Interactions of poly (vinyl alcohol) and poly (N-isopropylacrylamide) with tannic acid and pyrazole derivatives: The identification of H-bonds for revealing a “Two-Molecules” carrier system

Abstract

The interactions between tannic acid (TA), a natural bioactive polyphenol, and two biodegradable polymers have been investigated in the presence and absence of a pyrazole derivative through their NMR and UV–Vis spectra as well as quantum chemical calculations. The role of H-bonding ability of organic molecules is studied for clarifying the molecule carrier capability of the polymers. TA uses its hydrogen bonding capability controversially depending on the functional groups of the polymers in solutions/mixtures in DMSO. In the mixtures with poly (vinyl alcohol) (PVA), the color of TA is darkened, which associates to the appearance of a new UV–Vis band, the disappearance of two one-dimensional proton NMR signals, and some differences in the heteronuclear multiple bond correlation (HMBC) NMR spectra. The H-bond interactions in the mixtures of TA and PVA are mainly between oxygen of the polymer backbone and proton of OH of TA in addition to hydrogen bonding interaction between O atom of aromatic rings of TA and OH moiety of PVA as revealed by UV–Vis spectroscopy and computational results. Association of the pyrazole derivative in the mixture introduces a weak interaction between protons of OH belonging to outer core of TA and oxygen of CO of the pyrazole derivative. In the mixtures of TA with poly (N-isopropylacrylamide) (PNIPAM), a noticeable change is not observed in the UV–Vis and one-dimensional proton NMR spectra irrespective of the addition of the pyrazole derivative. However, the weak interactions between TA and the pyrazole derivative in mixtures with PNIPAM are apparent in their HMBC NMR spectra. PNIPAM and TA interact mainly through the proton of OH in the outer core of TA and the oxygen of CO of PNIPAM backbone as in the case of TA and the pyrazole derivative. They also interact through the CO in the central core of TA and proton of OH of PNIPAM backbone. To assess the effect of the different H-bond environments on the UV–Vis and NMR spectra, we have performed quantum chemical calculations employing B3LYP density functional approach. The present calculations appear quite useful in explaining the source of spectral differences in different media. Based on the spectral analysis, we suggest that PVA is a better molecule carrier with respect to PNIPAM.