Thionine-Leucothionine Synproportionation
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 981
It would appear that the caged complex and the intermediate radical-radical recombination represent two extreme cases on the time scale, caged complex formation involves only a short time interval between initial replacement and combination, while the radical-radical recombination requires a considerably longer time period.
M. E. Berg, W. M. Grant, R. W. Hekon, and E. P. Rack, J. fhys. Chem., 79, 1327 (1975). K-C. To, M. E. Berg, W. M. Grauer, and E. P. Rack, J . Phys. Chem., 80, 1411 (1976). M. E. Berg, A. Loventhal, D. J. Adelman, W. M. Grauer, and E. P. Rack, J. Phys. Chem., 81, 837 (1977). K-C. To, M. E. Berg, and E. P. Rack, J. Phys. Chem., 81, 1239 (1977). M. Henchman, D. Urch, and R. Wolfgang, Can. J. Chem., 38, 1722 (1960). D. Urch and R. Wolfgang, J . Am. Chem. SOC.,83, 2982 (1961). M. E. Berg, K-C. To, and E. P. Rack, Radiochim. Acta, in press. R. J. Abraham, "The Analysis of High Resolution NMR Spectra", Elsevier, Amsterdam, 1971. R. J. Abraham, M. A. Cooper, T. M. Sivern, P. F. Swinton, and H. G. Wedes, Org. Magn. Reson., 6, 331 (1974). Computer program for the calculation of dipole moments of molecules developed by E. L. Eliel et al., University of North Carolina. The authors also thank Professor E. L. Eliel, University of North Carolina, and his staff for their advice and assistance in carrying out the dipole measurements. E. G. Claeys, G. P. Van der Kelen, and 2. Eckhaut, Bull. SOC. Chim. Belg., 70, 462 (1961). The 3J" and '&(gauche) coupling constants of meso-l,2-dichloro-1,ddif/uoroethane can be used to calculate the conformational population of the racemic system because the only difference between these conformers is the relative position of the CI and F atoms whose physical characteristics are very similar, and the 3JHH coupling constants are not affected by the change of atoms. This is, however, not true for the 'J"(bans and 'J"(gauche) coupling constants. C. P. Smyth, "Dielectric Behavior and Structure", McGraw-Hill, New York, N.Y., 1955. R. C. Weast, Ed., "Handbook of Chemistry and Physics", 51st ed, The Chemical Rubber Co., Cleveland, Ohio, 1971. See, e.g., 0. Sinanoglu in "Molecular Associations in Biology", B. Pullman, Ed., Academic Press, New York, N.Y., 1968, p 427 f f . L. Onsager, J. Am. Chem. SOC.,58, 1486 (1936). I. Watanabe, S. Mizushima, and Y. Morino, J. Chem. SOC. Jpn., Pure Chem. Sect., 63, 1131 (1942). I . Watanabe, S. Mizushima, and Y. Mashiko, J . Chem. SOC.Jpn., 64, 962 (1943).
Acknowledgment. The authors thank Dr. H. M. Bell, Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Va., for valuable advise and assistance in the interpretation of the NMR results. References and Notes Work supported by the U S . Energy Research and Development Administration. L. Vasaros, H. J. Machulla, and G. Stocklin, J . fhys. Chem., 76, 501 (1972). C. M. Wai, C. T. Ting, and F. S. Rowland, J . Am. Chem. Soc., 96, 2525 (1974). R. S. Rowland, C. M. Wai, C. T. Ting, and G. Miller in "Chemical Effects of Nuclear Transformations", Vol. 1, IAEA, Vienna, 1965, p 333. C. M. Wai and F. S. Rowland, J. fhys. Chem., 71, 2752 (1967). C. M. Wai and F. S. Rowland, J. fhys. Chem., 74, 434 (1970). G. F. Palino and F. S. Rowland, Radiochim. Acta, 15, 57 (1971). H. J. Machulla and G. Stocklin, J . Phys. Chem., 78, 658 (1974). Y. Y. Su and H. J. Ache, J . Phys. Chem., 80, 659 (1976). J. Wu and H. J. Ache, J . Am. Chem. SOC., 99, 6021 (1977). A. E. Richardson and R. Wolfgang, J. Am. Chem. SOC.,92, 3480 (1970). E. R. Grant and J. W. Root, J . Chem. fhys., 64, 417 (1976). R. G. Manning and J. W. Root, J. Chem. Phys., 64, 4926 (1976). J. W. Root, UCD-34P158-74-1. R. G. Manning, S-H Mo, and J. W. Root, J . Chem. Phys., 67, 636 (1977). R. W. Helton, W. M. Grant, and E. P. Rack, Radiochim. Acta, 19, 44 (1973).
