Time-resolved EPR observation of blue-light-induced radical ion pairs in a flavin-Trp dyad.

A dyad of flavin and Trp bridged by a p-phenylamide linker was synthesized as an artificial model system to investigate molecular-based magnetic-field sensors relevant to blue-light photoreceptor proteins. The results demonstrated that intramolecular electron transfer generates a radical pair, only the triplet-born one of which has a microsecond lifetime at room temperature.

Long-range magnetic ordering at 5.5 K for cobalt(II)-hydroxide diamond chains isolated by 17 Å with α-phenylcinnamate.

[Co(4)(phcina)(6)(OH)(2)(H(2)O)(4)]·2H(2)O (1) (phcina = α-phenylcinnamate), obtained as pink needles by hydrothermal technique, consists of two edge-sharing pairs of octahedral CoO(6) connected by their apexes to form diamond-chains. The magnetic chains are isolated from one another without chemical bond at 17 Å by the nonmagnetic bulky organic ligand. It exhibits hidden canting below the long-range antiferromagnetic (AF) ordering at 5.5 K. A metamagnetic critical field of only 30 Oe at 2 K suffices to overcome the weak through space magnetic interaction between chains and reverse the moments within the chains to give a canted AF reaching only (1)/(4) of the required moment for all parallel moment alignment. The anisotropy measured on aligned crystals, using dc and ac modes, estimates g(||b) = 6.5 and g(⊥b) = 3.25, thus eliminating an Ising or a Heisenberg magnetic dimensionality while suggesting an easy-plane AF. Weak frequency dependence of the ac-susceptibilities below the transition temperature is associated with domain structure in the ordered state, and absence of it well above T(N) eliminates single-chain magnetism. It is inferred that the competition between the strong AF coupling between the edge-sharing pairs, which dominates the high temperature susceptibility, and the ferromagnetic coupling within the pairs, results in a canting of the moments within each chain, where the moments are out of the ac-plane by an estimated 13°. In zero-field the moments of adjacent chains are antiparallel resulting in a compensated system, i.e., hidden canting.

Exclusion of Anchor-Matched Peptide Nucleic Acid from Liquid-Ordered Domains by Hybridization with Complementary Flavin-Labeled DNA.

Membrane-anchored proteins and their mimics, such as peptide nucleic acids (PNAs), are known to partition preferentially into either lipid raft/liquid-ordered (lo) domains or into non-raft/liquid-disordered (ld) domains, depending on their lipophilic anchors. Here, anchor-matched PNA was demonstrated to be excluded from the lo microdomains of giant unilamellar vesicles by hybridization with the complementary flavin-labeled DNA. As shown in control experiments using Alexa Fluor 488-labeled DNA, which showed that the preferential partitioning was the lo domain, the domain distribution of PNA was not only dependent on the lipophilic anchor but also on the structure of the hybridized DNA or PNA pair. In such systems, the main factors that influence changes in the domain selectivity of the probes are most likely to also be interactivity (i.e., steric bulkiness), hydrophilicity, and self-assembling ability. These findings may have the potential to contribute to the elucidation of membrane-active peptides, the method of their activation, and their applications in medicine such as antimicrobial use, especially with regard to their actions at the interface between the lo and ld domains in cells.

Photogenerated Radical Pair between Flavin and a Tryptophan-Containing Transmembrane-Type Peptide in a Large Unilamellar Vesicle.

Electron-transfer (ET) reactions in biological systems, such as those with magnetic sensors based on flavoproteins and electron transport at biomembrane interfaces, are interesting and important issues that require understanding. As a model system of flavoproteins in biomimetic environments, we report the dynamics of the radical pair generated by photoinduced ET between riboflavin tetrabutylate (RFTB) and tryptophan (Trp) residues in a transmembrane-type polypeptide, both of which are distributed in a large unilamellar vesicle of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. The Trp residues locate near the hydrophilic membrane interface, as confirmed by a dual-fluorescence quenching assay. The fluorescence and transient absorption upon photoexcitation of RFTB indicate that ET from both the singlet and triplet excited states occurs at the hydrophilic interface, whereas the RFTB in the hydrophobic region does not contribute to ET. The ET efficiency and the magnetic field effect (MFE) on the RFTB anion increase significantly above the gel-to-liquid crystal phase transition temperature due to a decrease in microviscosity. The MFE analysis indicates that the radical pair generated from the triplet ET channel exhibits a long lifetime as those in micellar systems due to the strong cage effect of the vesicle.

A light-controlled molecular brake with complete ON-OFF rotation.

