Solar energy conversion in oxygen evolving organisms occurs in two separate reaction center protein complexes called photosystems I and II. In each, light induces the transfer of electrons, via a series of protein bound pigment acceptors, across a biological membrane. The very high efficiency of light induced electron transfer is related to the electronic and structural organization of these protein bound electron acceptors. To study the electronic and structural organization of these acceptors in the protein binding site we have used time-resolved visible and infrared difference spectroscopy. In photosystem I a highly reducing phylloquinone functions as an intermediary in electron transfer. Phylloquinone is bound to the protein in the, so called, A1 binding site, and it is the nature of this binding that makes phylloquinone so reducing. To probe the properties of the A1 binding site we have made use of a mutant cyanobacterial strain that allows different quinones to be easily incorporated into the A1 binding site. To verify the extent of quinone incorporation into the A1 binding site we have used nanosecond transient absorption spectroscopy in the visible spectral region. To probe the molecular properties of the introduced quinones in both the neutral and anion states, we have used microsecond time-resolved FTIR difference spectroscopy at 77 K. The time resolved FTIR difference spectra display a multitude of bands that are associated with both the quinone and the protein binding site. The convoluted nature of the spectra makes interpretation difficult. However, by comparing spectra obtained for photosystem I particles with four different quinones incorporated (phylloquinone, 2-methyl naphthoquinone, plastoquinone and dichloro naphthoquinone) we have been able to distinguish quinone infrared absorption bands from protein bands. To complement the experimental work and to aid in FTIR difference band assignment we have used quantum mechanical/molecular mechanics computational methods to simulate the infrared difference spectra associated with the different quinones in the protein binding site.


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