Role of methyl radicals in the growth of PAHs

The role of methyl radicals in the networking of sp² carbons has been explored through kinetic analysis of mass spectra of the gas-phase products of the pyrolysis of toluene and toluene/acetone mixtures. Pyrolytic reactions were performed in a flow tube reactor at temperatures of 1140–1320 K and a constant total pressure of 10.38 Torr with a residence time of 0.585 s. On addition of acetone, methyl substituted products and their derivatives were enhanced. Mass peaks were observed in several sequences at an interval of 14 mass units; these ions correspond to methyl substituted products formed as a result of hydrogen abstraction (−H) followed by methyl radical addition (+CH₃). Each major peak was usually preceded by a peak at two mass units lower, which was likely produced through dehydrogenation/dehydrocyclization (−H₂) of methyl substituted products. Detected species include a large number of alkyl, cyclotetrafused (CT), cyclopentafused (CP) mono-, di-, and polycyclic aromatic hydrocarbons (PAHs) along with primary PAHs. The analysis showed that MAC (methyl addition/cyclization) has a unique capacity to induce the sequential growth of hexagonal networks of sp² carbons from all fusing sites [1] of a PAH. Moreover, MAC was found capable of answering an important question in PAH growth, which is expansion of the CT → CP → hexagonal network for which other reported mechanisms are inefficient.     

Simultaneous determination of hydrophilic amino acid enantiomers in mammalian tissues and physiological fluids applying a fully automated micro-two-dimensional high-performance liquid chromatographic concept

A validated two-dimensional HPLC system combining a microbore-monolithic ODS column and a narrowbore-enantioselective column has been established for a sensitive and simultaneous analysis of hydrophilic amino acid enantiomers (His, Asn, Ser, Gln, Arg, Asp, allo-Thr, Glu and Thr) and the non-chiral amino acid, Gly, in biological samples. To accomplish this goal, the amino acids were first tagged with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) to the respective fluorescent NBD derivatives which were separated in the first dimension by a micro-reversed-phase column. The automatically collected fractions of the target peaks were then transferred to the second dimension consisting of a Pirkle type enantioselective column generating separation factors higher than 1.13 for all the enantiomeric target analytes. The system was validated using standard amino acids and a rat plasma sample, and analytically satisfactory calibration and precision results were obtained. The present 2D-HPLC system enables the fully automated determination of hydrophilic amino acid enantiomers in mammalian samples. The d-isomers of all the investigated 9 amino acids were found in rat urine but at various enantiomeric ratios.     

A Supramolecular Polymer of Nitroxide Radicals via Hydrogen Bonding

Summary: 2,2,6,6-Tetramethylpiperidinyl-N-oxy (TEMPO) is a robust nitroxide radical molecule under ambient conditions. We found that the TEMPO derivatives act as a proton acceptor to form an intermolecular hydrogen-bonding complex with many kinds of phenol or urea derivatives. ORTEP analysis of the crystals of TEMPO with the phenol derivatives indicated that hydrogen bonding could be formed between the oxygen of the nitroxide and the phenolic proton and the N O bond of the hydrogen-bonded TEMPO was lengthened in comparison to that of the free N O bond. The formation constant of the hydrogen-bonding complex of TEMPO with the phenol or urea derivatives in a chloroform solution was spectroscopically determined by IR to be 10–100 M⁻¹. Hydrogen bonding of the thelechelic bis-TEMPO derivatives with thelechelic bis-phenol or bis-urea derivatives provided a supramolecular structure. The estimated molecular weights of the supramolecules in the chloroform solution, based on DOSY-NMR spectroscopy, were 3000–4000. The potential of the nitroxide radical's supramolecule as a new functional material is also described.     

Inner-valence states of N2+ and the dissociation dynamics studied by threshold photoelectron spectroscopy and configuration interaction calculation

The N2(+) states lying in the ionization region of 26-45 eV and the dissociation dynamics are investigated by high-resolution threshold photoelectron spectroscopy and threshold photoelectron-photoion coincidence spectroscopy. The threshold photoelectron spectrum exhibits several broad bands as well as sharp peaks. The band features are assigned to the N2(+) states associated with the removal of an inner-valence electron, by a comparison with a configuration interaction calculation. In contrast, most of the sharp peaks on the threshold photoelectron spectrum are allocated to ionic Rydberg states converging to N2(2+). Dissociation products formed from the inner-valence N2(+) states are determined by threshold photoelectron-photoion coincidence spectroscopy. The dissociation dynamics of the inner-valence ionic states is discussed with reference to the potential energy curves calculated.     