A light-controlled molecular machine based on cyclic azobenzenophanes consisting of a dioxynaphthalene rotating unit and a photoisomerizable dioxyazobenzene unit bridged by methylene spacers is reported. In compounds 1 and 2, 1,5- and 2,6-dioxynaphthalene moieties, respectively, are linked to p-dioxyazobenzene by different methylene spacers (n=2 in 1a and 2; n=3 in 1b), whereas a 1,5-dioxynaphthalene moiety is bonded to m-dioxyazobenzene by bismethylene spacers in 3. In 1b and 2, the naphthalene ring can rotate freely in both the trans and cis states at room temperature. The rotation speed can be controlled either by photoinduced reversible trans-cis (E-Z) isomerization of the azobenzene or by keeping the system at low temperature, as is evident from its NMR spectra. Furthermore, for the first time, we demonstrate a light-controlled molecular brake, wherein the rotation of the naphthalene moiety through the cyclophane is completely OFF in the trans isomer of compound 3 due to its smaller cavity size. Such restricted rotation imparts planar chirality to the molecule, and the corresponding enantiomers could be resolved by chiral HPLC. However, the rotation of the naphthalene moiety is rendered ON in the cis isomer due to its increased cavity size, and it is manifested experimentally by the racemization of the separated enantiomers by photoinduced E-Z isomerization.

Structure of silver(I) complex prepared from azobenzenonaphthalenophane, photochemical coordination change of silver(I) and silver(I)-induced acceleration of Z-E thermal isomerization of azobenzene unit.

A Ag(I) complex was prepared from azobenzenonaphthalenophane as a photoreactive multidentate ligand. The coordination of Ag(I) was reversibly controlled by photoisomerization of the azobenzene unit, and at the same time, Ag(I) accelerated the Z-E thermal isomerization of the azobenzene unit.

Cesium Cation Complexation by a Flavin Receptor via Self-Assembly and Deprotonation.

This study focuses on the self-assembly of a new flavin compound and its scaffolding function for a Cs+ ion. 7,8-Dimethyl-10-[4'-(methoxycarbonyl)phenyl]-isoalloxazine (FlH-MB) displays self-assembly in a DMSO solution and has strong dependence on the solvent. In the DMSO solution, both the resulting scaffold and the deprotonation of FlH-MB were demonstrated to induce complex formation with a Cs+ ion, which was investigated by UV-vis, 1H NMR, and fluorescence titrations. This complex formation involves both Coulombic and cation-π interactions through the Fl- site in an Fl--MB dimer.

Gigantic Magnetic Field Effect on the Long-Lived Intermolecular Charge-Separated State Created at the Nonionic Bilayer Membrane.

For realization of low-cost organic photon-energy conversion, the supramolecular approach has been a focus of attention as a counter approach to precise synthesis of covalently linked donor (D)-acceptor (A) molecules. Here we report photogeneration of a long-lived (∼3 µs) intermolecular charge-separated (CS) state of metal porphyrins (D) and an alkyl viologen (A) at an interface of a vesicle membrane formed by self-assembly of nonionic surfactant and cholesterol molecules. The yield of escaped free radicals is negligibly low as in the case of CS states in covalently linked D-A systems. Furthermore, the transient concentration of the CS state dramatically increases by ∼100% upon application of a magnetic field of 250 mT at room temperature. The simulation of the spin dynamics of the CS state indicates that fast (∼107 s-1) spin-selective recombination and slow (105-106 s-1) dissociation-re-encounter dynamics are the key processes for the long CS-state lifetime and the gigantic magnetic field effect. It has turned out that such dynamics are sharply dependent on temperature and alkyl chain length of the viologen. The present results would lead to the development of future materials for light energy conversion, drug delivery, and microscopic bioprobes.

Fluorescence Imaging of Disrupted Interfaces between Liquid-Ordered and Liquid-Disordered Domains by a Flavin-Labeled PNA Duplex.

Lipid rafts and membrane-active peptides are attracting attention because they help understand basic membrane functions. In addition, we focus on flavoproteins playing some physiological roles and explore the model compounds. In this study, we demonstrate that a new flavin probe, composed of palmitoylated peptide nucleic acid (PNA) and its complementary PNA labeled with flavin, targets the liquid-ordered (lo) microdomains and disrupts its interfaces to liquid-disordered (ld) microdomains of giant unilamellar vesicles and can be visualized by using confocal laser scanning microscopy. Surprisingly, as shown in time-lapse images, vesiculation and probe aggregations appear in the lo-ld interfaces, which leads to local disruption of the membrane. We discuss a possible interpretation of the data based on a comparison with control experiments.

Transformation of cytosine to uracil in single-stranded DNA via their oxime sulfonates.

The novel transformation of cytosine oxime sulfonate (C*) to uracil sulfonate (U*) proceeded by a Co(II)-assisted benzoyl peroxide reaction, eventually leading to the conversion of cytosine to uracil.

Reaction of cytosine with bisulfite and hydroxylamine.

