Ryota Saito

Solvent and substituent effects on the fluorescent properties ofcoelenteramide analogues

Coelenteramide 1 is the light emitter in aequorin bioluminescence. To establish the fluorescent character of 1, the fluorescence properties of 1 and a series of its analogues, 3a–f, possessing a substituent R [= CF3, F, H, OCH3, OH, N(CH3)2] at the para-position on the 5-phenyl group have been investigated in solvents of various polarity. The fluorescence emission maxima of 1 and 3d–f, possessing an electron-donating group R [= OCH3, OH, N(CH3)2] shift to lower energy with increasing solvent polarity, while those of the analogues 3a–c (R = CF3, F, H) are independent of the solvent polarity. The linear correlation between the fluorescence maxima of 1 and 3d–f and the solvent polarity scales can be explained by formation of the singlet excited state with a charge-transfer (CT) character. The quantum yields of CT fluorescence of 1 and 3d–f have been found to be higher than those of 3a–c. These results indicate that the solvatochromic fluorescence of 1 originates from the CT excited state and the existence of an electron donating hydroxy group on the 5-phenyl group is essential for determining a wavelength and a high fluorescence quantum yield of aequorin bioluminescence.

Peptide-conjugated pterins as inhibitors of ricin toxin A.

Several 7-peptide-substituted pterins were synthesized and tested as competitive active-site inhibitors of ricin toxin A (RTA). Focus began on dipeptide conjugates, and these results further guided the construction of several tripeptide conjugates. The binding of these compounds to RTA was studied via a luminescence-based kinetic assay, as well as through X-ray crystallography. Despite the relatively polar, solvent exposed active site, several hydrophobic interactions, most commonly π-interactions not predicted by modeling programs, were identified in all of the best-performing inhibitors. Nearly all of these compounds provide IC₅₀ values in the low micromolar range.

Pterin-7-carboxamides as a new class of aldose reductase inhibitors.

Aldose reductase is related to the onset and progression of diabetic complications, such as neuropathy, retinopathy, angiopathy, and so on: therefore molecules that are capable of inhibiting the enzyme are potential drugs for treatment of diabetic complications. Epalrestat is the sole aldose reductase inhibitor that is clinically used, but still has some drawbacks. Thus, the development of new aldose reductase inhibitors is still desired. We have synthesized a series of new pterin-7-carboxamides, and evaluated their in vitro inhibitory activities against human aldose reductase. All newly synthesized compounds exhibited the inhibitory activity. Among them, 1a having a glycine side chain exhibits the highest activity comparable to that of sorbinil, a highly active aldose reductase inhibitor. Molecular docking of 1a on the active site of the enzyme indicated this compound interacts with amino acid residues that are specific to the enzyme and related to suppressing side effects. Based on these results, we proved perin-7-carboxamides to be a new class of aldose reductase inhibitors, and particularly compound 1a was found to be a good candidate for further biological investigations as a drug for treatment of diabetic complications with fewer side effects.

SYNTHESIS OF OXOVANADIUM(IV) AND ZINC(II) COMPLEXES OF 3-HYDROXY-4-(p-SUBSTITUTED)PHENYLTHIAZOLE-2(3H)-THIONES WITH A S2O2 COORDINATION MODE AND THEIR INSULIN-MIMETIC ACTIVITIES

Oxovanadium(IV) and zinc(II) complexes with five kinds of 3-hydroxy-4-(p-substituted)phenylthiazole-2(3H)-thiones as bidentate ligands were newly synthesized. Zinc(II) complexes showed approximately 15 times higher in vitro insulin-mimetic activities than that of ZnSO 4 as a positive control. Oxovanadium(IV) complexes also exhibited in vitro insulin-mimetic activities, in which a correlation between the activity and the Hammetts constant of the substituent R was found. Among zinc(II) complexes, bis[2,3-dihydro-2-thioxo-4-(p-nitrophenyl)-(6a) and bis[2,3-dihydro-2-thioxo-4-(p-chlorophenyl)-3-thiazololato]zinc(II) (6b) substantially lowered the blood glucose levels in KK-A y mice. Oral glucose tolerance tests for 6a and 6b indicated the improvement of the diabetic states of animals.

Allosteric Binding of Alkali Metal Ions to a Pseudocryptand Formed by a C-Pivot Tripodal Ligand Containing 3-Hydroxy-2(1H)-pyridinone and Ga(III)

A novel C-pivot tripodal hexadentate ligand (3,2-HOPOHL) composed of 3-hydroxy-2(1H)-pyridinone as a bidentate ligand, the ethyleneoxy chain as a spacer, and tris(carboxyric acid) as an anchor was synthesized. 3,2-HOPOHL recognized only Na + ion, suggesting that it pre-organized a cavity due to the electrostatic interaction among the 2(1H)-pyridinone rings. UV-VIS spectroscopic analysis indicated that 3,2-HOPOHL formed a stable intramolecular 1:1 Fe(III) complex in aqueous solution. The stability constant (log K) of 3,2-HOPOHL-Fe(III) complex was estimated to be 27.6 from the competitive reaction with EDTA. 1 H-NMR titration of 3,2-HOPOHL-Ga(III) complex with Na + and K + ions in CDCl 3 -CD 3 CN indicated the formation of 1:1 complexes. The binding constants of Na + - and K + -3,2-HOPOHL-Ga(III) complexes were estimated to be 3.3×10 3 and 7.8×10 3 M - 1 , respectively, the ion selectivity of K + toward Na + being more than two-fold.

