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

Abstract

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 approximately 13 and approximately 42 mus 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 (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.

FIGURE 1

Molecular structure of Per.

FIGURE 2

DADS of PCP obtained using a parallel model after 670-, 530-, and 480-nm excitation (from top to bottom). DADS1 (dashed line) and DADS2 (solid line) are associated with 13- and 42-μs lifetimes, respectively.

FIGURE 3

PCP carbonyl region of the raw time-resolved data at t₀ obtained after excitation at 670 (dashed line) and 530 nm (dotted line) compared to the second derivative of PCP FTIR absorption spectrum (solid line).

FIGURE 4

Comparison of the carbonyl modes of the PCP DADS at 670-nm, 530-nm, and 480-nm excitation.

FIGURE 5

PCP time traces for absorbance at 1720 cm⁻¹ with 670-nm (solid line), 530-nm (dashed line), and 480-nm (dotted line) excitations.

FIGURE 6

Differential absorbance signals of PCP obtained by step-scan FTIR. Raw data are t₀ time slices with 5-μs time resolution for 670-nm (dotted line), 480-nm (dashed line), and 530-nm (solid line) excitation; intensity 2 mJ/cm² at all wavelengths.