Koh-ichi Yamada

Relationship between tyrosinase inhibitory action and oxidation-reduction potential of cosmetic whitening ingredients and phenol derivatives

The oxidation-reduction potentials of cosmetic raw materials, showing tyrosinase inhibitory action, and phenolic compounds structurally similar to L-tyrosine were determined by cyclic voltammetry. The voltammograms obtained could be classified into 4 patterns (patterns 1–4). Pattern 1, characterized by oxidation and reduction peaks as a pair, was observed with catechol, hydroquinone or phenol, and pattern 2 exhibiting another oxidation peak in addition to oxidation and reduction peaks as a pair was found with arbutin, kojic acid, resorcinol, methyl p-hydroxybenzoate and L-tyrosine as the substrate of tyrosinase. Pattern 3 with an independent oxidation peak only was expressed by L-ascorbic acid, and pattern 4 with a reduction peak only at high potentials, by hinokitiol. The tyrosinase inhibitory activity of these compounds was also evaluated using the 50% inhibitory concentration (IC50) and the inhibition constant (Ki) as parameters. Hinokitiol, classified as pattern 4, showed the highest inhibitory activity (lowest IC50 and Ki). Hydroquinone showing the second highest activity belonged to pattern 1, which also included compounds showing no inhibition of tyrosinase activity. The inhibitory activity of compounds exhibiting pattern 2 was relatively low with Ki values being in the order of 10−4 M. Although there was no consistent relationship between oxidation-reduction potentials and tyrosinase inhibitory action, the voltammetry data can be used as an additional index to establish the relationship between the structure and the tyrosine inhibitory activity.

A second-order asymmetric transformation of racemic 2-hydroxymethyl[5]thiaheterohelicene into a single enantiomer upon uptake by bovine serum albumin

Abstract Racemic 2-hydroxymethylthieno[3,2-e:4,5-e′]di[1]benzothiophene (2-HT) with a labile helical structure was converted into a P enantiomer upon uptake by bovine serum albumin (BSA) in 1 % ethanol-water. The effect of the alteration of the molar ratio [2-HT]: [BSA] on UV and CD spectra of the 2-HT-BSA complex solution revealed that BSA possesses two different substratebinding sites with a distinct ability to recognize chirality of enantiomers of 2-HT. The stability of the 2-HT-BSA complex and the chirality recognition at the two sites of BSA were evaluated by the equilibrium constants and the thermodynamic parameters which were obtained from the temperature-dependence of CD-absorptional intensities. BSA pretreated at 50 ∼ 70 °C in an aqueous solution exhibited a great depression of ability in diseriminating chirality between enantiomers of 2-HT, though it possessed slightly altered ability for uptake of 2-HT, in comparison with untreated BSA.

Molecular chirality on a solid surface : thiaheterohelicene monolayer on gold imaged by STM

Abstract Imaging of molecule–molecule recognition of chirality on a solid surface was attempted using scanning tunneling spectrocopy (STM) under ultra-high vacuum (UHV) conditions. The target molecule in this work, thiaheterohelicene ([11]TH), is a fused product which consists of five benzene rings and six thiophene rings. It has a helical chirality which can be compared to a spring washer. A racemic mixture of [11]TH was vaporized to prepare an adsorbed layer onto an Au(111)/mica substrate under UHV conditions. Images were obtained with molecular resolution on wide terrace areas and faceted narrow steps. The images obtained in the terrace areas show that the right handed and the left handed isomers were randomly adsorbed on the surface with little interaction. On the other hand, the images obtained in the steps showed that molecules of the same chirality formed a tightly packed row. The high resolution STM image obtained for [11]TH enabled us to distinguish not only the domain structures but also the domain compositions caused by the structural difference of the substrate.

Chiral recognition by single bilayered phosphatidylcholine vesicles using [5]thiaheterohelicene as a probe

Incorporated into chiral phosphatidylcholine (PC) vesicles, [5]thiaheterohelicene with a labile helix that functioned as a probe, developed induced CD absorptions which manifested alterations in Cotton effects and intensities by chirality of PC applied, temperature variations, and coexistence of cholesterol or its 5α-epimer.

