Fish-Bite structure by three-dimensional hydrogen-bond acceptor: IR spectroscopy of pyrrole and N-methylpyrrole binary clusters

The N-H[ellipsis (horizontal)]π hydrogen-bonded (H-bonded) structures of pyrrole (Py) and N-methylpyrrole (NMPy) binary clusters have been studied by IR cavity ringdown spectroscopy and density functional theory calculations. The Py(1)-NMPy(1) cluster has an "L-shape" structure, which is formed by an ordinary H-bond between a N-H donor of Py and a π-electron cloud acceptor of NMPy. The Py(2)-NMPy(1) cluster has a "Cyclic" structure, which is also formed by ordinary N-H[ellipsis (horizontal)]π H-bonds as well as the weak C-H[ellipsis (horizontal)]π H-bond between the methyl CH group and the π cloud acceptor of Py. On the other hand, the Py(1)-NMPy(2) cluster shows an extraordinary structure, in which the single donor NH group is surrounded by a three-dimensional H-bond acceptor formed by two aromatic π electron clouds. We call the Py(1)-NMPy(2) cluster as the "Fish-Bite" structure. The Py(1)-NMPy(2) cluster exhibits a redshifted NH stretch by 157 cm(-1) from the Py monomer, which is larger than 94 cm(-1) of the Py(1)-NMPy(1) cluster. However, both Py(1)-NMPy(1) and Py(1)-NMPy(2) clusters have calculated IR intensities of 169 and 163 km∕mol, respectively. This result indicates that not only the N-H[ellipsis (horizontal)]π H-bonds but also the dipole-dipole interaction between Py and NMPy contributes to the Fish-Bite Py(1)-NMPy(2) cluster formation.     

A TEMPO-conjugated fluorescent probe for monitoring mitochondrial redox reactions

We report a mitochondrial targeted redox probe (MitoRP) that comprises a nitroxide radical (TEMPO) moiety and coumarin 343. Using isolated mitochondria in the presence/absence of substrates and inhibitors of oxidative phosphorylation, we demonstrated that MitoRP is a useful probe to monitor the electron flow associated with complex I.     

Theoretical study of dynamic electron-spin-polarization via the doublet-quartet quantum-mixed state (II). Population transfer and magnetic field dependence of the spin polarization

The population transfer to the spin-sublevels of the unique quartet (S = 3/2) high-spin state of the strongly exchange-coupled (SC) radical-triplet pair (for example, an Acceptor-Donor-Radical triad (A-D-R)) via a doublet-quartet quantum-mixed (QM) state is theoretically investigated by a stochastic Liouville equation. In this work, we have treated the loss of the quantum coherence (de-coherence) due to the de-phasing during the population transfer and neglected the effect of other de-coherence mechanisms. The dependences on the magnitude of the exchange coupling or the fine-structure parameter of the QM state are investigated. The dependence on the velocity of the population transfer (by the electron transfer or the energy-transfer) from the QM state to the SC quartet state is also clarified. It is revealed that the de-coherence during the population transfer mainly originates from the fine-structure term of the QM state in the doublet-triplet exchange coupled systems. This de-coherence leads to the unique dynamic electron polarization (DEP) on the high-field spin sublevels of the SC state, which is similar to the unique DEP pattern of the photo-excited triplet states of the reaction centers of photosystems I and II. The magnetic field dependence of the population transfer leading to the populations of the spin-sublevels of the SC states is also calculated. The possibility of the control of energy transport, spin transport and information technology by using the QM state is discussed based on these results. The knowledge obtained in this work is useful in the spin dynamics of any doublet-triplet exchange coupled systems.     

Platinum catalyzed 7-endo cyclization of internal alkynyl amides and its application to synthesis of the caprazamycin core

The scope and limitations of the platinum catalyzed 7-endo cyclization of internal alkynyl amides were investigated. Substitution of the alkyne with an aryl group gave better results, presumably because it stabilized the transition state. Applying the reaction to a secondary amide, the caprazamycin core was successfully synthesized from commercially available material in eight steps.     

Furanose ring conformations in a 1′-alkynyl C-nucleoside and the dinucleotide

A large-scale synthesis was carried out for alkynyl C-nucleotide oligomers in order to clarify the structural details of the artificial DNA. A liquid-phase synthesis by means of phosphoramidite methodology enabled us to handle ～0.1 g of the DNA oligomers possessing a pseudouracil derivative (T*) as a non-natural nucleobase. ¹H NMR measurements in aqueous media were carried out for the oligomers, succeeded in the accurate assignments of the protons accompanying with determination of the coupling constants (J values). These J values revealed that average conformation of the alkynylribose rings in the DNA was substantially biased toward the S-type conformers (²T₁/E₁/^OT₁). X-ray crystal analysis of the non-natural nucleoside also supported the S-type conformation (²E/²T₁) as seen in natural B-DNA.     

Hapalocyclamide: a novel phytotoxic hexapeptide of the cyanobacterium Hapalosiphon sp.

Hapalocyclamide, a novel oxazole-, thiazole- and thiazoline-containing cyclic hexapeptide, was isolated from the terrestrial cyanobacterium Hapalosiphon sp., and which showed phytotoxic activity on lettuce seedling growth. The gross structure of hapalocyclamide was established from spectroscopic data and chemical degradation. The absolute stereochemistry was determined by Marfey’s analysis. Hapalocyclamide was established as cyclo-thiazole-l-alanine-oxazole-d-alanine-d-thiazoline-d-phenylalanine.     

Medicinal Flowers. XXXV. Nor-oleanane-type and acylated oleanane-type triterpene saponins from the flower buds of Chinese Camellia japonica and their inhibitory effects on melanogenesis(1))

The methanolic extract and its 1-butanol-soluble fraction from the flower buds of Camellia japonica, cultivated in Yunnan Province, China, showed inhibitory effects on melanogenesis in theophylline-stimulated B16 melanoma 4A5 cells. From the 1-butanol-soluble fraction, a new 28-nor-oleanane-type and three new oleanane-type triterpene saponins, sanchakasaponins A-D, were isolated together with four known triterpene saponins. Their chemical structures were elucidated on the basis of chemical and physicochemical evidence. The inhibitory effects on melanogenesis in theophylline-stimulated B16 melanoma 4A5 cells and structure-activity relationships of the saponins were investigated.