Nallasamy Palanisami

Inhibitory effect of apocarotenoids on the activity of tyrosinase : Multi-spectroscopic and docking studies

In this present study, the inhibitory mechanism of three selected apocarotenoids (bixin, norbixin and crocin) on the diphenolase activity of tyrosinase has been investigated. The preliminary screening results indicated that apocarotenoids inhibited tyrosinase activity in a dose-dependent manner. Kinetic analysis revealed that apocarotenoids reversibly inhibited tyrosinase activity. Analysis of fluorescence spectra showed that apocarotenoids quenched the intrinsic fluorescence intensity of the tyrosinase. Further, molecular docking results implied that apocarotenoids were allosterically bound to tyrosinase through hydrophobic interactions. The results of the in vitro studies suggested that higher concentrations of bixin and norbixin inhibited tyrosinase activity in B16F0 melanoma cells. Our results suggested that apocarotenoids could form the basis for the design of novel tyrosinase inhibitors.

Synthesis, characterization and structures of diphenyldiaminosilanes bearing bulky substituents on nitrogen

Aminosilanes bearing bulky substituents on nitrogen centers, [(ArNH) 2 SiPh 2 ] (Ar=2,6- i Pr 2 C 6 H 3 (1), 2,4,6-Me 3 C 6 H 2 (2), 2,6-Et 2 C 6 H 3 (3)), have been prepared in good yields by the addition of dichlorodiphenylsilane to the corresponding substituted monolithiated aniline. The new compounds have been characterized by elemental analysis and IR, EI mass and NMR ( 1 H and 29 Si) spectroscopic studies. The solid-state structures of 1 and 3 have been determined by single crystal X-ray diffraction studies. The molecules have a C s symmetry and the two N≪H protons are approximately trans to each other. The amido nitrogen atoms show significant deviation from trigonal-planar geometry, as a result of which the observed Si≪ N bonds are marginally longer than those observed in aminosilanes with planar nitrogen atoms. Aminosilanes bearing bulky substituents on nitrogen centers, [(ArNH) 2 SiPh 2 ] (Ar=2,6- i Pr 2 C 6 H 3 (1), 2,4,6-Me 3 C 6 H 2 (2), 2,6-Et 2 C 6 H 3 (3)), are accessible in good yields by the addition of dichlorodiphenylsilane to the corresponding monolithiated substituted aniline. The molecules have a Cs symmetry due to the large steric crowding and the two N≪H protons are approximately trans to each other.

Eight membered cyclic-borasiloxanes: synthesis, structural, photophysical, steric strain and DFT calculations

Eight-membered cyclic borasiloxanes, Ph2Si[OBArO]2SiPh2 [Ar = 4-EtC6H4 (1), 4-tBuC6H4 (2), 2-PhC6H4 (3), 4-PhC6H4 (4) and β-C10H7 (Nap) (5); Ph = phenyl], were synthesized via the reaction of diphenylsilanediol with aryl boronic acid through a condensation reaction. The compounds were characterized using elemental analysis, FT-IR and NMR (1H, 13C, 29Si and 11B). The compounds 1, 3 and 5 were further confirmed using single crystal X-ray diffraction studies. This showed the eight-membered ring (B2O4Si2) configuration and that organic substituents occupied the axial and equatorial positions. Furthermore, non-covalent C–H⋯π and π⋯π interactions were observed in the crystal packing pattern. These borasiloxanes exhibited strong solid state fluorescence. The thermal behavior of the compounds 1–5 has been investigated using thermogravimetric analysis (TGA), which shows that the borasiloxanes 1 and 2 are thermally stable up to 220 °C and 180 °C respectively, whereas 3 and 4 are stable up to ∼120 °C and 5 is stable up to 230 °C. The band gap was calculated using the diffuse reflectance spectroscopic method. Compound 5 exhibits a low band gap (3.28 eV) which indicates that the naphthyl group shows more π-bonding delocalization within the molecule (strong intra-molecular charge transfer). The band gap decreases in the order of the compounds, 1 > 2 > 3 > 4 > 5. The theoretically computed band gap values were in good agreement with the experimentally observed trend. HOMO–LUMO analysis, TD-DFT, and the electrophilicity index, dipole moment and hyperpolarizability were computed using the B3LYP/6-31+G** method. The steric strain energies of the borasiloxanes and their degree of puckering conformation (O–Si–O, O–B–O and B–O–Si) were also analysed using DFT. This confirms that compound 3 has more strain, which is due to having a phenyl group in a sterically hindered ortho-position.

