Radiat. Phys. Chem. Vol. 23, No. 5, pp. 495-497, 1984 0146-5724/84 $3.00 + .00 Printed in Great Britain. Pergamon Press Ltd.

USE OF RIBOFLAVIN AS A STANDARD SOLUTE FOR ESTIMATING H-ATOM REACTION RATE CONSTANTS BY

COMPETITION KINETIC METHOD

KAMAL KISHORE, P. N. MOORTHY and K. N. RAO Chemistry Division, Bhabha Atomic Research Centre, Bombay-400 085, India

(Received for publication 12 May 1983)

Abstract--Riboflavin (RF) has been used as a standard solute for the determination of H atom rate constants by the competition kinetic method. In deaerated acidified aqueous solutions (pH ~ 1.7) containing t-butanol its radiolytic decomposition was found to closely correspond to G ( - R F ) = I/2GH. The rate constant for its reaction with H atoms was first evaluated by employing glucose as the standard (kH + glucose = 4.7 x 107 dm 3 mol- i s- i). The kH + RF thus obtained was 2.4 x 109 dm 3 tool- t s- i. Using this value the rate constant lor reaction of a number of other solutes have been determined and compared with literature values. The simplicity of the method vis-a-vis its limitation is discussed.

I N T R O D U C T I O N

RIBOFLAVIN has been used earlier 0) as a standard solute for determination o f O H radical reaction rate constants by competit ion kinetic method in radiolysis experiments. We have now explored the feasibility of its use as a standard solute for determination of H-a tom reaction rate constants by a similar method. H-a tom reaction rate constants are generally deter- mined by (i) competi t ion kinetic methods in steady

1 . ( 5 ) state radiolysis (2~) and pulse radio ysls, (ii) follow- ing the formation of solute transient (6) during pulse radiolysis if the absorption is in the accessible spectral region and (iii) ESR spectroscopy (7's) in conjunction with pulsed or steady state radiolysis.

In steady state radiolysis methods employing com- petition kinetics, the concentration of radiolytic products from the standard solute are measured, e.g. acetaldehyde from ethanol, (2) acetone from isopropanol °) and H2 from formate ~4) and the rate constants are calculated from the slopes and inter- cepts of the competi t ion kinetic plots. These involve post irradiation chemical treatment or separation to assay the products. If we can follow the change in concentration of the standard or the radiolytic prod- ucts by spectrophotometry without recourse to fur- ther chemical treatment, the method becomes much simpler. Riboflavin (RF) is one such solute whose concentration can be directly followed by measuring its absorbance at 445 nm as its reduction product (leucoriboflavin) does not absorb at this wavelength. The results of the study are presented in this paper.

E X P E R I M E N T A L Riboflavin used was from Sigma Chemicals and did not

show any impurities on TLC analysis. All other chemicals

employed were either BDH "AnalaR" or E. Merck "G.R" grade. Triple distilled water was used to prepare all the solutions. Argon used for flushing the solutions was the purest commercially available and the solutions contained less than 10-6moldm -3 oxygen as checked by polar- ography. A 6°Co ~,-source with a dose rate of 6 x 10~SeVdm-Ss -1 was used for irradiating the samples (2 rain.) so as to get measurable changes in absorbance. However, even with this dose the decomposition of the solutes was less than 30%. A Beckman DU spec- trophotometer was employed to measure absorbance of the solutions.

R E S U L T S A N D D I S C U S S I O N S

Irradiation of aqueous solutions leads to the for- mation of the primary reactive species OH, H and ea~. For studying competit ion for H-atoms alone, the irradiations were carried out under the following conditions: (i) the pH of the solutions was adjusted to ~ 1.7 using perchloric-acid to convert e~ quan- titatively to H atoms. The final reduction product formed by reaction of H atoms with riboflavin was found to be stable at this pH, i.e. it was not reoxidised by air to riboflavin as there was no post irradiation increase in the absorbance of the solutions at the 2m of riboflavin on keeping in contact with air. Hence after irradiation the samples can be brought into contact with air for absorbance measurements; (ii) OH radicals were scavenged using t-butanol. The t-butanol radicals are known to be inert (9) and hence are assumed not to react with riboflavin. About 5 x 10 -2 tool dm -3 t -butanol was sufficient to scav- enge all the O H radicals preferentially if the other solutes used are in the range of 1 0 - 4 - 1 0 - 3 m o l d m -3. Its H atom reaction rate constant (s) is 8 x l & d m 3 m o l - l s - ~ ; hence at the concentration of t-butanol used it cannot compete

