Dr. Michal Gwizdala Profile

Dr. Michal Gwizdala

at Univ of Pretoria

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Publications (2)

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Proceedings Article | 5 March 2021 Presentation + Paper

Impact of the intensity threshold on binary switching analysis in single molecule spectroscopy of phycobilisomes

Gonfa Tesfaye Assefa, Tjaart P. Krüger, Michal Gwizdala

Proceedings Volume 11650, 1165006 (2021) https://doi.org/10.1117/12.2583009

KEYWORDS: Lead, Single molecule spectroscopy, Proteins, Energy transfer, Switching, Spectroscopy, Quenching (fluorescence), Optical properties

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The functional dynamics of photosynthetic light-harvesting pigment-protein complexes play an essential physiological role. The dynamic switching between different states – associated with different spectroscopic signatures – allows these complexes to regulate the excitation energy flow in the photosynthetic apparatus as a direct response to changing environmental conditions. To a large extent, the robustness of the photosynthetic processes depends on this functional flexibility of the light-harvesting complexes. Single molecule spectroscopy (SMS) is a method of choice to investigate the spectroscopic properties of individual molecules or complexes and allows the direct observation of switches between emissive states. The fluorescence intensity dynamics of individual light-harvesting complexes are commonly analyzed using a two-state model, corresponding to an ON and OFF state, respectively. In this model, each switch across the intensity threshold that separates the two states is counted as an intensity switch. However, multichromophoric systems in general, and protein complexes binding multiple pigments in particular, are known to switch among various intensity levels, suggesting that the choice of the intensity threshold may significantly influence the two-state statistics. Here, we investigated the fluorescence data of a very large complex – the main light harvesting complex of cyanobacteria, the phycobilisome – and determined that the intensity threshold has only a limited impact on the binary switching analysis of the complex for an extended range of threshold positions.

Proceedings Article | 9 March 2015 Paper

Single-molecule exploration of photoprotective mechanisms in light-harvesting complexes

Hsiang-Yu Yang, Gabriela Schlau-Cohen, Michal Gwizdala, Tjaart Krüger, Pengqi Xu, Roberta Croce, Rienk van Grondelle, W. Moerner

Proceedings Volume 9331, 933109 (2015) https://doi.org/10.1117/12.2083628

KEYWORDS: Luminescence, Proteins, Quenching (fluorescence), Antennas, Molecular mechanisms, Light harvesting, Photosynthesis, Field programmable gate arrays, Switches, Physics

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Plants harvest sunlight by converting light energy to electron flow through the primary events in photosynthesis. One important question is how the light harvesting machinery adapts to fluctuating sunlight intensity. As a result of various regulatory processes, efficient light harvesting and photoprotection are balanced. Some of the biological steps in the photoprotective processes have been extensively studied and physiological regulatory factors have been identified. For example, the effect of lumen pH in changing carotenoid composition has been explored. However, the importance of photophysical dynamics in the initial light-harvesting steps and its relation to photoprotection remain poorly understood. Conformational and excited-state dynamics of multi-chromophore pigment-protein complexes are often difficult to study and limited information can be extracted from ensemble-averaged measurements. To address the problem, we use the Anti-Brownian ELectrokinetic (ABEL) trap to investigate the fluorescence from individual copies of light-harvesting complex II (LHCII), the primary antenna protein in higher plants, in a solution-phase environment. Perturbative surface immobilization or encapsulation schemes are avoided, and therefore the intrinsic dynamics and heterogeneity in the fluorescence of individual proteins are revealed. We perform simultaneous measurements of fluorescence intensity (brightness), excited-state lifetime, and emission spectrum of single trapped proteins. By analyzing the correlated changes between these observables, we identify forms of LHCII with different fluorescence intensities and excited-state lifetimes. The distinct forms may be associated with different energy dissipation mechanisms in the energy transfer chain. Changes of relative populations in response to pH and carotenoid composition are observed, which may extend our understanding of the molecular mechanisms of photoprotection.

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