PSII Inhibitory Activity of 2,4-Diamino-6-chloro-s-triazines with a Chiralsec-Butyl and/or α-Methylbenzyl Group

The influence of steric factors on the activity of chiral isomers on the N-α-methylbenzyl and/or N4-sec-butyl group of the 2,4-diamino-6-chloro-s-triazines as the inhibitor of the Hill reaction was examined. The (S)-isomers for either chiral center were more active than the corresponding (R)-isomers. The N2-(5)-α-methylbenzyl-s-triazine compound, in spite of a change of the substituent of the N4-amino group, exhibited considerably high potency; in particular the N4-ethyl derivative (19) was the most potent inhibitor toward PSII. The level of optical discrimination of the receptor site for the chiral isomer of the α-methylbenzyl-s-triazines was greater than those for the sec-butyl derivatives, and their rates varied with the change of steric hindrance of the substituent at the other amino group. The structure–activity relationship for the s-triazine compounds tested involved the binding direction of the s-triazine inhibitor being determined by the difference in size of the substituent at each amino group. The amino group with a small substituent offerring low steric hindrance is placed in a narrow binding region (N-region) of the receptor site in PSII protein, and the other amino group is placed in a wide region (W-region), the optical discrimination to each region depending on the steric suitability of the other one.