Hydration regulates thermodynamics of G-quadruplex formation under molecular crowding conditions

The effect of molecular crowding on the structure and stability of biomolecules has become a subject of increasing interest because it can clarify how biomolecules behave under cell-mimicking conditions. Here, we quantitatively analyzed the effects of molecular crowding on the thermodynamics of antiparallel G-quadruplex formation via Hoogsteen base pairs and of antiparallel hairpin-looped duplex (HP duplex) formation via Watson-Crick base pairs. The free energy change at 25 degrees C for G-quadruplex formation decreased from -3.5 to -5.5 kcal mol(-1) when the concentration of poly(ethylene glycol) 200 was increased from 0 to 40 wt %, whereas that of duplex formation increased from -9.8 to -6.9 kcal mol(-1). These results showed that the antiparallel G-quadruplex is stabilized under molecular crowding conditions, but that the HP duplex is destabilized. Moreover, plots of stability (ln K(obs)) of the DNA structures versus water activity (ln a(w)) demonstrated that the ln K(obs) for G-quadruplex formation decreased linearly as the ln a(w) increased, whereas that for duplex formation increased linearly with the increase in ln a(w), suggesting that the slope approximately equals the number of water molecules released or taken up during the formation of these structures. Thus, molecular crowding affects the thermodynamics of DNA structure formation by altering the hydration of the DNA. The stabilization of the DNA structures with Hoogsteen base pairs and destabilization of DNA structures with Watson-Crick base pairs under molecular crowding conditions lead to structural polymorphism of DNA sequences regulated by the state of hydration.     

Determination of the pion charge form factor at Q2=1.60 and 2.45 (GeV/c)2

The 1H(e,e'pi+)n cross section was measured at four-momentum transfers of Q2=1.60 and 2.45 GeV2 at an invariant mass of the photon nucleon system of W=2.22 GeV. The charged pion form factor (F(pi)) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F(pi) is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q2 by one sigma, but is still far from its perturbative quantum chromodynamics prediction.     

Solid‐Phase Synthesis of a Combinatorial Methylated (±)‐Epigallocatechin Gallate Library and the Growth‐Inhibitory Effects of these Compounds on Melanoma B16 Cells

We report on the solid‐phase synthesis of a combinatorial methylated (±)‐epigallocatechin gallate (EGCG) library and its biological evaluation. Epigallocatechin gallate (EGCG) and its methylated derivatives, which are members of the catechin family, exhibit various anti‐cancer effects. The solid‐phase synthesis of methylated EGCG involves the preparation of the α‐acyloxyketone by the coupling of a solid‐supported aldehyde with a ketone and an acid. The subsequent release and reductive etherification reaction of the solid‐supported α‐acyloxyketone provide the protected EGCG in good total yields. Sixty‐four methylated EGCGs were successfully prepared. The growth‐inhibitory effects of the methylated EGCG library were also examined. Although methylation of EGCG generally causes reduced growth inhibition, the growth‐inhibitory effect of 7‐OMe EGCGs was comparable to that of EGCG. The 7‐OMe EGCGs are attractive drug candidates because of their enhanced bioavailability.     

Efficient synthesis of the functional central core lactides, a constituent of antibiotic fattiviracins

An efficient convergent synthetic method has been developed for preparation of various stereoisomers and derivatives of fattiviracins via a common lactide. The synthetic route comprises seco acids prepared by the β-selective glycosylation of chiral 3-hydroxy-4-pentanoate obtained by enzymatic kinetic resolution. The regioselective protection of four individual hydroxy groups was achieved via the 4,6-O-benzylidenation of the glucose moiety from its TMS ethers. The dimeric cyclization of the seco acids under control of reaction concentration afforded the desired lactide without using KH. Our convergent synthetic method was successfully applied to direct installation of side chains to the lactide by cross metathesis to synthesize fattiviracin derivatives. We achieved improvements to the reported method with respect to: (1) synthesis of a convergent synthetic intermediate; (2) stereoselectivity in glycosylation; and (3) establishment of a low cost route suitable for large scale synthesis.     

Regioselective glycosylation of fully unprotected methyl hexopyranosides by means of transient masking of hydroxy groups with arylboronic acids

Facile, one-pot synthesis was developed for several β(1→2)-, β(1→3)- or β(1→4)-linked disaccharides from fully unprotected methyl hexopyranosides according to the molecular recognition by arylboronic acids. The methodology was successfully applied to facile, short step assembly of the trisaccharide fragment of type II arabinogalactan.