Mechanism of adiabatic primary electron transfer in photosystem I: Femtosecond spectroscopy upon excitation of reaction center in the far-red edge of the Q band.

The ultrafast primary charge separation in Photosystem I (PS I) excited by femtosecond pulses centered at 720 and 760nm was studied by pump-to-probe laser spectroscopy. The absorbance in the red edge of PS I absorption spectrum has an unusual exponential dependence on wavelength. The cutoff of short wavelength components of 760nm pulse allows direct excitation of reaction center chlorophyll molecules without involvement of light-harvesting antenna. The transient spectrum manifests the features of the primary ion-radical pair PA at time delay <180fs, followed by formation of the secondary pair PA with a characteristic time of 26ps. The obtained data are rationalized in the framework of adiabatic three-state model that includes the chlorophyll dimer P and two symmetrically arranged nearest chlorophyll molecules of A. The arrangement of chlorophylls results in strong electronic coupling between P and A. Excitation in the maximum of P absorption generates electronic states with the highest contribution from P*, whereas excitation in the far-red edge predominantly generates charge transfer state PA in both branches of redox-cofactors. The three-level model accounts for a flat-bottomed potential surface of the excited state and adiabatic character of electron transfer between P and A, providing a microscopic explanation of the ultrafast formation of PA and exponential decline of PS I absorption.