The synthesis of DNA oligomer containing N(4)-hydroxy-5,6-dihydrocytosine-6-sulfonate by the simultaneous addition of bisulfite and hydroxylamine under mild conditions is reported. To the best of our knowledge, the reaction of the oxime into the ketone, N(4)-hydroxy-5,6-dihydrocytosine-6-sulfonate into 5,6-dihydrouracil-6-sulfonate or uracil has not been reported. Moreover, this issue contains the difficulty in specific reactivity of oxime derivative from DNA bases include (5m)C toward reagent; for example, T also reacts to potassium permanganate known as a reagent for oxime into ketone. Here we examined the transformation of DNA containing N(4)-hydroxy-5,6-dihydrocytosine-6-sulfonate into 5,6-dihydrouracil 6-sulfonate and the following elimination of sulfonate.

Synthesis and reaction of DNA oligomers containing modified cytosines related to bisulfite sequencing.

The synthesis of DNA oligomers containing N(4)-hydroxy-5,6-dihydrocytosine-6-sulfonate by using ligand-induced base flipping of cytosine followed by the simultaneous addition of bisulfite and hydroxylamine is reported. In contrast to C, the flipped-out 5-methylcytosine was selectively oxidized over thymines and cytosines in the duplex by potassium permanganate. Ligand-induced base flipping is a convenient and powerful strategy for the synthesis of modified cytosines and 5-methylcytosines related to bisulfite sequencing at the predetermined site of DNA.

Enhanced in vivo antitumor efficacy of fenretinide encapsulated in polymeric micelles.

Fenretinide (N-(4-hydroxyphenyl)retinamide, 4-HPR) is a synthetic retinoid with high antitumor activity against a variety of malignant cells in vitro, and is a promising candidate for cancer chemoprevention and chemotherapy. To enhance the antitumor efficacy of 4-HPR in vivo, 4-HPR were encapsulated into polymeric micelles for tumor targeting by enhanced permeability and retention effects. 4-HPR encapsulated in poly(ethylene glycol)-poly(benzyl aspartate) block copolymer micelles were prepared by the evaporation method. The mean particle size of 4-HPR encapsulated in polymeric micelles was about 173 nm. After intravenous injection into tumor-bearing mice, the delivery of 4-HPR by polymeric micelles increased the blood concentration and enhanced the tumor accumulation of 4-HPR over the injection of the 4-HPR encapsulated in oil-in-water (O/W) emulsions. Tumor growth was significantly delayed following treatment by 4-HPR encapsulated in polymeric micelles, which demonstrated the improved in vivo antitumor efficacy of 4-HPR. In addition, 4-HPR encapsulated in polymeric micelles did not cause any body weight loss. These results suggest that polymeric micelles are a promising and effective carrier of 4-HPR in order to enhance tumor delivery and have potential application in the treatment of solid tumor.

The reaction of cytosine with bisulfite by base flipping from the duplex.

Methylation of cytosine at the C5-position in DNA plays a major role in epigenetic gene control. The detection of 5-methylcytosine was designed with the base flipping from the duplex using NC and CGG/CGG triad. Using bisulfite and hydroxylamine, the reaction of cytosine in 11-mer duplex produced the adduct of bisulfite and hydroxylamine, which was isolated by HPLC and identified by MALDI-TOF MS. The duplex containing (5m)C in (5m)CGG/(5m)CGG did not react with bisulfite and hydroxylamine under the same conditions. Further this method enables cytosine adjacent to mismatch guanine to react with bisulfite and hydroxylamine selectively.

Knotted network consisting of 3-threads and a zwitterionic one-dimensional polymorphs of trans-3-(3-pyridyl)acrylate of cobalt and nickel, MII(C8H6NO2)2(H2O)2.

We present the hydrothermal synthesis, characterization (IR, DT-TGA), single-crystal structures, and magnetic properties of two polymorphs of trans-3(3-pyridyl)acrylate of cobalt(II) and of nickel(II), M(II)(C(8)H(6)NO(2))(2)(H(2)O)(2). Hydrothermal reaction at 120 or 170 degrees C results exclusively in the different polymorphs. The infrared spectra and thermogravimetric analyses of the complexes are almost similar for the two polymorphs but show a difference between cobalt and nickel in energies of the vibrational modes and in the decomposition temperatures. The crystal structures of the two polymorphs are quite different; one crystallizes in a monoclinic space group and the other in a triclinic. This major difference is due to the different stereochemistry, cis or trans, of the coordination at the metal sites. When it is trans-MN(2)O(4), it results in the monoclinic cell consisting of a 3D-network of metals bridged by the ligands through single bonds (M-N and M-O). There is threading of three sublattices up to 2a x 4b x 2c, at which point the three sublattices are knotted into one infinite framework. When it is cis-MN(2)O(4), it results in the triclinic cell and consists of Zwitterionic linear chains of metals bridged by one single ligand via the pyridine and a bidentate carboxylate group and the other ligand is bonded only via the pyridine while its carboxylate end is free. All four compounds are paramagnetic with Weiss constants suggesting weak interactions.