Design and Synthesis of New Biomimetic Materials by Sol-Gel: A Cu(II)(histidine)(2) Complex Covalently Bonded on a Silica Matrix

The synthesis of a new histidine-silane derivative, Boc-His(Boc)-CONH-(CH(2))(3)Si(OEt)(3), is reported. Hydrolysis and co-condensation of this monomer with tetraethoxysilane, via the sol-gel procedure, results in a hybrid inorganic-organic material that bears histidine molecules covalently bonded on a silica matrix. 1D-ESEEM and 2D-HYSCORE studies of its Cu(II) complex show that the copper atom is coordinated by two inequivalent histidine imidazoles. The new Cu(II) material exhibits catalytic activity for DTBQ formation in the presence of dioxygen, with considerable turnover rates and yields. In addition it is highly recyclable and shows high specific surface area.

Reaction centre photochemistry in cyanide-treated photosystem II

EPR was used to study the triplet state of chlorophyll generated by radical pair recombination in the photosystem II (PSII) reaction centre. The spin state of the non-haem Fe2+ was varied using the CN--binding method (Y. Sanakis, V. Petrouleas, B.A. Diner, Biochemistry 33 (1994) 9922-9928) and the redox state of the quinone acceptor (QA) was changed from semi-reduced to fully reduced (F.J.E. van Mieghem, W. Nitschke, P. Mathis, A.W. Rutherford, Biochim. Biophys. Acta 977 (1989) 207-214). It was found that the triplet was not detectable using continuous wave EPR when QA- was present irrespective of the spin-state of the Fe2+. It was also found that the triplet state became detectable by EPR when the semiquinone was removed (by reduction to the quinol) and that the triplet observed was not influenced by the spin state of the Fe2+. Since it is known from earlier work that the EPR detection of the triplet reflects a change in the triplet lifetime, it is concluded that the redox state of the quinone determines the triplet lifetime (at least in terms of its detectability by continuous wave EPR) and that the magnetic state of the iron, (through the weakly exchange-coupled QA- Fe2+ complex) is not a determining factor. In addition, we looked for polarisation transfer from the radical pair to QA- in PSII where the Fe2+ was low spin. Such polarisation is seen in bacterial reaction centres under comparable conditions. In PSII, however, we were unable to find evidence for such polarisation of the semiquinone. It is suggested that both the short triplet lifetime in the presence of QA- and the lack of polarised QA- might be explained in terms of the electron transfer mechanism for triplet quenching involving the semiquinone which was proposed previously (F.J.E. van Mieghem, K. Brettel, B. Hillmann, A. Kamlowski, A.W. Rutherford, E. Schlodder, Biochemistry 34 (1995) 4798-4813). It is suggested that this mechanism may occur in PSII (but not in purple bacterial reaction centres) due the triplet-bearing chlorophyll being adjacent to the pheophytin at low temperature as suggested from structural studies (F.J.E. van Mieghem, K. Satoh, A.W. Rutherford, Biochim. Biophys. Acta 1058 (1992) 379-385).

A Mn(II)-Mn(III) EPR signal arises from the interaction of NO with the S1 state of the water-oxidizing complex of photosystem II

It was shown recently [Goussias, C., Ioannidis, N., and Petrouleas, V. (1997) Biochemistry 36, 9261-9266] that incubation of photosystem II preparations with NO at -30 degrees C in the dark results in the formation of a new intermediate of the water-oxidizing complex. This is characterized by an EPR signal centered at g = 2 with prominent manganese hyperfine structure. We have examined the detailed structure of the signal using difference EPR spectroscopy. This is facilitated by the observations that NO can be completely removed without decrease or modification of the signal, and illumination at 0 degree C eliminates the signal. The signal spans 1600 G and is characterized by sharp hyperfine structure. 14NO and 15NO cw EPR combined with pulsed ENDOR and ESEEM studies show no detectable contributions of the nitrogen nucleus to the spectrum. The spectrum bears similarities to the experimental spectrum of the Mn(II)-Mn(III) catalase [Zheng, M., Khangulov, S. V., Dismukes, G. C., and Barynin, V. V. (1994) Inorg. Chem. 33, 382-387]. Simulations allowing small variations in the catalase-tensor values result in an almost accurate reproduction of the NO-induced signal. This presents strong evidence for the assignment of the latter to a magnetically isolated Mn(II)-Mn(III) dimer. Since the starting oxidation states of Mn are higher than II, we deduce that NO acts effectively as a reductant, e.g., Mn(III)-Mn(III) + NO--> Mn(II)-Mn(III) + NO+. The temperature dependence of the nonsaturated EPR-signal intensity in the range 2-20 K indicates that the signal results from a ground state. The cw microwave power saturation data in the range 4-8 K can be interpreted assuming an Orbach relaxation mechanism with an excited state at delta = 42 K. Assuming antiferromagnetic coupling, -2JS1.S2, between the two manganese ions, J is estimated to be 10 cm-1. The finding that an EPR signal from the Mn cluster of PSII can be clearly assigned to a magnetically isolated Mn(II)-Mn(III) dimer bears important consequences in interpreting the structure of the Mn cluster. Although the signal is not currently assigned to a particular S state, it arises from a state lower than S1, possibly lower than S0, too.

