4600
J. Phys. Chem. 1988, 92, 4600-4606
Carotenoid Cation Radicals Produced by the Interaction of Carotenoids with Iodine Ruisong Ding, Janice L. Grant, Robert M. Metzger, and Lowell D. Kispert* Department of Chemistry, University of Alabama, Tuscaloosa, Alabama 35487-9671 (Received: October 28, 1987: In Final Form: January 20, 1988)
Electron paramagnetic resonance (EPR) and optical absorption spectra of 1,2-dichloroethaneand dichloromethane solutions of 8-carotene (C40H56)and iodine have been studied. EPR studies showed that 8-carotene cation radicals are formed. These radicals form complexes at 77 K, in highly concentrated (0.1 M) iodine solutions, with higher order polyiodide anions, Is-, 17-, 1s (as evidenced by g value shifts and line-width increases), while at room temperature and in dilute ( l ~ + - l FM) ~ solutions of iodine only If counterions are involved. In dilute solutions (total molar concentration = 2 X lo4 M), a Job plot showed the stoichiometry to be 2:3, indicating the reaction 2C40H56+ 312+ 2C,H5,'+ + 213-. The radical fraction (EPR) is -2% at 77 K and 0.4% at 300 K (for a 2 mM P-carotene/O.l M I2 solution); it increases at lower @-caroteneconcentrations. Dimers and trimers of p-carotene cation radicals are formed in dilute solution (I2 < lo4 M) if [12]/[j3-carotene]< 1, and an intense absorption band occurs at A,, 1030 nm. Canthaxanthin and o-apo-8'-carotenal with I2 produce new absorption bands with A,, < 1000 nm; the g values are independent of iodine concentration, suggesting a poorer ability to form complexes with the polyiodide anions. All three carotenoids react with other electron acceptors (7,7,8,8-tetracyanoquinodimethane (TCNQ), tetrachloro- 1,4-benzoquinone (chloranil), and Br2) according to the electron affinity of the acceptor used.
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Introduction Carotenoids not only serve as photoprotect devices and as light-harvesting pigments in photosynthetic systems14 but also are involved in other functions. For instance, molecular triad molecules, C-P-Q, consisting of porphyrins (P) covalently linked to both carotenoids (C) and quinones (Q) can achieve photodriven electron transfer in good ~ i e l d . ~Excitation -~ of the C-P-Q triad initiates a series of electron-transfer reactions that ultimately yield the charge-separated state C'+-P-Q', where C'+ is the carotenoid cation radical. Light excitation of chloroplasts produces absorption changes due to the formation of a carotenoid cation radical with lifetimes of 10-35 p s at the photosystem I1 reaction The production of carotenoid cation radicals has been previously studied by photochemicalloand radiolytic means."-13 Such studies have shown that in hexane solvent carotenoid cation radicals are formed on oxidation, but their lifetimes are only on the order of microseconds. In contrast, four studies have found that the chemical oxidation of @-carotene(C40H56, I, Chart I) both as a solid and in solution, with Iz (vapor or solution) as an electron acceptor, gives rise to products that are stable for several minutes or
CHART I
0
(111)
DDQ (1) Goedheer, J. C. Ann. Reu. Plant Physiol. 1972, 23, 87. (2) Sauer, K. Bioenergetics of Photosynthesis; Academic: New York, 1975; p 115. (3) Renger, G.; Wolff, Ch. Biochim. Biophys. Acta 1977, 47, 460. (4) Chessin, M.; Livingston, R.; Truscott, T. G. Trans. Faraday SOC.1966, 62, 1519. (5) Seta, P.; Bienvenue, E.; Moore, A. L.; Mathis, P.; Bensasson, R. V.; Liddell, P.; Pessiki, P. J.; Moore, T. A,; Gust, D. Nature (London) 1985, 316, 653. (6) Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J.