Rapid quench kinetic experiments on fructose 1,6-bisphosphatase demonstrate a stereospecificity for the alpha anomer of fructose 1,6-bisphosphate relative to the beta configuration. The beta anomer is only utilized after mutarotation to the alpha form in a process that is not enzyme catalyzed. Studies employing analogues of the acyclic keto configuration indicate that the keto form is utilized at a rate less than 5% that of the alpha anomer, a finding also confirmed by computer simulation of the rapid quench data. Chemical trapping experiments of the keto analogue, xylulose 1,5-bisphosphate, and the normal substrate suggest that interconversion of the acyclic and anomeric configurations is retarded by their binding to the enzyme. A hypothesis is advanced attributing substrate inhibition of fructose 1,6-bisphosphatase to possible binding of the keto species.
T h e kinetics of the process whereby the retention increases on heating previously neutron-irradiated potassium chromate have been investigated: T h e results cannot be explained by any unimolecular or bimolecular ~nechanism. nor by a combination of'a few sudh steps. The data are not compatible with the commoner diffusion-controlled mechanisms. Experiments on solid solutions of potassium chromate in potassium fluoberyllate suggest that the process involves the recombination of the fragments from individual r u p t~~r e events. A tentative hypothesis, analogous t o the theory of the formation of very thin oxide films on metals, is advanced and it accounts for all the existing data.
INTRODUCTIONThe salts of the oxy-anions, such as potassium chromate, are among the simplest compounds in which it is possible to study the chemical effects of radiative thermal neutron capture. Potassium permanganate was one of the first compounds in which these reactions, lcnown collectively as the SzilardChalmers' Effect, were investigated (1). The nature of the fragments formed by the recoil following the dissipation of the excitation energy of the compound nucleus formed by thermal neutron capture, in one or more photons, has been studied (13). Libby proposed that the metal-oxygen bonds in those oxyanions containing a normally electropositive central atom are brolcen heterolytically. Thus the recoil fragments formed in the permanganate include Mn03+, M1102+3, MnO3+', and possibly together with 0-= ions. The most convincing argument in favor of these fragments was based on the observation that upon solution of the irradiated permanganate in very allcaline solutions neai-ly all the activity \vas found as permanganate.Since it is illconceivable that a species could be produced which could be oxidized to the heptavalent state by water, the observation suggests that the fragments all contain heptavalent manganese. An alternative explanation is, however, possible. The fragments may be able to undergo rapid exchange with permanganate ions in very allcaline solution. For example, this might be the case for a fragment containing pentavalent manganese, and giving i\/Ii104-~ in alkaline solution.On Libby's hypothesis, the chromates should give CrO3, C I -O~+~, CrO+4, and possibly Cr+6 ions. When the irradiated chromate crystals are dissolved in water prior t o analysis, these fragments react with the water through hydration or reduction to give radioactive chromate and chromic ions respectively.T h e first two of the fragments suggested by the hypothesis for the chromates are ltnown to hydrolyze t o chromate ions without reduction. But CrO+4
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.