Photosynthetic O2 formation tracked by time-resolved x-ray experiments

Area of Science:

• Biochemistry
• Photosynthesis research
• Plant science

Background:

• Plants and cyanobacteria produce oxygen from water using sunlight via photosystem II.
• The established S-cycle model for oxygen evolution includes five states, but the S4 state remains uncharacterized.
• The S4 state is critical for the direct formation of dioxygen (O2).

Purpose of the Study:

• To identify and characterize the missing S4 state in the oxygen evolution process.
• To elucidate the mechanism of dioxygen formation at the S4 state.
• To extend the understanding of the S-state cycle in photosynthesis.

Main Methods:

• Utilized advanced X-ray techniques for real-time monitoring.
• Achieved 10-microsecond resolution to observe rapid redox and structural changes.
• Focused on the manganese metal centers within photosystem II.

Main Results:

• Observed the formation of the S4 state as an intermediate during O2 formation.
• Determined that S4 state formation involves a deprotonation process.
• Identified a subsequent S4' state formed by electron transfer, extending the cycle.

Conclusions:

• The enigmatic S4 state has been identified and characterized.
• Oxygen evolution involves deprotonation at the S4 state, challenging previous models.
• The findings expand the fundamental S-state cycle, offering new insights into photosynthesis.