New sterically hindered tin(IV) siloxane precursors to tinsilicate materials: synthesis, spectral, structural and photocatalytic studies

A series of sterically hindered tin(IV) siloxanes were synthesized by the reaction between tris(tert-butoxy)silanol/tri-phenylsilanol and organotin chlorides {[(tBu)2Sn(OSi(OtBu)3)2] (1), [(tBu)2Sn(OSi(OtBu)3)Cl] (2), [(n-Bu)2Sn(OSi(OtBu)3)2] (3), [(n-Bu)2Sn(OSi(OtBu)3)Cl] (4), [(Me)2Sn(OSi(OtBu)3)2] (5), [(Me)2Sn(OSi(OtBu)3)Cl] (6), [(tBu)2Sn(OSiPh3)2] (7), [(tBu)2Sn(OSiPh3)Cl] (8) whereas tBu = tertiary butyl; n-Bu = butyl; Me = methyl}. All the compounds were characterized by analytical and spectroscopic (FT-IR and 1H, 13C, 29Si, 119Sn NMR) methods. Compounds 1 and 7 were structurally characterized by single-crystal X-ray crystallography. The coordination geometry of tin (SnO2C2) is slightly distorted from tetrahedral due to sterically crowded ligands around the tin atom. In order to convert tinsilicate materials, compounds 1 and 3 were selected for thermolysis to give identical SnO2·2SiO2 materials at low temperature (∼350 °C). The degradation was investigated by thermal analysis (TGA/DTA), both compounds having butyl groups which facile eliminate to butene gas via a β-hydride elimination process (∼250 °C). The molecular route to oxide materials at low temperature described here represents an alternative to the sol–gel technique. The tinsilicate material was examined by several techniques including infrared, powder X-ray diffraction analysis (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The optical band gap of the material has been studied by UV-vis-NIR spectroscopy and the result is a low band gap value (Eg = 2.549 eV) due to the silicon oxide mixed with tin oxide. The prepared material acts as photocatalyst for the degradation of methylene blue (MB).

Correction: Non-covalent polyhedral oligomeric silsesquioxane-polyoxometalates as inorganic–organic–inorganic hybrid materials for visible-light photocatalytic splitting of water

A new series of visible light responsive and water-stable inorganic–organic–inorganic hybrid materials such as polyoxometalate (POM) and polyhedral oligomeric silsesquioxanes (POSS) with different metals was prepared, and these materials include {[PW12O40][(NH3CH2CH2CH2)(iBu)7Si8O12]3 (POM(W)–POSS), [PMo12O40][(NH3CH2CH2CH2)(iBu)7Si8O12]3 (POM(Mo)–POSS) and [PMo10V2O40][(NH3CH2CH2CH2)(iBu)7Si8O12]5 (POM(MoV)–POSS)}; these materials containing a cationic electron donor of amino-substituted POSS (POSS-NH2) on an anionic electron acceptor of POM have been developed for efficient and sustainable photocatalytic H2 production. It is interesting to note that the synthesized hybrid materials exhibit remarkable photocatalytic H2 production activities under visible-light irradiation. The maximum H2 production rate of 485 μmol h−1 g−1 is achieved using POM with heterometallic sites, i.e., (MoV)–POSS rather than that having homometallic sites. The enhanced photocatalytic H2 production is mainly due to improved charge carrier separation by electron-deficient metal sites and a red shift in the absorption, as revealed from photoluminescence and UV-vis spectra. Importantly, POM–POSS hybrid materials demonstrate excellent water stability during photocatalysis due to the hydrophobicity of the hybrid materials, which results from the integration of hydrophobic POSS-NH2 with POMs in contrast to free POMs. Additionally, the successful integration of POSS-NH2 into POMs is confirmed using FT-IR, NMR, ESI mass spectroscopy, and elemental and thermogravimetric analyses. The present study may open new possibilities in the design and development of stable POM organic/inorganic hybrid materials for solar energy conversion applications.

Synthesis and spectroscopic characterization of tris(tert-butoxy)siloxy titanium and hafnium complexes: Molecular precursor to [M/Si/O (M = Ti, Hf)] materials

GRAPHICAL ABSTRACT ABSTRACT The reaction of one equivalent of metallocene dichloride Cp2MCl2 (M = Ti, Hf) with two molar equivalents of sodium salt of tri(tert-butoxy)siloxane, (tBuO)3SiONa, in toluene yields Cp2Ti[OSi(OtBu)3]2 (1) and Cp2Hf[OSi(OtBu)3]2 (2). These compounds were characterized by analytical and spectroscopic techniques (C/H/N analysis, FT-IR, NMR, EI-MS, and thermal analysis). An independent thermolysis of 1 and 2 at low temperature resulted in the formation of the corresponding metal silicates MO2·2 SiO2 [M = Ti (3) and Hf (4)] as evidenced by infrared spectroscopy, powder X-ray diffraction analysis (PXRD), and scanning electron microscopy (SEM). These thermally stable amorphous metal silicate materials (3 and 4), albeit exhibiting moderate surface areas, are found to be microporous by N2 adsorption studies. UV-vis-NIR spectroscopy shows low energy band gap Eg = 2.63 eV for 3 and 2.99 eV for 4 due to the silicon oxide mixed with the corresponding transition metal oxides. The photocatalytic activity of titanium silicate material (3) has been explored in terms of degradation of methylene blue.