For thirty years the study of Quantitative Structure Activity Relationships (QSAR) has been an active area of research aimed at developing an understanding of the interactions be­ tween inhibitors of photosynthetic electron transport and the herbicide binding site in the Photosystem II (PS II) reaction centre. Many QSAR studies of PS II inhibitors with diverse chemical structures have emphasized the hydrophobic nature of the binding domain, with lipophilicity being the dominant determinant of

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... ll inhibition activity. The cyanoacrylate classes of PS II inhibitors also show a diversity of active structures and considerable variation in inhibition potency with minor alterations to structure. QSAR analysis and examination of chirality in cyanoacrylate inhibitors has also shown the importance of steric factors in determining activity. Different modes of binding for different classes of cyanoacrylates have been identified; a classical urea-type relationship between activity and hydrophobicity and another type of interaction in which the lipophilicity or electronic nature of phenyl substituents plays little part and the size of the substituents is of primary importance. Because size and shape are parameters of great importance in determining the topography of a binding site, QSAR studies of flexible PS II inhibitors such as cyanoacrylates will continue to be important in elucidating the intricacies of inhibitor/PS II interactions.