Multivariate analysis of Photosystem II fluorescence quenching by substituted benzoquinones and naphthoquinones

The present investigation examines the action of substituted benzoquinones and naphthoquinones as inhibitors of chlorophyll fluorescence in barley chloroplasts. Stern-Volmer analyses have been used to determine the fractions of chlorophyll fluorescence accessible to quinone as a measure of quinone-membrane solubility and the Stern-Volmer quenching constant as a measure of quinone intrinsic activity. Correlations between quinone structural properties and experimentally measured chlorophyll fluorescence parameters have been made to determine the contribution of lipophilic and electronic factors to quinone solubility and intrinsic activity. In particular, 1,4-benzoquinones must have hydrophobic substituents or hydrogen atoms at positions 2, 3, 5 and 6 for fluorescence quenching activity. The magnitude of the Stern-Volmer quenching constant is affected by the electronic character of hydrophobic substituents, with a maximum value observed in the presence of both electron-releasing and electron-withdrawing moieties. Similarly, the lipophilic character of 2,3-substituents of 1,4-naphthoquinones determines the fraction of chlorophyll fluorescence accessible to quinone, with a maximum accessibility observed for hydrophobic substituents. Naphthoquinone intrinsic activity is also correlated with the electronic character of the substituents and is observed to be at a maximum value with only electron-withdrawing substituents.