12894
J. Phys. Chem. B 2001, 105, 12894-12901
Isotope Effects in FTIR Difference Spectra of the Photosynthetic Oxygen-Evolving Catalytic Site Determined by ab Initio Calculations on Model Compounds Gad Fischer*,† and Tom Wydrzynski‡ Department of Chemistry, The Faculties, and Research School of Biological Sciences, The Australian National UniVersity, Canberra, ACT 0200 Australia ReceiVed: May 30, 2001; In Final Form: September 27, 2001
The oxidation of water and the production of O2 during photosynthesis occur at a manganese-containing catalytic site within photosystem II. A number of questions remain regarding water substrate interactions and the nature of the other ligands to the catalytic site. Through the use of difference spectra, FTIR spectroscopy is being exploited in the mid-IR range to address these questions. Here, we investigate the isotope-induced effects in three regions of the mid-IR range S2/S1 difference spectrum of photosystem II (Noguchi, T.; Sugiura, M. Biochemistry 2000, 39, 10943 and Noguchi, T.; Inoue, Y.; Tang, X.-S. Biochemistry 1999, 38, 10187) by ab initio Hartree-Fock and density functional calculations, using the 6-31G(d), 6-31+G(d,p), and 6-311++G(d,p) bases. The first region contains the OH stretching modes of water. From a determination of the coupling between the two stretching modes and knowledge of the H2O/D2O isotope-induced shifts, the frequency of the symmetric stretching mode has been determined. It is shown that in the S2 state, one OH is strongly H-bonded and the other much more weakly, whereas in the S1 state the H-bonding for both OHs is more similar. The second region involves frequencies of two ring modes near 1100 cm-1. Calculations of N-deuteration and 15N isotope-induced shifts both confirm that the histidine interaction with the catalytic site is represented by 5-methylimidazole. The third region investigated spans the frequencies characteristic of the stretching vibrations of the carboxyl group. An apparent deuterium-induced frequency upshift of a band near 1600 cm-1, and attributed to the carboxyl group, requires that the two CO bonds of the carboxyl group are similar.
Introduction The site for water oxidation and the production of O2 during photosynthesis is located within photosystem II (PSII), a transmembrane, chlorophyll-containing protein complex found in the thylakoid membranes of higher plants, algae, and cyanobacteria. Although there is considerable information on the mechanism of photosynthetic O2 evolution, several important questions remain. In particular, it is still unknown as to how the substrate water binds during the four-step oxidation cycle and what is the exact nature of the ligand structure around the manganese-containing catalytic site. In the original Kok S-state hypothesis (where the catalytic site cycles through five states, defined as S0, S1, S2, S3, and S4, before releasing O2), it was implied that the two substrate water molecules entered the reaction sequence during the final S3 to S4 to S0 transition1. Although this notion has been considered throughout the literature, the most recent bioinorganic models predict that the substrate water binds already in the S0 state.2-10 Recent rapid 18O isotope exchange measurements have shown that one substrate water molecule is bound in the S0 state through to the S3 state, whereas the second substrate water molecule is bound in at least the S3 state.10 The results left open the possibility that the second substrate water molecule may only bind to the catalytic site after the formation of the S3 state. It is generally agreed that the O-O bond forms during the S3 to S4 to S0 transition. * To whom correspondence should be addressed. Tel: +61 2 6125 3043. Fax: + 61 2 6125 0760. E-mail:
[email protected] † Department of Chemistry, The Faculties. ‡ Research School of Biological Sciences.
Following the discovery of the S2-state, Mn multiline EPR signal in PSII,11 many attempts have been made to try and identify substrate water ligand interactions with the catalytic Mn. In particular, the Mn EPR signals were measured of PSII samples suspended in various isotopes of water. In one study, a weak hyperfine broadening (
