OPTICAL ABSORPTION AND ELECTRON SPIN RESONANCE SPECTRA OF CATION RADICALS OF DIMERIC CHLOROPHYLL a IN LOW-TEMPERATURE MATRICES

Abstract— A chlorophyll (Chl) a solution in 3-methylpentane at 77 K exhibits an absorption spectrum with a distinct peak at 706 nm in the red-band region. The formation of the 706 nm absorbing species (S706) was reversible with respect to temperature change; no chemical change was observed. γ-Irradiation of the rigid 3-methylpentane solution at 77 K yields an absorption spectrum which can be ascribed to S706⁺ and S706⁻. When carbon tetrachloride, an electron scavenger, was added to the solution, the absorption of S706⁺ survived, which has peaks at 850 and 956 nm. It is assumed that the S706 is hydrogen-bonded dimeric Chi a , which may be regarded as a model of P700 in photosynthesis. Cation radicals of monomeric Chi a were formed in a γ-irradiated sec -butyl chloride solution at 77 K, and an absorption spectrum with peaks at 730 and 813 nm was recorded. ESR spectra of the cation radicals of S706 and monomeric Chi a are of a similar shape but their linewidths are 7.5 and 11.0 Gauss, respectively. The linewidth narrowing observed for S706⁺ is clear evidence for the assumption that S706 is dimeric Chi a. Comparison was made of the absorption spectrum of S706⁺ with the light-induced spectrum of P700 reported earlier.     

A simple and efficient method for synthesis of 5-substituted 2'-deoxyuridine nucleosides using metal-halogen exchange reaction of 5-iodo-2'-deoxyuridine sodium salt

Treatment of the sodium salt of 2'-deoxy-3', 5'-bis-O-(tert-butyldimethylsilyl)-5-iodouridine (3) with n-BuLi effected regioselective lithiation at the 5-position and the following reaction with various electrophiles afforded 5-substituted 2'-deoxyuridines including 1b, the precursor of stable spin-labeled 1a, in good yields.     

The photochemical reactions of N,N-dialkyl α,β-unsaturated amides

Upon benzene-sensitized irradiation N,N-dibenzyl α,β-unsaturated amides 1a-1c cyclized to the corresponding 2-azetidinones 2a-2c in good yields via intramolecular hydrogen abstraction by the β-C atom. Under the same conditions N,N-diisopropyl amides 1e and 1f were found to undergo a novel photoreaction to give N-isopropyl saturated amides via the abstraction. Irradiation of N,N-diethyl and dimethyl amides gave neither 2-azetidinones nor N-monosubstituted amides.     

Synthesis of δ-thiolactams by the aza-Diels-Alder reaction of in situ generated allenyltrimethylsilylthioketenes with imines

Allenyltrimethylsilylthioketenes, generated in situ through [3,3] sigmatropic rearrangement of trimethylsilylethynyl propargyl sulfides, underwent facile [4+2] cycloaddition with imines to afford the corresponding δ-thiolactams. The resulting 2-trimethylsilyl-4-methylenetetrahydroquinolidine-2-thione, obtained by the [4+2] cycloaddition using piperideine as a dienophile, was transformed into (±)-lupinine in six steps.     

Precise coulometric titration of sodium thiosulfate and development of potassium iodate as a redox standard

In this paper, we determine the effective purity of potassium iodate as a redox standard with a certified value linked to the international system of units (SI units). Concentration measurement of sodium thiosulfate solution was performed by precise coulometric titration with electrogenerated iodine, and an assay of potassium iodate was carried out by gravimetric titration based on the reductometric factor of sodium thiosulfate assigned by coulometry. The accuracy of the coulometric titration method was evaluated by examining the current efficiency of iodine electrogeneration, stability of sodium thiosulfate solutions and dependence on the amount of sodium thiosulfate solution used. The measurement procedure for gravimetric titration of potassium iodate with sodium thiosulfate was validated based on determination of a reference material of known purity (potassium dichromate determined by coulometry with electrogenerated ferrous ions) using the same gravimetric method. Solutions of 0.2 and 0.5 mol/L sodium thiosulfate were stable over 17 days without stabilizer. Investigation of the dependency of titration results on the amount of sodium thiosulfate solution used showed no significant effects, no evidence of diffusion of the sample, and no effect of contamination appearing during the experiment. Precise coulometric titration of sodium thiosulfate achieved a relative standard deviation of less than 0.005% under repeating conditions (six measurements). For gravimetric titration, the results obtained for the effective purity of potassium dichromate were sufficiently close to its certified value to allow confirmation of the validity of the gravimetric titration was confirmed. The relative standard deviation of gravimetric titration for potassium iodate was less than 0.011% (nine measurements), and a redox standard with a certified value linked to SI units was developed.     

Fabrication of wood-inspired high-performance composites through fermentation routes

In this paper, we developed a microbial route to fabricate wood-inspired biomimetic composites comparable to natural wood. Focusing on the chemical composition of woody biomass, we performed in situ bioprocessing of bacterial cellulose (BC) imbibed in modified cationic lignin (Catlig), which exhibited significant bioactivity in improving the microbial growth dynamics. The structural and morphological characteristics were enhanced by the formation of hydrophobic and electrostatic interactions between BC and Catlig during biosynthesis. Microbially derived BC/Catlig composites exhibited enhanced thermal stability and crystallinity, with oriented cellulose fibers. The tensile properties, toughness, and specific strength of BC/Catlig composites were comparable to those of a heavy wood species (Zelkova serrata) under hydrated conditions and synthetic soft materials.

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Confirmation of the crystal structure of poly(p‐phenylene benzobisoxazole) by the X‐ray structure analysis of model compounds and the energy calculation

To check the previously proposed crystal structure of poly(p‐phenylene benzobisoxazole) [PBO], we performed an X‐ray structure analysis for single crystals of low molecular weight model compounds with the following chemical formulas: Both of these two model compounds show essentially the same molecular and subcell structures as those of PBO: the molecular chains take an almost perfect planar conformation and are packed together with a relative height between the adjacent chains of about 3 Å along the chain axis, although for the polymer the chains are shifted by the same value but in a disordered mode with respect to the direction of the shift (upward or downward), different from the regular packing in model compounds. These structural features are reproduced well with energy calculations. Structural ordering in PBO fibers caused by heat treatment at high temperatures, as clarified by X‐ray diffraction measurement, are interpreted on the basis of the energy calculations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1296–1311, 2001