Interactions between Methylene Blue and Pullulan According to Molecular Orbital Calculations

Abstract

In our previous study, we developed a colorimetric indicator based on methylene blue (MB)-dyed pullulan thin films for detection of hydroxyl radicals as a novel indicator to detect active oxygen species with improved oxidative ability; however, information concerning intermolecular interactions between MB and pullulan remain insufficient. In this study, we identified MB-pullulan interactions using molecular orbital (MO) calculations for used a truncated pullulan model represented by five glucose units to investigate the large and complex pullulan structure. The results identified MB-pullulan interactions at the HF/6-31G(d) level, revealing 10 MB-pullulan complex structures with full geometry optimization. Moreover, structural and energy analyses predicted the existence of the hydrogen-bonding interactions between pullulan hydroxyl groups and MB nitrogen, carbon, or sulfur atoms. The results of MO analysis suggested the formation of hydrogen bonds between S, C, and N atoms in the MB benzene ring with a pullulan OH group groups, resulting in a lower electronic energy in the complex structure relative to that observed in the individual molecules.