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Triplet State Dynamics in Peridinin-Chlorophyll-a-Protein: A New Pathway of Photoprotection in LHCs?

Maxime T. A. Alexandre ^*, Daniel C. Lührs ^*, Ivo H. M. van Stokkum ^*, Roger Hiller , Marie-Louise Groot ^*, John T. M. Kennis ^* and Rienk van Grondelle ^*

^* Department of Biophysics and Physics of Complex Systems, Division of Physics and Astronomy, Faculty of Sciences, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands; and Department of Biology, Macquarie University, Sydney, New South Wales 2109, Australia

Correspondence: Address reprint requests to R. van Grondelle, E-mail: R.van.Grondelle{at}few.vu.nl.

This work investigates the interaction of carotenoid and chlorophyll triplet states in the peridinin-chlorophyll-a-protein (PCP) of Amphidinium carterae using step-scan Fourier transform infrared spectroscopy. We identify two carotenoid triplet state lifetimes of 13 and 42 µs in the spectral region between 1800 and 1100 cm^–1 after excitation of the ‘blue’ and ‘red’ peridinin (Per) conformers and the Q_y of chlorophyll-a (Chl-a). The fast and slow decaying triplets exhibit different spectral signatures in the carbonyl region. The fast component generated at all excitation wavelengths is from a major conformer with a lactone stretching mode bleach at 1745 cm^–1. One (1720 cm^–1) and two (1720 cm^–1 and 1741 cm^–1) different Per conformers are observed for the slow component upon 670- and 530–480-nm excitation, respectively. The above result implies that ³Per triplets are formed via two different pathways, corroborating and complementing visible triplet-singlet (T-S) spectra (Kleima et al., Biochemistry (2000), 39, 5184). Surprisingly, all difference spectra show that Per and Chl-a modes are simultaneously present during the ³Per decay, implying significant involvement of ³Chl-a in the ³Per state. We suggest that this Per-Chl-a interaction via a delocalized triplet state lowers the ³Per energy and thus provides a general, photoprotection mechanism for light-harvesting antenna complexes.