Fourier transform infrared studies on charge-transfer interactions of plastoquinones and α-tocopherol quinone with their hydroquinone forms and monog

Abstract

Fourier transform infrared (FTIR) spectra of plastoquinone-9 (PQ-9), plastoquinone-3 (PQ-3), α-tocopherol quinone (α-TQ), their reduced forms, monogalactosyldiacylglycerol (MGDG), quinone-hydroquinone Q-QH 2) and Q-QH 2-MGDG mixtures in thin films have been measured. The spectra were recorded directly on samples evaporated from hexane and after precooling at liquid nitrogen temperature. It was found that pure QH 2 molecules are hydrogen bonded and occur mainly in polymeric forms, but in mixtures with corresponding quinones, they show only the presence of dimers. The quinones in Q−QH 2 mixtures in a liquid state do not form hydrogen bonds, but after solidification most of their carbonyl groups are hydrogen bound to hydroquinones, which facilitates the formation of charge-transfer (CT) complexes. Hydrogen bonds in CT complexes between PQ-9 and PQH 2-9 were found to occur also in MGDG matrix. There were also some indications found on Q−QH 2 interactions in a liquid state. The peak responsible for C−O stretching vibrations of hydroxyl groups in liquid Q−QH 2 mixtures is strongly reduced, and after solidification there appeared two new peaks due to stretching vibrations as a result of hydrogen bonding. Only for PQ-9 and PQH 2-9 in solid state did crystallization bands appear below 900 cm −1, which originate from skeletal vibrations of the isoprenoid side chain of both compounds. The possibility of the existence of CT complexes of different stoichiometry in apolar solutions is also considered.