Spin-lattice relaxation of the phyllosemiquinone radical of photosystem I

The spin-lattice relaxation time (T1) of the phyllosemiquinone anion radical, A1-, of the photosystem I (PSI) reaction center, were measured between 4.5 and 85 K by electron spin-echo spectroscopy. The selective removal of the iron-sulfur centers, FA, FB, and FX, from PSI allowed the measurement of the intrinsic T1 of the A1- radical. The temperature dependence of the intrinsic (T1)-1 for A1- was found to be approximately T1.3 +/- 0.1. The spin-lattice relaxation of the reduced form of iron-sulfur center FX was also measured at low temperatures, in FA/FB-depleted PSI membranes. It was found that the fast-relaxing FX center enhances the spin-lattice relaxation of the phyllosemiquinone due to dipolar coupling. The effect of the reduced forms of FA/FB on the T1 of the phyllosemiquinone was minor compared to the effect of FX. By analyzing the data with a dipolar model in the light of limitations imposed by other information present in the literature, the distance between the phyllosemiquinone and FX in PSI is estimated to be 14.8 +/- 4 A.

ESEEM study of the phyllosemiquinone radical A1.- in 14N- and 15N-labeled photosystem I

The phyllosemiquinone radical of the photosystem I reaction center has been studied by electron spin echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of ESEEM data of the semiquinone in 14N- and 15N-labeled PSI and numerical simulations demonstrate the existence of two protein nitrogen nuclei coupled to the semiquinone. One of the 14N couplings is characterized by a quadrupolar coupling constant e2qQ/4h of 0.77 MHz, an asymmetry parameter eta of 0.18, and a hyperfine coupling tensor with an almost pure isotropic hyperfine coupling, i.e. (Axx, Ayy, Azz) = (1.3, 1.3, 1.5 MHz). The second nitrogen coupling is characterized by a quadrupolar coupling constant e2qQ/4h of 0.45 MHz, an asymmetry parameter eta of 0.85, and a weak hyperfine coupling tensor with a dominant anisotropic part, i.e. (Axx, Ayy, Azz) = (-0.2, -0.2, 1.5 MHz). On the basis of a comparison of the 14N-ESEEM data with 14N-NQR and 14N-ESEEM data from the literature, the first coupled nitrogen is assigned to the indole nitrogen of a tryptophan residue. The coupling of the second nitrogen is much weaker and therefore more difficult to assign. However, the simulated spectrum best describes an amino nitrogen of a histidine, although the amide group of an asparagine or glutamine cannot be ruled out. The possible origins of teh nitrogen hyperfine coupling are discussed in terms of the amino acid residues thought to be close to the semiquinone in PSI.