Indirect Measurement of the Thionine-Leucothionine Synproportionation Rate Constant by a Photochemical Perturbation Technique Peter D. Wildes and Norman N. Lichtin" Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received November IO, 1977) Publication costs assisted by the National Science Foundation
The absorption spectrum of semithionine (S) in acidic solutions in water and in 50 v/v % aqueous CH3CN has been measured from 380 to 880 nm by a flash photolytic technique. Absorption maxima occur at 390 and 770 nm. No absorption at these values of ,A, attributable to S was detectable in 35-95% photobleached acidic iron-thionine solutions to M in thionine (T) in the photostationary state. These data show that leucothionine (L) is the principal form of reduced T in photobleached iron-thionine solutions under the conditions of observation. Pseudo-first-order specific rates of return to dark equilibrium of photobleached acidic iron-thionine solutions vary linearly with initial (dark) concentration of T at constant initial (dark) concentration of Fe(II1). A relationship has been derived by which rate constants for synproportionation of T and L to S, kz,and the apparent second-order rate constant for oxidation of L by Fe(III), kl, can be evaluated from the observed linear dependence. Combination of values of k-2 with previously determined values of kz, the specific rate of disproportionation of S, provides values of the equilibrium constant Kz = [SI2/[L][T]= k_2/2kz,(0.4-0.8) X in water and (0.09-1.8) X low6in 50 v/v % aqueous CH,CN. These values of K z have been used in conjunction with E"' at pH 2 for the two-electron reduction of T to L to determine Eo'TIS = 0.196 f 0.004, E"'S,L = 0.570 f 0.005 V vs. NHE in water and E"'T,s = 0.166 i 0.005, Eo'SIL= 0.530 f 0.005 V vs. NHE in 50 v/v % aqueous CHSCN, both media 0.01 M in sulfuric acid.
Introduction Complete characterization of the redox processes of an organic reagent involving a two-electron transformation with a radical intermediate stage requires a knowledge of the rate constants for all one-electron transfer processes. 0022-3654/78/2082-0981$01 .OO/O
Among these processes are disproportionation of the radical intermediate and its reverse, synproportionation of the fully reduced and fully oxidized states. A complete set of such reactions is illustrated for the spontaneous reaction of leucothionine (L) with ferric ions in acid so0 1978 American Chemical
Society
982
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
P. D. Wildes and N. N. Lichtin
lution by eq 1-3.
H
L (leucothionine)
I
L
i
H
S (semithionine) 2 s t Ht
L
t
(2)
HzNQ(g-JiH2
T (thionine) S t Fe(II1) tT
iFe(I1)
+ H+
(3)
Knowledge of the equilibrium constant, K z , for the disproportionation-synproportionation reaction, eq 2 , is also useful in determining the standard half-cell redox potentials for the one-electron transformations of the organic material. The first estimate of K z for the thionine system was published by Michaelis in 1940.’ Using a potentiometric titration technique, Michaelis concluded that in aqueous buffer solutions in the range of pH el-7, half-reduced solutions of thionine contain an amount of semithionine equal to about 5 1 0 % of the total dye concentration, i.e., Kz = [S]2,’[L][T]e 0.01-0.05. Later authors have used this value in calculations relating to the use of the ironthionine system in photogalvanic device^.^-^ On the basis of photochemical studies of the ironthionine system, however, other workers have concluded that K2 1 X lo-* M a cell consisting of two Pyrex plates separated by 80-km thick teflon spacers was constructed. The latter two cells were positioned at a 4 5 O angle to the photolyzing light beam from a 150-W xenon lamp and also at a 45’ angle to a weak probing light beam fixed a t a 90’ angle to the photolyzing light. The probe beam was passed through an Oriel filter monochromator to produce a narrow wavelength band centered at 600 nm, then through the sample cell. The transmitted probe beam was then passed through a Schoeffel Model ClOO miniature grating monochromator and was detected with an RCA 1P28A photomultiplier tube connected to a Tektronix Model 569 storage oscilloscope. The photolyzing light beam was filtered with a Corning 0-53 glass filter to remove UV wavelengths. The intensity of the beam was adjusted using neutral density filters to produce different extents of bleaching a t the photostationary state. The spectrum of semithionine in acidic solutions was determined by monitoring changes in absorbance of iron-thionine solutions at 10 nm intervals following flash excitation using a conventional flash photolysis apparatus with optical monitoring. Deaerated solutions containing 2X M thionine, 0.01 M Fe(II), M Fe(III), and 0.01-0.1 M acid were flashed in a cylindrical 10-cm quartz cell.
Results and Discussion It is difficult to directly determine the concentration of semithionine present in photobleached iron-thionine solutions a t the photostationary state. Hatchard and Parker reported a partial absorption spectrum for semithionine in aqueous 0.1 M sulfuric acid obtained by monitoring changes in absorption of an iron-thionine solution following flash e ~ c i t a t i o n . ~Using a similar technique, we have extended the spectrum in acidic aqueous solution through the range 380-880 nm (Figure 1). Similar absorbance changes were observed following flash excitation of acidic iron-thionine solutions in 50 v/v % acetonitrile,’water. Using the cross-illumination technique we have been unable to detect any increase in absorbance a t 390 or 770 nm ,,A,( of S) in 35-95% photobleached iron-thionine solutions to M in thionine a t photostationary state under conditions where a change of 10% would have been detectable. Thus, the conclusion reached by Hardwick.15 based on kinetic evidence, that S is the principal form of reduced thionine present in photobleached iron-thionine solutions is almost certainly incorrect. Using kinetic data previously obtained for the reactions involved in the iron-thionine photoredox system, we have calculated photostationary state compositions over a range of initial solution compositions and conditions of illumination.13J4 The calculations indicate that in solutions bleached to an extent varying from 5 to 9570,less than 1% of the total dye concentration is present as S. If we assume that during the return from the photo-
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978 983
Thionine-Leucothionine Synproportionation r r
I
I
I
I
-I . 0)
D
Y
A , nm
S (----) and T (-) in 0.1 M aqueous perchloric acid. The tT scale for T is larger than that for S by a factor of 4, as indicated by the arrows. Figure 1. Absorption spectra of
stationary state to dark equilibrium [SI,
