Combined NMR and quantum chemical studies on the interaction between trehalose and dienes relevant to the antioxidant function of trehalose

In a previous study (Oku, K.; Watanabe, H.; Kubota, M.; Fukuda, S.; Kurimoto, M.; Tujisaka, Y.; Komori, M.; Inoue, Y.; Sakurai, M. J. Am. Chem. Soc. 2003, 125, 12739), we investigated the mechanism of the antioxidant function of trehalose against unsaturated fatty acids (UFAs) and revealed that the key factor relevant to the function is the formation of OH...pi and CH...O hydrogen bonds between trehalose and the cis double bonds of the UFA. Here, we investigate whether such intriguing interactions also occur between this sugar and cis double bonds in other unsaturated compounds. For this purpose, we selected various diene compounds (1,3-butadiene, 1,3-pentadiene, 1,4-pentadiene, and 2,5-heptadiene) as interaction partners. All NMR experiments performed, including 1H-1H NOESY measurements, indicated that trehalose selectively interacts with the cis-olefin proton pair in the above diene with a 1:1 stoichiometry, and the C-3 (C-3') and C-6' (C-6) sites of the sugar are responsible for the interaction. Similar interactions were not observed for the mixtures of the diene and other saccharides (neotrehalose, kojibiose, nigerose, maltose, isomaltose, sucrose, maltitol, and sorbitol). Quantum chemical calculations revealed that the OH-3 and OH-6 groups bind to the olefin double bonds of the diene through OH...pi and CH...O types of hydrogen bonds, respectively, and the stabilization energy of the resulting complex is 5-6 kcal mol(-1). These results strongly support the above NMR results. Finally, the activation energies were calculated for the hydrogen abstraction reactions from the activated methylene group of heptadiene. In particular, when the reaction was initiated by a methyl radical, the activation energy was only 10 kcal mol(-1) for the free heptadiene, but on complexation with trehalose it drastically increased to ca. 40 kcal mol(-1). This indicates that trehalose has a significant depression effect on the oxidation of the diene compounds. These results strongly support the antioxidant mechanism deduced in the previous study and indicate that the formation of unique multiple hydrogen bonds between trehalose and cis-olefin bonds is rather a general event not confined to the case of UFA.