeric units of the main light-harvesting complex (LHCII) of plants, it is shown that the amount of information that can be extracted from an intensity time trace increases considerably, thereby enlarging the possibility to reveal new phenomena. It is demonstrated how shot noise can lead to substantial deviations and misleading interpretations when the conventional two-state kinetic model for intensity fluctuations is applied. By first resolving the accessed intensity levels, the artifactual effect of shot noise is sufficiently reduced. The technique is particularly applicable to the analysis of fluorescence intermittency from multichromophoric systems. © 2011 American Chemical Society

Fluorescence intermittency from the main plant light-harvesting complex: sensitivity to the local environment

The time-resolved fluorescence intensity fluctuations from single, immobilized complexes of the main light-harvesting complex (LHCII) of plants were investigated in different pH environments close to room temperature and under different light conditions. The efficiency of light harvesting, which was represented by complexes typically residing for long periods in strongly fluorescing states, was significantly reduced by decreasing the pH or increasing the incident photon flux. The same environmental changes significantly increased the switching frequency between strongly and weakly fluorescing states. The environmental dependence became more evident when the various accessed intensity levels were first resolved, a technique that significantly reduced the obscuring effect of shot noise. The strong environmental sensitivity suggests that the immediate environment of an LHCII complex can modulate the amount of energy dissipation. A simple model illustrates how this may be achieved: the dynamic equilibrium between the strongly and weakly fluorescing states can be shifted by environmentally controlling the conformational diffusion on the potential energy surface of LHCII

How Photosynthetic Proteins Switch

Recent time-resolved studies have revealed the switching behavior of single photosynthetic light-harvesting complexes. In this work, we suggest a diffusion-controlled model describing essential protein dynamics underlying this switching. The calculated blinking statistics are compared with the experimental result and not only reproduce the power-law behavior at intermediate times, but also follow the experimentally observed deviations from such behavior on a shorter time scale. We propose that the coupling of fast protein dynamics to a specific slow coordinate is at the basis of regulatory switching. © 2012 American Chemical Society

Fluorescence blinking of single major light-harvesting complexes

Recent time-resolved studies have revealed the switching behavior of single photosynthetic light-harvesting complexes. In this work, we suggest a conceptual diffusion-controlled model, which is able to describe essential protein dynamics underlying this switching phenomenon. The calculated blinking statistics is compared with the experimental results measured under various experimental conditions and not only reproduces the power-law behavior at intermediate times, but also follows the experimentally observed deviations from such behavior on a shorter timescale. We find that even under ordinary light-harvesting conditions, some antenna complexes are quenched and their fraction noticeably increases in a more acid environment. As a result, the lability of the protein scaffold allows the coexistence of light-harvesting and excitation-quenching states and therefore gives rise to regulatory switching known as non-photochemical quenching. © IOP Publishing and Deutsche Physikalische Gesellschaft

Controlled Disorder in Plant Light-Harvesting Complex II Explains Its Photoprotective Role

The light-harvesting antenna of photosystem II (PSII) has the ability to switch rapidly between a state of efficient light use and one in which excess excitation energy is harmlessly dissipated as heat, a process known as qE. We investigated the single-molecule fluorescence intermittency of the main component of the PSII antenna (LHCII) under conditions that mimic efficient use of light or qE, and we demonstrate that weakly fluorescing states are stabilized under qE conditions. Thus, we propose that qE is explained by biological control over the intrinsic dynamic disorder in the complex—the frequencies of switching establish whether the population of complexes is unquenched or quenched. Furthermore, the quenched states were accompanied by two distinct spectral signatures, suggesting more than one mechanism for energy dissipation in LHCII