Chiral discrimination in the transport of ketoprofen and ibuprofen esters through an aqueous phase mediated by various serum albumins

Serum albumins that act as carriers discriminated between enantiomers of alkyl esters of ketoprofen and ibuprofen in transport in the O/W/O (oil/water/oil) system using a U-shaped cell. The transport rate and the preferred enantiomer of the esters were substantially affected by pH, temperature, and species of albumin. Among five serum albumins studied, bovine serum albumin (BSA) showed the largest rate constant and rat serum albumin (RSA) manifested the highest enantioselectivity. Regarding enantiomer selectivity in transport overall, it is anticipated that the ester uptake step plays an important role for BSA, whereas the ester release is the key step for RSA.

Crystal and molecular structures of tetrathia[7]heterohelicene: racemate and enantiomer

The crystal and molecular structures of the racemate and enantiomer tetrathia[7]heterohelicene have been determined in order to elucidate their molecular geometries and crystal packing modes. The racemate and enantiomer both crystallize from benzene in the orthorhombic form with Z= 4. The former belongs to the space group Pbcn with a= 15.383(2), b= 14.195(2), c= 7.767(1)A, and the latter belongs to the space group P212121 with a= 16.975(2), b= 11.236(1), c= 9.186(1)A. The racemate molecule retains C2 symmetry in the crystal, while the enantiomer lacks this symmetry, primarily because of differences in the dihedral angles between the successive aromatic rings. In the racemate, the molecules are stacked in a column with alternating chirality, while in the enantiomer the molecules are confined orthogonally to each other, resulting in strain. These packing modes are consistent with the observed geometrical deformation of the enantiomer, and provide a clue to the problem of chirality recognition.

Induction and reversal of chirality of heterohelicene by charge transfer interaction in aqueous SDS micelles

2-Hydroxymethylthieno[3,2-e∶4,5-e′ ]di[1]benzothiophene and 2-(2,4,5,7-tetranitrofluoren-9-ylideneaminooxy)propionic acid in SDS micelles initially formed a 1∶2 complex with a negative Cotton effect around 360 nm, which with time or sonication gradually changed to a 1∶1 complex giving a reversed Cotton effect.

Total conversion of racemic [5]thiaheterohelicene into a single enantiomer by a diastereoisomeric charge-transfer complex formation

Racemic thieno[3,2-e:4,5-e′]di[1]benzothiophene was totally converted into a single enantiomer on crystallizing the charge-transfer complex with (S)-2-(2,4,5,7-tetranitro-9-fluorenylidene-amino-oxy)propionic acid; the structure of the complex was determined by X-ray crystallographic analysis.

Conversion of racemic 2-hydroxymethyl[5]thiaheterohelicene into a single enantiomer on the uptake by bovine serum albumin

Racemic 2-hydroxymethylthieno[3,2-e:4,5-e′]di[1]benzothiophene with a labile helical structure was converted into a (P) enantiomer upon uptake by bovine serum albumin, in which the existence of two substrate-binding sites with a distinct ability to recognize chirality was revealed.

The conformations and 1H nuclear magnetic resonance parameters of dibenzylideneacetone

Dibenzylideneacetone, (C6H5CHCH)2CO, with two degrees of rotational freedom imparted by the two single bonds between the olefin and the carbonyl group was studied to determine its stable conformers by 1H n.m.r. spectroscopy and by an INDO molecular orbital method. A solvent effect induced by benzene on the trans-olefinic protons and a least squares analysis of the temperature dependence of the internal chemical shift (δAB) between the olefinic protons have revealed support of the theoretical calculations of δAB that the major conformation is s-cis,cis and the minor one is s-cis,trans. An INDO calculation of the conformers concerned confirmed this deduction together with the absence of the s-trans,trans form. δAB is a characteristic parameter of the conformation with the values 0.5 and 1.5 p.p.m. for the s-cis and s-trans forms, respectively, in the olefinic portions of dibenzylideneaċetone.