Triplet state dynamics in peridinin-chlorophyll-a-protein:: A new pathway of photoprotection in LHCs?

This work investigates the interaction of carotenoid and chlorophyll triplet states in the peridinin-chlorophyllaprotein ( PCP) of Amphidinium carterae using step-scan Fourier transform infrared spectroscopy. We identify two carotenoid triplet state lifetimes of similar to 13 and similar to 42 mu s in the spectral region between 1800 and 1100cm(-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 (3)Per 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 (3)Per decay, implying significant involvement of (3)Chl- a in the (3)Per state. We suggest that this Per-Chl- a interaction via a delocalized triplet state lowers the (3)Per energy and thus provides a general, photoprotection mechanism for light-harvesting antenna complexes.