Neutral and charged forms of inubosin B in aqueous solutions at different pH and on the surface of Ag nanoparticles

Abstract

One of the strategies to combat neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, or to heal brain injuries is based on the regeneration of neurons. Inubosin B is acridine alkaloid with neuroregenerative potential. The understanding of its physico-chemical properties and structure in solution can be useful for drug formulations. We have determined pKa,1 5.30 and pKa,2 9.45 constants for inubosin B. The presence of both acidic and basic sites in the molecule of inubosin B does not lead to formation of zwitterion. In neutral form, inubosin B exists as “a free base”, which is lipophilic and easily extractable to chloroform and n-octanol. Moreover, by comparison of experimental and calculated UV–vis and NMR spectra, we have proven the existence of inubosin B in “free base” form and revealed the conformers of inubosin B in water. Inubosin B was adsorbed on silver surface providing the strongest surface enhanced Raman (SERS) spectrum in neutral pH. Based on relative intensity conservation and density functional theory (DFT) calculation, we propose that inubosin B is adsorbed on the silver surface in free base form.