PARENT SESSION

Symposium S8D Artificial Photosynthesis and Biological Hydrogen Production
Friday September 3rd, 2004 8:30 AM-10:30 AM Room 510A
Chair: Tom Moore
Co-Chair: Leif Hammarström

Tetrapyrrole singlet excited state quenching by covalently linked carotenoids of different conjugation length. Rudi Berera¹, Rodrigo Palacios², Mikas Vengris¹, Gerdenis Kodis², Rienk van Grondelle¹, Herbert van Amerongen³, Devens Gust², Thomas A. Moore², Ana L. Moore², John Kennis^*,1, ¹ Department of Biophysics, Division of Physics and Astronomy, Faculty of Sciences, Amsterdam, The Netherlands² Department of Chemistry and Biochemistry and the, Tempe, Arizona, USA³ Laboratory for Biophysics, Wageningen, The Netherlands

ABSTRACT- We present results from pump-probe spectroscopy on artificial light-harvesting dyads made up of a zinc-phthalocyanine (Pc) covalently linked to a carotenoid with nine or ten conjugated double bonds, with the aim to investigate how carotenoids of different length affect the excited state decay of tetrapyrroles. The decay of the Qy state (680 nm), populated upon excitation of the Qx band (610 nm), was followed in THF, acetone and DMSO. The singlet excited state decay of Pc with no carotenoid attached was 3.1 ns in THF, 2 ns in acetone and 4.9 ns in DMSO. The Pc singlet-excited state lifetime in the 10 double-bond carotenoid dyad was 270 ps in THF, 56 ps in acetone and 64 ps in DMSO whereas for the 9 double-bond dyad we found Pc lifetimes of 3.1 ns, 571 ps and 310 ps, respectively. Thus, in THF the lifetime of Pc linked to the 10 double-bond carotenoid is significantly quenched, whereas the lifetime of Pc linked to the 9 double-bond carotenoid is not quenched at all. In the more polar solvents acetone and DMSO both carotenoids have a quenching ability, the 10 double-bond carotenoid being a much stronger quencher than the 9-double bond carotenoid. A change in the carotenoid length is a key factor in the process leading to thermal dissipation of excess energy in Photosystem II, known as non-photochemical quenching, where the activation of the quenching process is necessarily linked to the conversion of violaxanthin (9 double-bonds) to zeaxanthin (11 double bonds). Whether zeaxanthin plays a direct role in the process is a matter of debate. Our results show that a change of only one double bond in the conjugated system of carotenoids can lead to a significant change their quenching abilities, suggesting a direct role of zeaxanthin in the quenching of Chl singlet-excited states.

KEY WORDS: nonphotochemical quenching, carotenoid, photoprotection, artificial antenna