Spectral Hole Burning in Cyanobacterial Photosystem I with P700 in Oxidized and Neutral States

Abstract

We explored the rich satellite hole structures emerging as a result of spectral hole burning in cyanobacterial photosystem I (PSI) and demonstrated that hole burning properties of PSI, particularly at high resolution, are strongly affected by the oxidation state of the primary donor P700, as P700 effectively quenches the excitations of the lowest-energy antenna states responsible for fluorescence. Obtaining better control of this variable will be crucial for high-resolution ensemble experiments on protein energy landscapes in PSI. The separate nonphotochemical spectral hole burning (NPHB) signatures of various red antenna states were obtained, allowing for additional constraints on excitonic structure-based calculations. Preliminary evidence is presented for an additional red state of PSI of T. elongatus peaked at 712.6 nm, distinct from previously reported C708 and C715 states and possibly involving chlorophyll B15. Excitation at wavelengths as long as 800 nm results in charge separation at cryogenic temperatures in PSI also in Synechocystis sp. PCC 6803. Both the "P700 minus P700" holes and nonphotochemical spectral holes were subjected to thermocycling. The distribution of barriers manifesting in recovery of the "P700 minus P700" signature contains two components in sample-dependent proportions, likely reflecting the percentages of F and F clusters being successfully prereduced before the optical experiment. The barrier distribution for the recovery of the lower-energy nonphotochemical spectral holes resembles those observed for other pigment-protein complexes, suggesting similar structural elements are responsible for NPHB. Higher-energy components exhibit evidence of "domino effects" such as shifts of certain bands persisting past the lower-energy hole recovery. Thus, conformational changes triggered by excitation of one pigment likely can affect multiple pigments in this tightly packed system.

The structure of a red-shifted photosystem I reveals a red site in the core antenna

“…Another suggestion is that red Chls play a role in photoprotection. It was shown that when P700 is oxidized and the RC is closed, emission from the red sites is significantly reduced, potentially minimizing the formation of Chls triplets in PSI 21 , 22 .…”

Structural basis for the adaptation and function of chlorophyll f in photosystem I

“…It is well-known that cyanobacterial PSI core binds red-shifted Chls (red Chls) whose energy levels are lower than P700. It has been suggested that red Chls consist of 7-11 molecules of Chl a per 96 Chl a based on the absorption spectra of PSI at cryogenic temperatures [33][34][35] . At least four Chl a clusters are necessary to explain the absorption spectrum of red Chls.…”

“…13). Several red Chl a candidates have been reported based on hole-burning analysis 33 . In the present structure, Chls a836B-a835B-a834B and a841A-a842A form two Chl a clusters, and thus can be considered as candidates for the red Chl a clusters (Fig.…”

“…This "red trap" may therefore also work in the H. hongdechloris PSI. Another candidate for the red Chl a is a839A-a840A 33 (Fig. 4b), which is also conserved between white and far-red PSI.…”

Symmetry breaking in photosystem I: ultrafast optical studies of variants near the accessory chlorophylls in the A- and B-branches of electron transfer cofactors

“…2, red line). The bleach at 704 nm represents a mixture of the excited long-wavelength Chl in the LHA [41,42] and the main band of P 700 [43]. The appearance of a bleach at 690 nm was previously attributed to an ultrafast (< 100 fs) reduction of the primary electron acceptor A 0 treated as excitonically coupled dimers Chl 2A -Chl 3A and Chl 2B -Chl 3B [20] in that fraction of PS I complexes in which the special pair P 700 was directly excited in the far-red spectral region [12,13].…”