Specific Inclusion Mode of Guest Compounds in the Amylose Complex Analyzed by Solid State NMR Spectroscopy

The inclusion compound formation between linear amylose of molecular weight 102500 (AS100) and p-aminobenzoic acid (PA) during the sealed-heating process was investigated by powder X-ray diffractometry, infrared spectroscopy and solid state NMR spectroscopy. Sealed-heating of AS100 and PA at 100 °C for 6 h provided an inclusion compound with 61-helix structure, while a 71-helix structure was found when sealed-heating was carried out at 150 °C for 1 h. The formation of an inclusion compound was not observed when sealed-heating was performed at 50 °C for 6 h. The 71-helix inclusion compound maintained its structure even during storage at high temperature while the 61-helix inclusion compound decomposed and returned to the original Va-amylose upon heating to 180 °C. Quantitative determination revealed that one PA molecule could be included per one helical turn of AS100 for both 61-helix and 71-helix inclusion compounds. Solid state NMR spectroscopy suggested that PA molecules were included in the amylose helix core in the 71-helix inclusion compound, while in the case of 61-helix inclusion compound, PA molecules were accommodated in the interstices between amylose helices. Moreover, the inclusion compound formation by sealed-heating of AS100 was also observed when using PA analogues as guest compounds. The binding ratio of AS100 and PA analogues varied depending on the size of guest molecules.