HF-EPR, Raman, UV/VIS Light Spectroscopic, and DFT Studies of the Ribonucleotide Reductase R2 Tyrosyl Radical from Epstein-Barr Virus

Epstein-Barr virus (EBV) belongs to the gamma subfamily of herpes viruses, among the most common pathogenic viruses in humans worldwide. The viral ribonucleotide reductase small subunit (RNR R2) is involved in the biosynthesis of nucleotides, the DNA precursors necessary for viral replication, and is an important drug target for EBV. RNR R2 generates a stable tyrosyl radical required for enzymatic turnover. Here, the electronic and magnetic properties of the tyrosyl radical in EBV R2 have been determined by X-band and high-field/high-frequency electron paramagnetic resonance (EPR) spectroscopy recorded at cryogenic temperatures. The radical exhibits an unusually low g1-tensor component at 2.0080, indicative of a positive charge in the vicinity of the radical. Consistent with these EPR results a relatively high C-O stretching frequency associated with the phenoxyl radical (at 1508 cm−1) is observed with resonance Raman spectroscopy. In contrast to mouse R2, EBV R2 does not show a deuterium shift in the resonance Raman spectra. Thus, the presence of a water molecule as a hydrogen bond donor moiety could not be identified unequivocally. Theoretical simulations showed that a water molecule placed at a distance of 2.6 Å from the tyrosyl-oxygen does not result in a detectable deuterium shift in the calculated Raman spectra. UV/VIS light spectroscopic studies with metal chelators and tyrosyl radical scavengers are consistent with a more accessible dimetal binding/radical site and a lower affinity for Fe2+ in EBV R2 than in Escherichia coli R2. Comparison with previous studies of RNR R2s from mouse, bacteria, and herpes viruses, demonstrates that finely tuned electronic properties of the radical exist within the same RNR R2 Ia class

PH-triggered dethreading-rethreading and switching of cucurbit[6]uril on bistable [3]pseudorotaxanes and [3]rotaxanes

A series of water-soluble [3]rotaxanes-(n + 2) and [3]pseudorotaxanes-(n+2) with short (propyl, n = 1) and long (dodecyl, n = 10) aliphatic spacers have been prepared in high yields by a 1,3-dipolar cycloaddition reaction catalyzed by cucurbit[6]uril (CB6). The pH-triggered dethreading and rethreading of CB6 on these pseudorotaxanes was monitored by 1H NMR spectroscopy. A previously reported [3]rotaxane-12 that is known to behave as a bistable molecular switch has two recognition sites for CB6, that is, the diaminotriazole moieties and the dodecyl spacer. By changing the pH of the system, it is possible to observe more than one state in the shuttling process. At low pH values both CB6 units are located on the diaminotriazole moieties owing to an ion-dipole interaction, whereas at high pH values both of the CB6 units are located on the hydrophobic dodecyl spacer. Surprisingly, the CB6 units shuttle back to their initial state very slowly after reprotonation of the axle. Even after eighteen days at room temperature, only about 50% of the CB6 units had relocated back onto the diaminotriazole moieties. The rate constants for the shuttling processes were measured as a function of temperature over the range from 313 to 333 K and the activation parameters (enthalpy, entropy, and free energy) were calculated by using the Eyring equation. The results indicate that this [3]rotaxane behaves as a kinetically controlled molecular switch. The switching properties of [3]rotaxane-3 have also been studied. However, even under extreme pH conditions this rotaxane has not shown any switching action, which confirms that the propyl spacer is too short to accommodate CB6 units. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA

A Heck-type coupling for the synthesis of novel bridged metallochlorin-fullerene C-60 dyads

A short and convenient synthesis of metallochlorin-C-60 dyads based on a Heck-type hetero coupling at the 3(2) position of a chlorin is described. p-Bromobenzaldehyde was treated with Zn-metalated 13(2)-demethoxycarbonylmethylpheophorbide a, using a palladium acetate/LiCl catalyst mixture under phase-transfer conditions in DMF at 70 degrees C. The resulting asymmetric olefin was obtained in a high trans/cis ratio. The desired trans isomer was separated and subsequently transformed into a donor-acceptor dyad by a 1,3-dipolar cycloaddition to C-60 in the presence of sarcosine in refluxing toluene. The resulting dyads are expected to undergo efficient photoinduced electron transfer and can potentially be utilized in solar energy conversion devices. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

A Heck-type coupling for the synthesis of novel bridged metallochlorin-fullerene C-60 dyads

A short and convenient synthesis of metallochlorin-C-60 dyads based on a Heck-type hetero coupling at the 3(2) position of a chlorin is described. p-Bromobenzaldehyde was treated with Zn-metalated 13(2)-demethoxycarbonylmethylpheophorbide a, using a palladium acetate/LiCl catalyst mixture under phase-transfer conditions in DMF at 70 degrees C. The resulting asymmetric olefin was obtained in a high trans/cis ratio. The desired trans isomer was separated and subsequently transformed into a donor-acceptor dyad by a 1,3-dipolar cycloaddition to C-60 in the presence of sarcosine in refluxing toluene. The resulting dyads are expected to undergo efficient photoinduced electron transfer and can potentially be utilized in solar energy conversion devices. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Fusion and planarization of a quinoidal porphyrin dimer.

The crystal structure, near-infrared spectrum and electrochemistry of a quinoidal triply-linked porphyrin dimer are compared with those of its singly-linked precursor; fusing the two porphyrins planarizes the pi-system and reduces the optical HOMO-LUMO gap while increasing the gap between the first oxidation and reduction potentials

An artificial supramolecular photosynthetic unit

A biomimetic light harvesting and energy conversion unit is constructed by a modular self-organization methodology. A self-assembled antenna aggregate mimics the supramolecular light-harvesting structures of phototrophic bacteria, and an embedded reaction center model acts both as an energy acceptor and a charge transfer dyad. This unit can be viewed as a conceptual approach to self-assembled supramolecular solar cells, utilizing bio-inspired materials as a starting point for advanced solar energy conversion units. (C) 2007 Elsevier B.V. All rights reserved

Photophysical properties of gallium hydroxyl tetratolylporphyrin and 13(2)-demethoxycarbonyl-(gallium hydroxyl)-methyl-pheophorbide alpha

Two metal tetrapyrroles containing gallium, gallium hydroxyl tetratolylporphyrin and 13(2)-demethoxycarbonyl-(gallium hydroxyl)methyl pheophorbide a (Ga-(OH)-chlorin), were synthesized from their respective free bases using Ga(III)-acetylacetonate in a phenol melt. Their photophysical properties were investigated and the quantum yields of different monomolecular deactivation processes were determined. For Ga-(OH)-porphyrin S-2-fluorescence was observed and quantified. In contrast. for Ga-(OH)-chlorin no S-2-fluorescence was observed. Both compounds should be useful as efficient photosensitizers in photodynamic therapy. (c) 2005 Elsevier B.V. All rights reserved