Monolayer study of plastoquinones, α-tocopherol quinone, their hydroquinone forms and their interaction with monogalactosyldiacylglycerol. Charge-transfer complexes in a mixed monolayer

The surface pressure-area isotherms of pure plastoquinone-9 (PQ-9), plastoquinone-3 (PQ-3), α-tocopherol quinone (α-TQ), their reduced (hydroquinone) forms and mixtures of these molecules with monogalactosyldiacylglycerol (MGDG) have been studied by a monolayer technique. The collapse pressures of all hydroquinones (QH₂) were higher than those of the corresponding quinones (Q), the difference being highest between PQ-9 and PQH₂-9. The limiting molecular areas of hydroquinones were higher than those of the corresponding quinones except for α-TQH₂. All Q-QH₂ mixtures showed miscibility throughout the whole range of the components' ratios. There was no deviation from the addivitiy rule observed for any of the Q-QH₂ mixture, as well as for the mixtures of MGDG with PQ-3, PQH₂-9, α-TQ and α-TQH₂. On the other hand, PQ-9/MGDG and PQH₂-3/MGDG mixtures showed positive and negative deviations, respectively. All the isotherms of Q-MGDG and QH₂-MGDG mixtures showed a kink point above the collapse pressure of the Q or QH₂ examined, indicating that with the increase in surface pressure, Q or QH₂ were gradually squeezed out from the monolayer. The percent content of Q and QH₂ in the monolayer as a function of surface pressure was also calculated. The hydroquinones were more difficult to remove from monolayers than the corresponding quinones, and among the investigated quinones, PQ-9 was most easily and α-TQ most difficultly squeezed out. The surface pressure-are isotherms of the three-component mixtures of PQ-9/PQH₂-9/MGDG showed a shift to lower molecular areas in comparison with the corresponding two-component mixtures, especially at higher surface pressures. This indicates that the presence of PQ-9 lowered the PQH₂-9 content in the monolayer, especially at higher pressures, which was explained by charge-transfer complex formation upon interaction of PQ-9 with PQH₂-9. The comparison of surface potential-area isotherms of PQ-9/PQH₂-9/MGDG mixtures with those of the corresponding binary mixtures also suggest charge-transfer complex formation upon interaction of PQ-9 content in the monolayer, especially at quinones and quinols in the thylakoid membrane and significance of charge-transfer interactions in functioning of PQ-9 has been discussed.