May 15, 2012

Fast structural changes (200-900ns) may prepare the photosynthetic manganese complex for oxidation by the adjacent tyrosine radical

Biochimica Et Biophysica Acta
André KlaussHolger Dau


The Mn complex of photosystem II (PSII) cycles through 4 semi-stable states (S(0) to S(3)). Laser-flash excitation of PSII in the S(2) or S(3) state induces processes with time constants around 350ns, which have been assigned previously to energetic relaxation of the oxidized tyrosine (Y(Z)(ox)). Herein we report monitoring of these processes in the time domain of hundreds of nanoseconds by photoacoustic (or 'optoacoustic') experiments involving pressure-wave detection after excitation of PSII membrane particles by ns-laser flashes. We find that specifically for excitation of PSII in the S(2) state, nuclear rearrangements are induced which amount to a contraction of PSII by at least 30Å(3) (time constant of 350ns at 25°C; activation energy of 285+/-50meV). In the S(3) state, the 350-ns-contraction is about 5 times smaller whereas in S(0) and S(1), no volume changes are detectable in this time domain. It is proposed that the classical S(2)=>S(3) transition of the Mn complex is a multi-step process. The first step after Y(Z)(ox) formation involves a fast nuclear rearrangement of the Mn complex and its protein-water environment (~350ns), which may serve a dual role: (1) The Mn- complex entity is prepared for the subsequent proton ...Continue Reading

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