Femtosecond visible transient absorption spectroscopy of chlorophyll-f-containing photosystem II.

The recently discovered, chlorophyll-f-containing, far-red photosystem II (FR-PSII) supports far-red light photosynthesis. Participation and kinetics of spectrally shifted far-red pigments are directly observable and separated from that of bulk chlorophyll-a We present an ultrafast transient absorption study of FR-PSII, investigating energy transfer and charge separation processes. Results show a rapid subpicosecond energy transfer from chlorophyll-a to the long-wavelength chlorophylls-f/d The data demonstrate the decay of an ∼720-nm negative feature on the picosecond-to-nanosecond timescales, coinciding with charge separation, secondary electron transfer, and stimulated emission decay. An ∼675-nm bleach attributed to the loss of chl-a absorption due to the formation of a cation radical, PD1+•, is only fully developed in the nanosecond spectra, indicating an unusually delayed formation. A major spectral feature on the nanosecond timescale at 725 nm is attributed to an electrochromic blue shift of a FR-chlorophyll among the reaction center pigments. These time-resolved observations provide direct experimental support for the model of Nürnberg et al. [D. J. Nürnberg et al., Science 360, 1210-1213 (2018)], in which the primary electron donor is a FR-chlorophyll and the secondary donor is chlorophyll-a (PD1 of the central chlorophyll pair). Efficient charge separation also occurs using selective excitation of long-wavelength chlorophylls-f/d, and the localization of the excited state on P720* points to a smaller (entropic) energy loss compared to conventional PSII, where the excited state is shared over all of the chlorin pigments. This has important repercussions on understanding the overall energetics of excitation energy transfer and charge separation reactions in FR-PSII.

Femtosecond infrared spectroscopy of chlorophyll f-containing photosystem I.

The recent discovery of extremely red-shifted chlorophyll f pigments in both photosystem I (PSI) and photosystem II has led to the conclusion that chlorophyll f plays a role not only in the energy transfer, but also in the charge separation processes [Nürnberg et al., Science, 2018, 360, 1210-1213]. We have employed ultrafast transient infrared absorption spectroscopy to study the contribution of far-red light absorbing chlorophyll f to energy transfer and charge separation processes in far-red light-grown PSI (FRL-PSI) from the cyanobacterium Chroococcidiopsis thermalis PCC 7203. We compare the kinetics and spectra of FRL-grown PSI excited at 670 nm and 740 nm wavelengths to those of white light-grown PSI (WL-PSI) obtained at 675 nm excitation. We report a fast decay of excited state features of chlorophyll a and complete energy transfer from chlorophyll a to chlorophyll f in FRL-PSI upon 670 nm excitation, as indicated by a frequency shift in a carbonyl absorption band occurring within a 1 ps timescale. While the WL-PSI measurements support the assignment of initial charge separation to A-1+˙A0-˙ [Di Donato et al., Biochemistry, 2011, 50, 480-490] from the kinetics of a distinct cation feature at 1710 cm-1, in the case of FRL-PSI, small features at 1715 cm-1 from the chlorophyll cation are present from sub-ps delays instead, supporting the replacement of the A-1 pigment with chlorophyll f. Comparisons of nanosecond spectra show that charge separation proceeds with 740 nm excitation, which selectively excites chlorophyll f, and modifications in specific carbonyl absorption bands assigned to P700+˙ minus P700 and A1-˙ minus A1 indicate dielectric differences of FRL-PSI compared to WL-PSI in one or both of the two electron transfer branches of FRL-PSI.

Excited State Frequencies of Chlorophyll f and Chlorophyll a and Evaluation of Displacement through Franck-Condon Progression Calculations.

We present ground and excited state frequency calculations of the recently discovered extremely red-shifted chlorophyll f. We discuss the experimentally available vibrational mode assignments of chlorophyll f and chlorophyll a which are characterised by particularly large downshifts of 13¹-keto mode in the excited state. The accuracy of excited state frequencies and their displacements are evaluated by the construction of Franck-Condon (FC) and Herzberg-Teller (HT) progressions at the CAM-B3LYP/6-31G(d) level. Results show that while CAM-B3LYP results are improved relative to B3LYP calculations, the displacements and downshifts of high-frequency modes are underestimated still, and that the progressions calculated for low temperature are dominated by low-frequency modes rather than fingerprint modes that are Resonant Raman active.