RPC Vol. 23, No. 5~A 495

496 K. KISHORE et al.

for H-atom in presence of riboflavin ( ~ 10-4 mol dm 3) and other solutes. The solutions were flushed with argon to remove oxygen which could otherwise compete for H-atoms and also react with riboflavin radicals. The final reduction product present after irradiation was found to be leucoriboflavin. This compound can be generated by the chemical reduction of riboflavin in acidic solution using H2 over platinised asbestos; its absorption spectrum has a 2m at 270nm (Em=l.4× 104dm3mol ' cm -~) but no absorption at 445nm, the 2,. of riboflavin'. The absorption spectrum of a riboflavin solution irradiated under the above condi- tions was found to closely agree with that of chem- ically produced leucoriboflavin after point-by-point correction of the former for residual riboflavin in the solution and from the corrected absorbance at 270 nm and the E270 value given above, G(RFH2) was calculated and found to correspond to 1/2 GH. This supports the assumption made above that the t- butanol radicals formed by OH scavenging do not react with the riboflavin radicals formed by H-atom reaction and the following reaction scheme can be written

(1) H20 ~ H, OH, eaq, H202, H2

(2) ea~ + H ~ ~ H

(3) (CH3)3COH + OH~(~H2 - C(CH3)2OH q- H20

(4) R F + H k , , R F H

(5) S + H k5 , p

(6) 2 RFH ----} RF + RFH2.

As expected on the basis of this scheme, measured G ( R F H 2 ) = G ( - RF) = 1/2GH, and a plot of 1/2 G ( - RF) vs [S]/[RF] is expected to be linear with intercept equal to UGH and slope equal to intercept × ks/k4: knowing k4 we can calculate k,. Such competition plots are shown in Fig. 1. These were found to have an intercept = 1/GH, thus sup- porting the above mechanism. The rate constant for the reaction of H-atoms with riboflavin is not known and hence was evaluated first by using glucose as a standard solute whose rate constant with H-atoms is well known. Is/ From the competition kinetic plot shown in Fig. 1, k R v + . was estimated to be 2.4x 10~dm3mol ~s ~. Using this value the rate constants for H-atom reactions with other solutes were estimated by the same method. The competition kinetic plots for various systems are shown in Fig. 1. The values obtained for the rate constants are given in Table 1 and compared with the reported values. The value for k . . . . dione+H is not known but the value obtained by us is close to that reported for similar compound benzoquinone) ~°~

However, this method was not found suitable in

1.5

I 1 .0

0.5

/

7

0.0 I I 1 I. 1 50 100 [S._J.] 150 200 P50 --,,- a

00 0.s 1.0 [Rv] 1.5 2.0 Z S ~ b

FIG. 1. Competition kinetic plots for the deterrhination of H-atom reaction rate constants by riboflavin method. Scale-a is for Glucose, TI +, Ascorbic Acid, Glycerol and BrOf. Scale b is for Thionine and

Menadione

H-atom reaction rate constants by competition kinetic method

TABLE 1. H-ATOM REACTION RATE CONSTANTS DETERMINED BY

THE RIBOFLAVIN METHOD

kH+RF = 2.4 X 109dm3 mol -I s - '

Solute Rate const. Reported Ref. determined by rate const. riboflavin dm 3 mol- m s- t method. dm 3 mol - m s- '

Glycerol 3.2 x 107 3.6 x 107 (8) Ascorbic

acid 1.8 x l0 s 1.1 x l0 s (8) Thionine 1.0 x l0 I° 8.5 x l09 (1 l) Tl ÷ 1.3 x l0 s 1.2 x l0 s (10) Menadione 5.2 x 109 3.1 x 109t (10) BrO3- 1.6 x 107 2.0 x 107 (10)

tFor benzoquinone.

the case of compounds such as thymine, benzo- quinone and 103-. In such cases either there was no change in G ( - RF) with changing [S]/[RF] (as in the case of thymine) or G ( - R F ) ~ 0 in presence of other solutes as in the case of benzoquinone and IO~-. It was further observed that compounds whose one electron redox potentials are either much more negative or much more positive as compared to riboflavin ( E ° ~ = - 0 . 2 1 V v s N H E ) v2) posed such problems whereas compounds whose redox poten- tials are close to that of riboflavin gave good com- petition kinetic plots. Thus when the compound concerned has a very negative reduction potential, as in the case of thymine (E ° ' = -- 1.32 V vs NHE) , v2) the radical produced by H atom reaction with the compound can reduce riboflavin just as efficiently as the H atom and hence little or no change in G ( - RF) with changing [S]/[RF] is expected as was observed experimentally. On the other hand when the com- pound under investigation has a very positive reduc-

497

tion potential relative to riboflavin, the semiriboflavin radical formed by H atom reaction can be reoxidized to riboflavin by the test compound resulting in no observable loss of riboflavin; this was the case with benzoquinone (E ° ' = + 0.293 V vs N H E ) v2) and IO3- (E °1 = + 0.20 V vs N H E , value inferred from the Etl 2 value given in Ref. 13). The other compounds investigated have redox potentials within + 0.2 V of the value for riboflavin and reactions of the type discussed above are relatively unimportant in these cases. It is further interesting to note that although menadione and benzoquinone have similar structures the former ( E ° l = NOV vs N H E ) °2~ gives a good competit ion kinetic plot whereas the latter does not, for reasons discussed above. Their H atom reaction rate constants, however, are close to each other.

R E F E R E N C E S I. K. KISnORE, P. N. MOORTHV and K. N. RAO, Radiat.

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