Yiannis Sanakis

The S1YZ. metalloradical intermediate in photosystem. II: an X- and W-band EPR study

Visible light illumination at liquid He temperatures of photosystem II (PSII) membranes poised in the S1-state, results in the production of a metalloradical signal with resonances at g = 2.035 and g ∼ 2.0 at X-band (J. H. A. Nugent, I. P. Muhiuddin, and M. C. W. Evans, Biochemistry, 2002, 41, 4117–4126). A similar signal has been obtained by near IR excitation of samples poised in the S2 state (D. Koulougliotis, J.-R. Shen, N. Ioannidis, and V. Petrouleas, Biochemistry, 2003, 42, 3045–3053). The signal has been attributed to the magnetic interaction of the tyrosyl Z radical with the Mn cluster in the S1 state. In an effort to obtain further information about the interactions of tyrosine Z with the Mn cluster, and about the integer-spin S1 state we have employed EPR spectroscopy at two frequencies, X and W-band. The spectrum at W band is characterized by novel resonances at g = 2.019, g ∼ 2.00 and g = 1.987. For the analysis of the spectra at the two microwave frequency bands a spin Hamiltonian has been applied under the following basic assumptions: The S1 state of the Mn cluster is characterized by two low lying spin states Sa = 0 and 1. The major features of the spectra are attributed to the interaction of the Sa = 1 state with the spin Sb = 1/2 of the tyrosyl radical. Potential contributions from the Sa = 0 state are suppressed under the present experimental conditions. A satisfactory fit reproducing all features of the spectra is achieved with the same set of fitting parameters for the signals at both bands. An anisotropic ferromagnetic exchange interaction results from the fit with the coupling value being of the same order of magnitude with the value of the zero field splitting term of the Mn cluster (S = 1).

Q-band Electron Paramagnetic Resonance Studies of the S3 State of the OEC of Photosystem II

We have applied Q-band EPR spectroscopy to study the S3 state of the Mn cluster in untreated and in methanol treated PSII membranes, extending earlier bimodal studies at X band EPR. Prominent EPR signals are observed in both sets of samples. A simultaneous analysis of the spectra at both frequency bands indicates that the spin associated with the signals of S3 is S = 3 rather than S = 1. An S = 3 spin state is also indicated in the presence of methanol, but with significantly different zero field splitting parameters, which explain the absence of X-band signals in this case.

Iron-substituted cubic silsesquioxane pillared clays: Synthesis, characterization and acid catalytic activity

Novel pillared structures were developed from the intercalation of iron-substituted cubic silsesquioxanes in a sodium and an acid-activated montmorillonite nanoclay and evaluated as acid catalysts. Octameric cubic oligosiloxanes were formed upon controlled hydrolytic polycondensation of the corresponding monomer (a diamino-alkoxysilane) and reacted with iron cations to form complexes that were intercalated within the layered nanoclay matrices. Upon calcination iron oxide nanoparticles are formed which are located on the silica cubes (pillars) and on the surfaces of the clay platelets. Acid activation of the nanoclay was performed in order to increase the number of acid active sites in the pristine clay and thus increase its catalytic activity. A plethora of analytical techniques including X-ray diffraction, thermal analyses, Fourier transform infrared, electron paramagnetic resonance, Raman, Mössbauer and X-ray photoelectron spectroscopies and porosimetry measurements were used in order to follow the synthesis steps and to fully characterize the final catalysts. The resulting pillared clays exhibit a high specific area and show significant acid catalytic activity that was verified using the catalytic dehydration of isopropanol asa probe reaction.