The Energy Transfer Yield between Carotenoids and Chlorophylls in Peridinin Chlorophyll a Protein Is Robust against Mutations

Abstract

The energy transfer (ET) from carotenoids (Cars) to chlorophylls (Chls) in photosynthetic complexes occurs with almost unitary efficiency thanks to the synergistic action of multiple finely tuned channels whose photophysics and dynamics are not fully elucidated yet. We investigated the energy flow from the Car peridinin (Per) to Chl a in the peridinin chlorophyll a protein (PCP) from marine algae Amphidinium carterae by using two-dimensional electronic spectroscopy (2DES) with a 10 fs temporal resolution. Recently debated hypotheses regarding the S₂-to-S₁ relaxation of the Car via a conical intersection and the involvement of possible intermediate states in the ET were examined. The comparison with an N89L mutant carrying the Per donor in a lower-polarity environment helped us unveil relevant details on the mechanisms through which excitation was transferred: the ET yield was conserved even when a mutation perturbed the optimization of the system thanks to the coexistence of multiple channels exploited during the process.

Keywords: 2DES; N89L; PCP; carotenoid; chlorophyll; energy transfer; light harvesting; peridinin; photosynthesis; two-dimensional electronic spectroscopy.

Figure 1

(a) Structure of the carotenoid Per. (b) Crystallographic structure of WT PCP, with the Per molecules colored in orange and the Chl a molecules colored in green. (c) Absorption spectra of WT PCP (blue) and its N89L mutant (green). The spectrum of the exciting pulses used in 2DES measurements is also shown (orange area).

Figure 2

Absorptive 2DES maps of (a) WT PCP and (b) N89L PCP at different population times (t₂); to make the evolution of the populations more evident, the oscillating contributions to the signal were attenuated using a Savitzky–Golay filter. The markers pinpoint relevant coordinates discussed in the main text.

Figure 3

(a) Vertical cuts of the 2DES maps of WT PCP at excitation frequency 17,066 cm⁻¹ (indicated with the black line) at different population times; to ease the visualization of the signal trends, the oscillating contributions to the signal were attenuated using a Savitzky–Golay filter. Temporal traces of (b) the ET signals (extracted at (17,066, 15,100) cm⁻¹, triangle), (c) the higher frequency ESA signals (extracted at (17,066, 16,700) cm⁻¹, square) and (d) the lower frequency ESA signals (extracted at (17,066, 15,850) cm⁻¹, circle) for the WT and the mutant samples. Thick solid lines represent the fittings performed according to the multiexponential model described in Section S3.1 (only the non-oscillating components are shown for clarity).

Figure 4

Fourier transform of the oscillating components of the 2DES signal, mediated along excitation and detection frequency axes. They are plotted as square moduli of the Fourier transform amplitudes and normalized on the intensities of the 1225 cm⁻¹ component.

Figure 5

(a) Energy level diagram illustrating the different dynamic processes included in the kinetic model used to interpret the temporal dynamics of the experimental traces. Dashed lines indicate events occurring on a time scale faster or comparable with the time resolution of the experiment, namely, the initial optical excitation and the decay of the S₂ state via (i) the CI that transferred the population from S₂ to S₁ and (ii) the initial torsional movements that promoted the formation of distorted structures. $k_{S^{\prime },R}$, $k_{S_1{Q}_y}$, and $k_{S^{\prime }{Q}_y}$ represent the kinetic constants for the rise of S’, the S₁/ICT$\to$Q_y ET, and the S’$\to$Q_y ET, respectively. (b) Contributions to the rise of the ET signal, as determined by the fitting of the experimental data using the kinetic model in panel (a).