Synthesis of New 3-Hydroxy-4(1H)-Pyridinone Directly Attached to Quinoxaline and Its Fluorescence Property upon Complexation to Metal Ions

A new fluorophore (3), in which the bidentate ligand, 3-hydroxy-2-methyl-4(1H)-pyridinone is directly attached to the fluorescent 2,3-dimorpholino-quinoxaline at C-6 position. The bidentate ligand (3) formed 3:1 complexeswith Fe(III), Al(III), Ga(III), and Cr(III). The fluorescence was efficiently quenched by the metal complex formation via the Perrin model of static quenching, the quenching efficiency being in order of Fe(III) >> Al(III)> Ga(III) > Cr(III). The fluorescence was recoverd by removal of Fe(III) with the Nbenzoyl analogue of a naturally occurring siderophore, desferrioxamine B.

Facile detection of specific RNA-polypeptide interactions by MALDI-TOF mass spectrometry.

A simple method for the detection of specific RNA‐polypeptide interactions using MALDI‐TOF mass spectroscopy is described. Instead of direct observation of the RNA‐polypeptide complex, we attempted the indirect observation of the binding event by focusing on the disappearance of the free polypeptide signal upon interaction with RNA. As a result, specific binding of the Rev‐response element (RRE) RNA of the HIV with two RRE‐binding peptide aptamers, DLA and RLA peptides, as well as the bacteriophage λ boxB RNA with the λ N peptide was observed. We also show that specific RNA‐binding peptides can be identified from a mixture of peptides with varying RNA‐binding affinity, showing that the method could be applied to high‐throughput screening from simple peptide libraries. The method described in this study provides a quick and simple method for detecting specific RNA–polypeptide interactions that avoids difficulties associated with direct observation of RNA and RNA–polypeptide complexes, which may find various applications in the analysis of RNA–polypeptide interactions and in the identification of novel RNA‐binding polypeptides. Copyright

Synthesis and biological evaluation of 4-hydroxy-5-oxo-2,5-dihydro-1H-pyrrole-3-carboxamides and their zinc(II) complexes as candidate antidiabetic agents

The newly designed 1-(arylmethyl)-2,5-dihydro-4-hydroxy-5-oxo-1H-pyrrole-3-carboxamides (1a–e) were synthesized by using N-tritylated acrylamide as a starting material, and their zinc(II) complexes (10a–e) were readily prepared by simply mixing 1a–e and ZnSO4 in the presence of LiOH. The aldose reductase (AR) inhibitory activity of 1a–e was evaluated to reveal important structural features for 2,5-dihydro-5-oxo-1H-pyrrole derivatives to exhibit high AR inhibitory activity. The in vitro insulin–mimetic activity of these complexes was evaluated from 50% inhibitory concentrations (IC50) on free fatty acid (FFA) release from isolated rat adipocytes treated with epinephrine. Four out of the five newly synthesized complexes exhibited higher in vitro insulin-mimetic activities than ZnSO4, a positive control. This study proved that the newly synthesized N2O2-coordination-type zinc(II) complexes based on pyrrole-3-carboxamide derivatives (1a–d) are potential hypoglycemic agents.

α-Methylphenacyl thioesters as convenient thioacid precursors

α-Methylphenacyl (Mpa) thioesters are described as precursors of thioacids. Mpa thioesters are accessible via the condensation of carboxylic acids and phenacyl thiol, which is easily prepared without column chromatography. The Mpa thioesters are selectively deprotected by reduction with zinc dust in the presence of conventional thioacid protecting groups. In addition, the Mpa group exhibits orthogonal reactivity to the Boc group. These features are expected to facilitate the preparation of complex thioacids, including those in peptides.

Non-CRET-based green chemiluminescence of imidazopyrazinone modified by 2,3,6,7-tetrahydro-1H,5H-benzo〔i,j〕quinolizine as a strong electron-donating unit

A new imidazopyrazinone derivative, 1, having a 2,3,6,7-tetrahydro-1H,5H-benzo[i,j]quinolizin-9-yl (julolidin-9-yl) group at the 6-position, was synthesized and exhibited a largely red-shifted chemiluminescence in diglyme containing acetate buffer caused by a strong electron donation from the julolidin-9-yl group. The maximum wavelength was observed at 523 nm, which represents the most red-shifted light achieved only by the electron-donating effect from the 6-position of the imidazopyrazinone skeleton without any aids of extended π-conjugation systems or longer-wavelength-light emitting fluorophores.