The nature of the excited state of the reaction center of photosystem II of green plants: a high-resolution fluorescence spectroscopy study

Abstract

We studied the electronically excited state of the isolated reaction center of photosystem II with high-resolution fluorescence spectroscopy at 5 K and compared the obtained spectral features with those obtained earlier for the primary electron donor. The results show that there is a striking resemblance between the emitting and charge-separating states in the photosystem II reaction center, such as a very similar shape of the phonon wing with characteristic features at 19 and 80 cm-1, almost identical frequencies of a number of vibrational modes, a very similar double-Gaussian shape of the inhomogeneous distribution function, and relatively strong electron-phonon coupling for both states. We suggest that the emission at 5 K originates either from an exciton state delocalized over the inactive branch of the photosystem or from a fraction of the primary electron donor that is long-lived at 5 K. The latter possibility can be explained by a distribution of the free energy difference of the primary charge separation reaction around zero. Both possibilities are in line with the idea that the state that drives primary charge separation in the reaction center of photosystem II is a collective state, with contributions from all chlorophyll molecules in the central part of the complex.

Figure 1

Nonselectively excited emission spectrum of PSII RC at 5 K. Excitation was broad-banded (15 nm fwhm) at 487 nm. The spectral bandwidth of detection was 0.25 nm.

Figure 2

Selectively excited emission spectrum of PSII RC at 5 K. The spectral bandwidth of detection was 0.25 nm. Excitation light was provided with a laser (bandwidth 1 cm⁻¹) at 680.1, 681.3, 682.1, 682.9, and 684.0 nm.

Figure 3

Line-narrowed emission spectra of PSII RC at 5 K. Measuring conditions are the same as in Fig. 2. The spectrum is the average of five spectra, excited at 684.0–686.1 nm. Also shown is the vibronic region (×5 magnification, upper spectrum). (Insets) Magnification of the PW and the 1,500–1,720 cm⁻¹ region.

Figure 4

Experimental (solid line, as in Fig. 1) and simulated (dotted line) nonselectively excited emission spectrum of PSII RC. Parameters of the simulation: S = 1.6; IDF, two Gaussians at 683.6, width 70 cm⁻¹ (fwhm) and at 680.6 nm, width 80 cm⁻¹ (fwhm) with relative (area) contribution of 0.91. (Inset) A magnification of the main peak. A simulation (dashed line) is shown with a single Gaussian at 681.7 nm, width 110 cm⁻¹ (fwhm) as IDF and S = 1.6. It should be noted that below 685 nm the dotted and solid curve and above 685 nm the dashed and dotted curves almost coincide. For details see text.

Figure 5

Experimental (solid line) and simulated (dotted and dashed lines) emission spectra selectively excited at 680.1, 681.3, 682.1, 682.9, and 684.0 nm. The IDF used for the simulations consists of two Gaussians at 683.6, width 70 cm⁻¹ (fwhm) and at 680.6 nm, width 80 cm⁻¹ (fwhm) with a relative (area) contribution of 0.91. (A) S = 1.6. (B) S = 1.2 (dotted line) and 2.0 (dashed line). For details see text.