Absorbance difference spectra of the S-state transitions in Photosystem II core particles

Area of Science:

• Photosynthesis research
• Plant biochemistry
• Bioenergetics

Background:

• The oxygen evolving complex (OEC) in Photosystem II (PS II) is crucial for water splitting during photosynthesis.
• Understanding the redox states (S-states) of the OEC is key to elucidating the mechanism of water oxidation.
• The minimal Kok model describes a four-step S-state cycle (S0-S3) but may not fully capture OEC dynamics.

Purpose of the Study:

• To investigate the redox changes of the OEC in PS II core particles using UV-region absorbance difference spectroscopy.
• To evaluate the validity of the minimal Kok model in explaining observed absorbance oscillations.
• To determine difference spectra for successive S-state transitions and identify contributing factors.

Main Methods:

• Absorbance difference spectroscopy in the UV-region.
• Analysis of absorbance change oscillations induced by saturating flashes.
• Comparison of experimental data with the minimal Kok model.
• Determination of difference spectra for S0→S1, S1→S2, and S2→S3 transitions.

Main Results:

• Oscillations in absorbance changes could not be fully explained by the minimal Kok model alone.
• A slow equilibration of the S3 state with an inactive state and low quantum efficiency for the first turnover were proposed.
• Difference spectra revealed distinct spectral features for each S-state transition, influenced by Chl bandshifts.
• The S1→S2 transition spectrum suggested Mn(III) to Mn(IV) oxidation, while S2→S3 resembled data from PS II membranes.

Conclusions:

• The minimal Kok model requires modifications to accurately describe OEC redox behavior.
• Slow S3 equilibration and initial low quantum efficiency are critical factors in OEC function.
• UV-region spectroscopy provides insights into S-state transitions, with distinct spectral signatures and contributions from bandshifts and electrostatic effects.