The Chelating Tendency of Riboflavin.

Abstract

Feb., 193J NOTES 309 M and (CeH&PO = 1.60 X A I ; therefore, a plot of 1/OD-ODwhere OD is the optical density, vemus time is the proper derived function for a second-order reaction with the reactants a t equal equivalent con~eiitrations.~ The upper abscissa has been added to show the t.emperature in the reaction cell as a function of time. The slope of Fig. 1 a t any given temperature is then corrected for the change in density of the solvent with temperature and for a small change in the molar extinction coefficient of sulfur with temperature. The resultant Arrhenius plot (Fig. 2) of log corrected slope versim 1 / T gives an activation energy of 16.5 f 0.4 kcal./mole, in agreement with 16.0 f 0.2 kcal./mole. TABLE I RATE CONSTANTS IN BENZENE T e m p . , 'C.O k2, 1. mole-' sec.-'b Ref. methode 7.35 7.50 x 10-4 0 Ultraviolet 25.00 4.40 x 10-3 8,9 Ultraviolet titr. 35.00 11.3 x 10-3 8,9 Ultraviolet titr. a =t0.02. Rate = k?(Ss)((C6H&P). Ultraviolet analysis of sulfur, titr. iodometric analysis of the phosphine.The main requirements for the use of this method are (i) the rate of reaction is moderately slow a t the lowest temperature; (ii) the temperature in the cell must be uniform; (iii) the activation energy does not vary with temperature; (iv) the boiling point of the solvent cannot be exceeded; and (v) Beer's law must be obeyed even though the molar extinction coefficient may change with temperature. Requirement (ii) restricts the size of the cell and the rate of heating. If the activation energy varies greatly with temperature one would be unable to obtain the correct order of the reaction and the rate constants. The main disadvantage of this method is that only a small percentage of reaction is used to determine the rate at any one temperature. The advantage is obtaining the rate, the order of the reaction, the frequency factor and the activation energy in a single rapid experiment. ExperimentalThe purification of sulfur, triphenylphosphine and benzene has been reported previously.* The use of the ultraviolet absorption spectrum of sulfur to study this reaction a t a constant temperature and with conventional techniques will be subject to a forthcoming publication.@ The thermostated cell compartment for a Beckman D U spectrophotometer has been discussed.1O The brass jacket was carefully made to ensure good thermal contact with a square Corex cell. To the top of the cell was sealed a 7 mm. Pyrex t8ube (15 cm. in length) through which a multi-junction thermocouple was placed into the cell just above the light path The output of the thermocouple was applied to a Speedomax recorder. The reactants were mixed and placed in the cell (total volume 1 to 2.6 4). Water, circulated a t the rate of one gallon per minute through the compartment, was slowly heated. Optical density measurements were manually recorded every 30 seconds at 345 mp. The heating rate does not enter into the graphic analysis of the data but averaged 0.5 degree per minute to cover the range of 23 to 77". Other heating rates can be used to cover only ...

Transcriptomes of the Extremely Thermoacidophilic Archaeon Metallosphaera sedula Exposed to Metal “Shock” Reveal Generic and Specific Metal Responses

“…Violet, white, and green represent above average, average, and below average transcription levels, respectively. involvement in redox reactions (85,86), with an important connection to scavenging of reactive oxygen species being observed in Ashbya gossypii (87). Zn is known to induce the expression of proteins with protective functions (88) and is essential in redox homeostasis (77).…”

Mechanism of riboflavin-catalyzed oxidations

“…Also, reduced indophenol (112) and p-toluene diamine (113) (155) and Barclay Riboflavin has been reported to form chelates with metal ions. The early work by Albert (40,195,19&) (197) restates the possibility of an interaction with metal ions with the exception of ferrous ion. The value given in the latter report for the stability constant of the ribo flavin -Cu"^ chelate is not in agreement with the maximum value obtained by Hemmerich and Fallab (198) nor are the published titration curves in agreement with theoretical titration curves.…”

“…The is the part of these schemes which is hard to conceive and is, perhaps, the core of oxidative phosphorylation. Grabe et al, (186,197,198,217) The << -phenyl glycine used was recrystallized from material on hand.…”

Mechanism of riboflavin-catalyzed oxidations

“…This work has been quite well reviewed and serious doubts raised as to its validity. A more recent report (197) restates the possibility of an interaction with metal ions with the exception of ferrous ion. The value given in the latter report for the stability constant of the ribo flavin -Cu"^ chelate is not in agreement with the maximum value obtained by Hemmerich and Fallab (198) nor are the published titration curves in agreement with theoretical titration curves.…”

“…Similar mechanisms were proposed and demonstrated by Harrison is the part of these schemes which is hard to conceive and is, perhaps, the core of oxidative phosphorylation. Grabe et al, (186,197,198,217) Such a set of compounds would only differ as to reduction potential, i.e., only the side chains differ. Figure 8 shows the possible structures for these compounds.…”