Charge-transfer complexes of plastoquinone and α-tocopherol quinone in unsaturated fatty acids and temperature dependences of their absorption spectra

Abstract

It has been found that plastoquinone (PQ) and α-tocopherol quinone (α-TQ) form quinhydrone-type charge-transfer (CT) complexes on PQH 2 and α-TQH 2, respectively, in oleic, linoleic and linolenic acid throughout the whole range of temperatures examined, i.e., from −30°C to 60°C. The CT spectra of the PQ-PQH 2 complex at room temperature showed one peak at 371.8–383 nm and a shoulder at 454–494 nm, depending on the fatty acid used. The corresponding CT spectra for the α-TQ—α-TQH 2 complex were similar in shape with one peak at 371.4–378.1 nm and a shoulder at 439–469 nm. The temperature dependences of CT spectra were not large above the melting point of fatty acids. On the other hand, the changes in CT spectra in the frozen state were striking. A long-wavelength shift of both CT bands (up to 90 nm) and an increase in intensities of the bands, especially of the long-wavelength band, was observed, and as a result the intensity ratio of both bands was reversed with the decrease in temperature. The observed effects were explained in terms of different orientational isomers of the complex, the relative quantities of which varied with temperature changes.