Analytical Sciences

¹¹B Solid-state NMR Investigation of the Rhamnogalacturonan II-borate Complex in Plant Cell Walls

Tsunenori KAMEDA,*¹ Tadashi ISHII,*² Toshiro MATSUNAGA,*³ and Jun ASHIDA*⁴

*1 National Institute of Agrobiological Sciences, Tsukuba, Ibaraki 305-8634, Japan
*2 Forestry and Forest Products Research Institute, Tsukuba, Ibaraki 305-8687, Japan
*3 National Agricultural Research Center for Kyushu Okinawa Region, Nishigoshi, Kumamoto 861-1192, Japan
*4 Varian Technologies Japan Ltd., Tokyo 108-0023, Japan

The boron in plant cell walls, which is water-insoluble and in the solid state, is solubilized by pectinase digestion to give a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex. To clarify the nondestructive structure of boron present in plant cell walls (as represented by sugar beet fiber), we performed 192- and 96-MHz ¹¹B solid state NMR measurements. The use of a high field magnet frequency of 192-MHz enabled us to observe ¹¹B isotropic chemical shifts at -9.7 and -9.6 ppm for dRG-II-B and sugar beet fiber in the solid state, respectively, demonstrating that the boron in isolated dRG-II-B and in plant cell walls is present as a borate-diol ester (1:2). The observation of the magnetic field dependence of the chemical shift and lineshape for the borate-diol ester (1:2) by quadrupolar interaction suggested that the borate complex had a distorted tetrahedral boron structure.

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