A Hidden State in Light-Harvesting Complex II Revealed By Multipulse SpectroscopyThe Journal of Physical Chemistry B

About

Authors
Bart van Oort, Rienk van Grondelle, Ivo H. M. van Stokkum
Year
2015
DOI
10.1021/acs.jpcb.5b01335
Subject
Physical and Theoretical Chemistry / Materials Chemistry / Surfaces, Coatings and Films

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Article

A Hidden State in Light-Harvesting Complex

II – Revealed by Multipulse Spectroscopy

Bart van Oort, Rienk van Grondelle, and Ivo H.M. van Stokkum

J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b01335 • Publication Date (Web): 27 Mar 2015

Downloaded from http://pubs.acs.org on April 8, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical

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A Hidden State in Light-Harvesting Complex II – Revealed By

Multipulse Spectroscopy

Bart van Oort*, Rienk van Grondelle, Ivo H. M. van Stokkum

Department of Physics and Astronomy

Faculty of Sciences

VU University Amsterdam

De Boelelaan 1081 1081 HV Amsterdam

The Netherlands and

Institute for Lasers, Life and Biophotonics

Faculty of Sciences

VU University Amsterdam

De Boelelaan 1081 1081 HV Amsterdam

The Netherlands. * corresponding author: fax: +31 (0)20 598 7999 phone: +31 (0)20 598 6383 e-mail: b.f.van.oort@vu.nl

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Abstract

Light-harvesting complex II (LHCII) is pivotal both for collecting solar radiation for photosynthesis, and for protection against photodamage under high light intensities (a process called non-photochemical quenching, NPQ). Aggregation of LHCII is associated with fluorescence quenching, and is used as an in vitro model system of NPQ. However, there is no agreement on the nature of the quencher and on the validity of aggregation as a model system.

Here, we use ultrafast multi-pulse spectroscopy to populate a quenched state in unquenched (unaggregated) LHCII. The state shows characteristic features of lutein and chlorophyll, suggesting that it is an excitonically coupled state between these two compounds. This state decays in approximately 10 ps, making it a strong competitor for photodamage and photochemical quenching. It is observed in trimeric and monomeric LHCII, upon reexcitation with pulses of different wavelengths and duration.

We propose that this state is always present, but is scarcely populated under regular excitation conditions. Under higher excitation conditions it may become more accessible, and then form a quenching channel. The same state may be the cause of fluorescence blinking observed in single-molecule spectroscopy of LHCII trimers, where a small sub-population is in an energetically higher state where the pathway to the quencher opens up.

Introduction

Solar light captured by protein-bound pigments drives the majority of the earth’s primary production by photosynthesis. In plants most pigments are found in light-harvesting complexes (LHCs): membrane-bound pigment-protein complexes 1 . The major LHC is

LHCII, a trimeric complex of three gene products (Lhcb1, Lhcb2 and Lhcb3), binding 8 chlorophyll (Chl) a, 6 Chl b and 3-4 xanthophyll (Xan) molecules per monomeric unit 2 .

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These pigments work together to perform both light-harvesting and protection against photodamage (photoprotection).

At low light intensity, the energy of photons absorbed by LHCII and the functionally related minor (CP24, CP26 and CP29) LHCs and core (CP43 and CP47) LHCs is transferred with high efficiency to the reaction center (RC) of photosystem II (PSII), where it induces charge separation 1,3 . The resulting electron holes are filled by electrons derived from water splitting.

The resulting electrons are transferred via an electron transport chain to photosystem I (PSI)

RC. In PSI they fill the electron holes created by charge separation induced upon photon absorption by PSI pigments 4 . PSI feeds electrons into the biosynthetic pathway. In the process of PSII to PSI electron transfer, protons are transported across the thylakoid membrane, thereby driving the synthesis of ATP. This linear electron transport chain ultimately ensures the conversion of photonic energy into chemical energy.