Low Anion Concentrations Research Articles

Mechanism of octahedral substitution reactions in non-aqueous solutions. Part V. The chloride anation of cis- and trans-chloro-methanolobis(ethylenediamine)cobalt(III) cations in methanol

The trans-[Co en2CH3O Cl]+ cation has been characterised as its perchlorate and tetraphenylborate. Addition of acid to its methanol solution immediately generates the labile trans-[Co en2CH3OH Cl]2+ cation which, in the absence of co-ordinating anions, isomerises to a mixture that contains mainly the cis-isomer. Addition of chloride to the solution of the trans complex leads to the formation of some trans-[Co en2Cl2]+ but does not reduce the rate of concurrent isomerisation. The rate of disappearance of trans-[Co en2CH3OH Cl]2+ is independent of chloride-ion concentration and so the fraction undergoing anation also does not change with chloride concentration. The rate of anation of the equilibrium mixture is much slower and depends upon the chloride concentration at low anion concentration but reaches a limiting maximum when the chloride concentration is high enough. In addition, the steric course of this anation depends on chloride concentration, the product consisting mainly of the trans-isomer at low halide concentration but containing >50% of cis-isomer at high chloride ion concentrations. It is suggested that this change in steric course arises from the reversibility of the isomerisation of the chloro-methanolocomplexes so that, at low chloride-ion concentrations a large portion of the equilibrium mixture anates by way of the more labile trans complex and thereby yields the trans-dichloro-product. The general behaviour is accounted for in terms of a pre-equilibrium association between the reagents followed by a dissociative “interchange” mechanism for the actual act of substitution.Neither the reactivity of the chloro-methanolo-complexes nor the steric courses of their anation allow one to postulate their existence as necessary intermediates in the chloride exchange reactions of cis-[Co en2Cl2]+ in methanol. Consequently it is possible to rule out solvolysis, or solvent-assisted dissociation, as the rate-determining step of this reaction.

Metabolism and morphogenesis in a new Blastocladiella.

1. One phase of morphogenesis (production of resistant sporangia) in the life cycle ofBlastocladiella has been investigated experimentally in some detail, and the underlying biochemical transformations have, in part, been elucidated. Conversely, morphogenesis has been made to serve as a tool in physiological studies. Elevated concentrations of CO2 sources induce R.S. spores to develop directly into resistant-sporangial plants instead of gametophytes. The phenomenon, furthermore, is dependent upon additional factors found in a non-standardized peptone, but not in vitamin-free casein hydrolysate. Such factors can be effectively replaced by certain Krebs-cycle intermediates (e.g.a-ketoglutarate and citrate) and oxidative-decarboxylation inhibitors or carbonyl reagents (e.g. arsenite and semicarbazide). Biotin completely eliminates the effect ofa-ketoglutarate in inducing the formation of R.S. plants; this offers strong indirect evidence for the role of biotin in the oxidative decarboxylation ofa-ketoglutarate. Blastocladiella is heterotrophic for thiamine. The remarkable fact that it can be replaced by a combination of bicarbonate, acetate, and pantothenate is suggestive of the substitution of a Coenzyme A-dependent acetylation reaction for the decarboxylation of pyruvate during growth. It has been concluded that retardation of the oxidative decarboxylation ofa-ketoglutarate, and simultaneous accumulation of its precursors back to the citrate stage, is directly or indirectly related to the formation of R.S. The strong lactate production, the thiamine heterotrophy, the supposedly weak C2-forming mechanism associated with it, the probably-functional Wood-Werkman reaction, and the by-passing of the B1-deficiency with a source of CO2, acetate, and pantothenate are further discussed with relation to the biosynthetic mechanism underlying the morphogenetic pattern. 2. The germination of resistant sporangia, a very precise mechanism, is separable into two distinct phases; one, a period (e.g. 60 min. atca. 20° C.) terminating with cracking of the thick pitted wall, and the other, a subsequent period which leads to the final discharge of spores. Relatively low concentrations of anions, as well as different temperatures, exert a pronounced differential influence on the two processes.