SOLVATOCHROMISM AND THERMOCHROMISM STUDY OF HOMOPTEROCARPIN

Abstract

Major studies on Homopterocarpin has been focused majorly on its biological uses, little or no study has been carried out on the spectroscopic and photophysical properties of the compound. Understanding the influence of micro-environment on the spectroscopic properties of Homopterocarpin is very important for a better understanding of the solute-solvent interactions in biological systems. Homopterocarpin (3,9-Dimethoxypterocarpan) a natural compound, is an isoflavonoid extracted from Pterocarpus erinaceus stem bark. The antioxidant properties of this compound was studied in solvents of varying polarities. The UV-visible and steady state spectral was also studied in serial solvents of different polarities. The solvation characteristics of Homopterocarpin in its ground and excited states were investigated in fourteen solvents of different polarity under physiological conditions using UV-Visible and fluorescence spectroscopy at 25 °C. The organic solvents are methanol, acetone, benzene, acetic acid, diethylether, 2-propanol, 1-butanol, dimethylsulfoxide (DMSO), hexane, acetonitrile, chloroform, toluene, ethanol and distilled water. The effect of the various solvents on the antioxidant properties of the thin layer chromatographic purified homoptercarpin evaluated using 2,2-diphenyl-1- picrylhydrazyl (DPPH), 2,2-Azino-bis-3-ethylbenzothiazoline-6-sulphonic Acid (ABTS) revealed that Homopterocarpin has highest DPPH radical scavenging activity of 66.24% in acetic acid, a polar protic solvent. In ABTS, ethanol enhanced Homopterocapin antioxidant activity while DMSO could not. vii The steady-state absorption and fluorescence spectral shifts were analyzed using linear regression model proposed by Kamlet-Taft approach. The result obtained indicated that both non-specific and specific solute-solvent interactions play roles in solvatochromism of Homopterocarpin molecule. Spectroscopic studies indicate also that the nature of the absorbing and emitting states is significantly different in protic, aprotic and non-polar solvents. The results of the UV-Visible absorption spectroscopy showed a positive/bathochromic shift with increasing solvent polarity, indicating a π-π* transition electronic transition. Kamlet-Taft analysis of the UV-Vis absorption spectra shows that Homopterocarpin exhibits a hypsochromic shift with increasing solvent dipolarity which was offset by a bathochromic shift due to the hydrogen bond basicity and hydrogen bond acidity of the solvent resulting in a net bathochromic shift when the dependent factor is the microscopic polarity parameter. Diethyl ether had the highest bathochromic effect of 7.81% while water caused an insignificant hypsochromic shift (0.0009%). Kamlet- Taft analysis revealed Homopterocarpin qualitative spectra is a function of its microenvironment solvents hydrogen bond acidity and dipolarisability. Solvents with hydrogen bond basicity created hypsochromic effect. This infers that Homopterocarpin is a hydrogen bond acceptor compound which uses its methyl groups to achieve an excited state and uses its highly methylated structure to form dipole interactions with its environment. The calculated dipole moment of the excited state (Ue) was 6.14 x 10-23 Cm which was much higher than the corresponding ground state dipole moment (Ug) value of 2.33 x 10-22cm. Thermochromism study revealed that Homopterocarpin is thermochromic in nature and might influence its antioxidant capability.