eastern Thailand. With the exception of the arene oxide 6b, all of the compounds postulated in Scheme IV2' have been isolated from this plant. These are the benzyl benzoates (19 and 37),38 the o-(hydroxybenzy1)flavanones (14 and 46),% the missing link dienes (22 and 23),31 and the cyclohexene oxides (2, 24, and 25)31 as well as their various metabolites (30-33).35836 These findings have put the Cole and Bates biogenetic pathway (Scheme IV) on firm ground.Last of all it might be mentioned that the latest addition (unpublished) to the family of naturally occurring cyclohexene oxides is a new cyclohexene diepoxide, boesenoxide 86, from Boesenbergia sp. (Zingibereceae) from Thailand.5454) Personal communication with Dr. P. Tuntiwachwuttikul of the Thailand.~e a e ) ,~~ neither this compound nor any of its relatives (bearing the cis diepoxide structure) has been found in the Uvaria plant. This has raised the suspicion that despite their rich pool of benzyl benzoates (e.g., 36-43) and dienes (e.g. 18), the Uvaria plants probably lack the biological means to synthesize the cis diepoxides. The possibility that these mono-and diepoxides should arise from different biogenetic pathways is considered unlikely since it has been established that crotepoxide ( l ) , cr-senepoxide (2), pipoxide 16, zeylenol (281, and ferrudiol (30) all have identical 2S,3R absolute configurations, a fact highly suggestive of a unified biosynthesis via a common key intermediate, the diene 18.53 Results from the study of Piper hookeri, from which crotepoxide (l), pipoxide 16, and pipoxide chlorohydrin 85 have been isolated,26 lend further support to these deductions.Deserving special mention among the Uvaria species is Uvaria ferruginea which was collected from north-(52) 0. Pancharoen, V. A. Patrik, V. Reutrakul, P. Tuntiwachwuttikul, (53) G . R. Shulte, B. Canem, Tptrahpdron L~t t . 1982.23, 4299.Many macromolecules are able to bind a variety of ligand molecules to one or more specific sites. The importance of this phenomenon lies in the fact that the binding of one ligand often influences the binding strength of the macromolecule toward a subsequent ligand (or ligands). When this happens, one speaks of cooperative binding. This effect is the basis of enzyme control and many other vital biological processes. If we were to elaborate on this theme, we could only repeat what has been ably presented elsewhere. '-j In view of the importance of cooperative binding, it is not surprising that much effort should have been devoted not only to the elucidation of the mechanism by which the phenomenon might arise in a specific case' but also to the development of general methods by which cooperative binding can be recognized and subjected to mathematical or graphical representation. What is surprising is the seeming lack of coordination between papers dealing with different aspects of the topic, so that there is even some confusion about the very definition of the term "cooperativity". Wrong, or, at least, misleading statements seem never to have been chal...
The reaction between tropaeolin 0 and hydroxyl ions has been reinvestigated in the pH range between 10.8 and 12.2 at 25°C and an ionic strength i o f 0.1 M , using the T-jump method. The rate constants obtained from results at pH 2 11.66 are in satisfactory agreement with previous data. At lower pH. however, the rate is found to increase with decreasing hydroxyl-ion concentration. A mechanism which is compatible with these results is proposed and discussed. The dissociation constant of the last proton at Z = 0.1 M has been redetermined and found to be in satisfactory agreement with values given in the literature. In trod uctionThe reaction between a n acid and the hydroxyl ion usually proceeds at a diffusion-controlled rate [ l ] . Among the exceptions to this rule are some weak organic acids in which the proton to be transferred forms a n intramolecular hydrogen bridge [2,3]. Such a proton cannot form a bridge to a surrounding water molecule, and the extremely fast recombination mechanism with OH-, operative in the reaction of "normal" acids [ l ] , becomes blocked. T h e observed comparatively low rate may then be equal either to the encounter rate, multiplied by the-small-equilibrium constant between the "open7' and the hydrogen bonded forms, or to the rate a t which the hydrogen bridge is broken [2-41. Alternatively, it may be the rate of formation of a n activated complex which already contains the hydroxyl ion but in which the bridge, though seriously weakened, is not yet broken [5,6]. Some support for the first view has recently been derived from the influence of "structure-making" substances on the rate of deprotonation of Alizarin Yellow R [7]. I n this paper, \ve present the results of a reinvestigation, in a slightly wider p H range than employed by previous investigators [3,, of the rate of deprotonation of tropaeolin 0 (2,4-dihydroxy-4'-sulfonateazobenzene) which has the formula T h e question seems still under discussion [8].and which we shall call HL"; its deprotonated form will be L3-453 0 1975
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