Cochineal Red A is a negatively charged synthetic azo food colorant and a potential carcinogen. We present here the study of binding of Cochineal Red A with two homologous serum albumins, human (HSA) and bovine (BSA), in aqueous pH 7.4 buffer by optical spectroscopic techniques. Protein intrinsic fluorescence quenching by Cochineal Red A occurs through ground-state static interaction and its binding with BSA is stronger than with HSA. The magnitudes of thermodynamic parameters suggest that dye binding occurs principally via electrostatic complexation. Site-marker competitive binding shows that Cochineal Red A binds primarily to site I of serum albumins. Circular dichroic spectra indicate that dye binding results in some conformational modification of serum albumins. Increased ionic strength of the medium results in lowering of binding. This study provides an important insight into possible means of removal of dye toxicity.

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Formation of ion pair between charged molecule and protein can lead to interesting biochemical phenomena. We report the evolution of thermodynamics of the binding of tartrazine, a negatively charged azo colorant, and serum albumins with salt. The dye binds predominantly electrostatically in low buffer strengths; however, on increasing salt concentration, affinity decreases considerably. The calculated thermodynamic parameters in high salt indicate manifestation of nonelectrostatic interactions, namely, van der Waals force and hydrogen bonding. Site-marker competitive binding studies and docking simulations indicate that the dye binds with HSA in the warfarin site and with BSA at the interface of warfarin and ibuprofen binding sites. The docked poses indicate nearby amino acid positive side chains, which are possibly responsible for electrostatic interaction. Using the Debye-Hückel interionic attraction theory for binding equilibria, it is shown that, for electrostatic binding the calculated free energy change increases linearly with square root of ionic strength. Also UV-vis, fluorescence, CD data indicate a decrease of interaction with salt concentration. This study quantitatively relates how ionic strength modulates the strength of the protein-ligand electrostatic interaction. The binding enthalpy and entropy have been found to compensate one another. The enthalpy-entropy compensation (EEC), general property of weak intermolecular interactions, has been discussed.

The photochemistry of methane caged within amorphous solid water (ASW) is interesting as a model for studying interstellar and high-altitude atmospheric pathways for the formation of more complex hydrocarbons. Here, we report on the photoreactivity of clean methane and in the presence of oxygen molecules, known as electron capture species, within two 50 monolayer-thick D2O-ASW films adsorbed on Ru(0001) substrate under ultrahigh vacuum conditions. Irradiation by 248 nm UV photons (5.0 eV), where none of the involved molecules absorb these photons in the gas phase, leads to the formation of diverse hydrocarbons. In all cases, the presence of oxygen results in significantly enhanced reactivity. The dissociative electron attachment mechanism with electrons generated within the metal substrate is thought to largely govern the photoreactivity in this system. Methyl radicals as the primary photoproducts subsequently react with the surrounding water and neighboring methane as well as with the stable O2 – anion. Postirradiation temperature-programmed desorption measurements revealed cross sections for hydrocarbon formation in the range of 10–20 to 10–21 cm2. Possible mechanisms underlying the formation of various hydrocarbons and carbon dioxide as the final oxidation product are discussed.