A new non-heme iron environment in Paracoccus denitrificans adenylate kinase studied by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopy

Adenylate kinase from the Gram-negative bacterium Paracoccus denitrificans (AKden) has structural features highly similar to those of the enzyme from Gram-positive organisms. Atomic absorption spectroscopy of the recombinant protein, which is a dimer, revealed the presence of two metals, zinc and iron, each binding most probably to one monomer. Under oxidizing conditions, the electron paramagnetic resonance (EPR) spectrum of AKden at 4.2 K consists of features at g = 9.23, 4.34, 4.21, and 3.68. These features are absent in the ascorbate-reduced protein and are characteristic of a S = 5/2 spin system in a rhombic environment with E/D = 0.24 and are assigned to a non-heme Fe3+ (S = 5/2) center. The zero-field splitting parameter D (D = 1.4 +/- 0.2 cm-1) was estimated from the temperature dependence of the EPR spectra. These EPR characteristic as well as the difference absorption spectrum (oxidized minus reduced) of AKden are similar to those reported for the non-heme iron protein rubredoxin. Nevertheless, the redox potential of the Fe2+/Fe3+ couple in AKden was measured at +230 +/- 30 mV, which is more positive than the redox potential of the non-heme iron in rubredoxin. Binding of cyanide converts the iron from the high-spin (S = 5/2) to the low-spin (S = 1/2) spin state. The EPR spectrum of the non-heme Fe3+(S = 1/2) in the presence of cyanide has g values of 2.45, 2.18, and 1.92 and spin-Hamiltonian parameters R/lambda = 7. 4 and R/mu = 0.56. The conversion of the non-heme iron to the low-spin (S = 1/2) state allowed the study of its local environment by electron spin echo envelope modulation spectroscopy (ESEEM). The ESEEM data revealed the existence of 14N or 15N nuclei coupled to the low-spin iron after addition of KC14N or KC15N respectively. This demonstrated that iron in AKden has at least one labile coordination position that can be easily occupied by cyanide. Other possible magnetic interactions with nitrogen(s) from the protein are discussed.

Spin-lattice relaxation of the pheophytin, Pheo-, radical of photosystem II

The spin-lattice relaxation times (T1) of the pheophytin anion radical, Pheo-, of the PSII reaction center, were measured between 5 and 80 K by electron spin-echo spectroscopy. The Pheo- was studied in Mn-depleted PSII reaction centers in which the primary quinone, QA, was doubly reduced. The selective conversion of the non-heme Fe2+ into its low-spin (S = O) state, in CN-treated PSII, allowed the measurement of the intrinsic T1 of the Pheo- radical. The temperature dependence of the intrinsic (T1)-1 was found to be approximately T1.3 +/- 0.1. In Mn-depleted PSII membranes the high-spin (S = 2) non-heme iron, enhances the spin-lattice relaxation of Pheo-. By analyzing the data with a dipolar model, the dipolar interaction (k1d) between the Pheo and the Fe2+ (S = 2) is estimated over the temperature range 5-80 K. Comparison with the dipolar coupling between the iron and the tyrosine, YD+, shows that the Pheo is much closer to the iron than the YD+ in the PSII reaction center. By scaling the reported Fe(2+)-YD+ distance by the ratio [k1dPheo-]/[k1dYD+], we estimate the Fe(2+)-Pheo- distance in PSII to be 20 +/- 4.2 A. This distance is close to the Fe(2+)-BPheo- distance in the bacterial reaction center, and this result provides further evidence that the acceptor sides of the reaction centers in PSII and bacteria are homologous.

ESEEM study of the plastoquinone anion radical (QA.--) in 14N- and 15N-labeled photosystem II treated with CN

The nonheme iron of the photosystem II reaction center was converted to its low-spin state (S = 0) by treatment with CN-. This allowed the study of the plastoquinone, QA- anion radical by electron spin-echo envelope modulation (ESEEM) spectroscopy. A comparative analysis of the ESEEM data of QA- in 14N- and 15N-labeled PSII demonstrates the existence of a protein nitrogen nucleus coupled to the QA-. The 14N coupling is characterized by a quadrupolar coupling constant e2qQ/4h = 0.82 MHz, an asymmetry parameter eta = 0.45, and hyperfine coupling constant A approximately 2.1 MHz. The 15N hyperfine coupling is characterized by T = 0.41 MHz and alpha iso approximately 3.3 MHz. The possible origins of the nitrogen hyperfine coupling are discussed in terms of the amino acids thought to be close to the QA- in PSII. Based on a comparison of the 14N ESEEM with 14N-NQR and 14N-ESEEM data from the literature, the most likely candidate is the amide nitrogen of the peptide backbone of Ala261 of the polypeptide D2, although the indole nitrogen of Trp254 and the imino nitrogen of His215 of D2 also remain candidates.