Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione] is derived from the rhizomes of Curcuma longa. Although early studies concluded that curcumin exists predominantly as a keto-enol tautomer, 1b, in several recent articles the solution structure of curcumin has been represented as a beta-diketone tautomer, 1a. We have investigated the structure of curcumin in solvents ranging in polarity from CDCl3 to mixtures of DMSO-d6 in water, and in buffered aqueous DMSO-d6 solutions with pH values varying from 3 to 9. The solution structure of curcumin was determined on the basis of NMR techniques, including DEPT, HMQC, HMBC, and COSY. The results of the NMR studies show definitely that curcumin exists in solution as keto-enol tautomers, 1b.
In the presence of CI-, the severity of ammonia-induced inhibition of photosynthetic oxygen evolution is attenuated in spinach thylakoid membranes (Sandusky, P.O. and Yocum, C.F. (1983) FEBS Lett. 162, 339-343). A further examination of this phenomenon using steady-state kinetic analysis suggests that there are two sites of ammonia attack, only one of which is protected by the presence of CI-. In the case of Tris-induced inhibition of oxygen evolution only the CI-protected site is evident. In both cases the mechanism of CI-protection involves the binding of CI-in competition with the inhibitory amine. Anions (Br-and NO3) known to reactive oxygen evolution in Ci-depleted membranes also protect against Tris-induced inhibition, and reactivation of Cl-depleted membranes by CI-is competitively inhibited by ammonia. Inactivation of the oxygen-evolving complex by NH 2OH is impeded by C!-, whereas CI-does not affect the inhibition induced by so-called ADRY reagents. We propose that CI-functions in the oxygen-evolving complex as a ligand bridging manganese atoms to mediate electron transfer. This model accounts both for the well known Ci-requirement of oxygen evolution, and for the inhibitory effects of amines on this reaction.
Biophys. Acta 766, 603-611) has documented a competition between chloride and ammonia or Tris for a binding site within the oxygen-evolving complex of Photosystem II. This competition is in fact a general property of inhibitory amines which is related to their nucleophilicity; this in turn suggests that the binding site is associated with a metal. Only ammonia, of all amines tested, is able to occupy a second binding site which is unrelated to the site of chloride binding; this sterically hindered site may be identical to the site already described for binding of hydroxylamine, hydrazine, and certain of their derivatives (Radmer, R. and Oilinger, O. (1983) FEBS Lett. 152, 39-43). When the interaction between amines, chloride and the inhibitory halide fluoride was examined, steady-state kinetic plotting procedures revealed that amines and fluoride compete for the chloride binding site; binding of one inhibitor precludes the binding of the other. It was also observed that the intensity of inhibitor binding to the oxygen-evolving complex was influenced by the electron acceptor present during assays; stronger inhibition was observed with a PS ll-specific electron acceptor (2,5-dichloro-p-benzoquinone) than with an acceptor (ferricyanide) which requires electron transport to the reducing terminus of Photosystem L These results are interpreted in terms of a model which proposes that the binding site for chloride on the oxidizing side of Photosystem II resides within the pool of functional manganese associated with the oxygen-evolving complex of Photosystem II.
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