Ultrafast Dynamics of the Photo-Excited Hemes b and cn in the Cytochrome b6f Complex

The dynamics of the heme b and cn within the cytochrome b6f complex are investigated by means of ultrafast broad-band transient absorption spectroscopy. On one hand, the data reveals that, subsequent to visible light excitation, part of the b hemes undergo a pulse limited photo-oxidation, with the liberated electron supposedly being admited in one of the adjacent aromatic amino acid. The photo-oxidation is followed by a charge recombination in about 8.2 ps. Subsequent to the charge recombination, the heme is promoted to an vibrationally excited ground state that relaxes in about 4.6 ps. On the other hand, the heme cn undergoes an ultrafast ground state recovery in about 140 fs. Interestingly, the data also shows that, contrarely to previous beliefs, Chl a is involved in the hemes photochemistry. Indeed, subsequently to the hemes excitation, the Chl a bleaches and recovers its ground state in 90 fs and 650 fs, respectively. The Chl a bleaching allegedly corresponds to the formation of a short lived Chl a anion. Beyond the previously suggested structural role, this study gives unique evidences that Chl a is directly involved in the photochemistry of the hemes.