-C.; Chachaty, C.; Bensasson, R. V.; Land, E. J.; Doizi, D.; Liddell, P. A,; Lehman, W. R.; Nemeth, G. A.; Moore, A. L. Nature (London) 1984, 307, 630. (7) Gust, D.; Moore, T. A,; Liddell, P.A.; Nemeth, G. A,; Makings, L. R.; Moore, A. L.; Barrett, D.; Pessiki, P. J.; Bensasson, R. V.; Raigee, M.; Chachaty, C.; De Schryver, F. C.; Van de Anweraer, M.; Holzwart, A. R.; Connolly, J. S . J. Am. Chem. SOC.1987, 109, 846. (8) Mathis, P.; Rutherford, A. W. Biochim. Biophys. Acta 1984, 767, 217. (9) Schenck, C. C.; Diner, B.; Mathis, P.; Satoh, K. Biochim. Biophys. Acta 1982, 680, 216. (10) Mathis, P.; Vermeglio, A. Photochem. Photobiol. 1972, 15, 157. (11) Dawe, E. A.; Land, E. J. J. Chem. SOC.,Faraday Trans. 1975, 71, 2162. (12) Almgren, M.; Thomas, J. K. Photochem. Photobiol. 1979, 31, 329. (13) Lafferty, J.; Roach, A.; Sinclair, R. S.; Truscott, T. G.; Land, E. J. J . Chem. Soc., Faraday Trans. 1 1977, 73, 416. (14) Lupinski, J. H. J. Phys. Chem. 1963, 67, 2725. (15) Ioffee, N. T.; Engovatov, A. A,; Mairanovskii, V. G. Zh. Obsch. Khim. 1976, 46, 1638. (16) Ebrey, T. G. J. Phys. Chem. 1967, 71, 1963. (17) Matsuyama, T.; Sakai, H.; Yamaoka, H.; Maeda, Y . J. Chem. Soc., Dalton Trans. 1982, 229.
Chloranil
CI 03 E
o O CI= R O
In the first of the four studies, LupinskiI4 suggested in 1963 that the @-carotenel1, product is a charge-transfer complex. H e established that when solutions of p-carotene and iodine(< lo4 M, in 1,2-dichloroethane) are mixed, two new bands in the near UV, at 290 nm and 360 nm, as well as an intense absorption band in the near IR, at 1000 nm, are produced. The absorptions in the near UV were assigned to I< ions, while the band in the near IR was attributed to a complex of stoichiometry C40H56.21Z. Additional experiments showed the complex to be of ionic form, which suggested the equilibrium
-
C40H56 + 212 e C40H56.212
[ C ~ O H ~ ~ I ] ' * I ~ -(1)
LupinskiI4 proposed that the [C40H561]+ion exists as a charge-transfer complex, C&IS6.-I+. He discounted the possibility of a radical cation, since electron paramagnetic resonance (EPR) absorptions were not detected at room temperature in a 5 X lo4 M solution of p-carotene and iodine in 1,2-dichloroethane. The
0022-3654/88/2092-4600$01.50/00 1988 American Chemical Society
Carotenoid Cation Radicals carbonium ion [C40H561]+was similarly excluded because the IR spectrum of solid @-carotene-I2 does not exhibit a C-I band; a dication was also excluded because it would not exhibit absorptions around 1000 nm. In the second study, Ebrey16 suggested that the band at 1000 nm may be due to a shift in the absorption maximum of @carotene and is not a Mulliken donor-acceptor charge-transfer band. He suggested that if @-caroteneis allowed several isoenergetic resonance structures in the ground state, then the A, would be -1 100 nm (compared with the observed A, of 451 nm if @-carotenehas only a single resonance structure). Thus, on addition of iodine to @-carotene,several bond configurations of @-carotenebecome possible, causing the bonds to become nearly equal, with no bond alternation and a shift of the & of @-carotene to much longer wavelengths. However, if the near-IR band is due to several isoenergetic resonance structures of @-carotene,then the absorption band peak maximum should not depend on the acceptor used; in fact, for @-carotenefilms exposed to the vapors of various acceptor molecules, Mallik et al.I8*l9found that the & does vary with the acceptor used. In the third study, Ioffee et al.I5 suggested that the @-carotene/12 product is a carbonium ion. They found that reaction of @-carotene with a strong acid (for example, CF,COOH) also gives rise to an absorption band in the near IR (Imm= 990 nm), due to the production of a carbonium ion formed by protonation of @-carotene. This result indicated that the @-carotene/12product might be the diamagnetic cationic species, [C40H561]+,analogous to a carbonium ion. However, the authors detected a weak EPR signal between -40 and -80 OC when @-caroteneand iodine were mixed in 1,2-dichloroethane solvent, and the true nature of the product was left unresolved. In the fourth study, Matsuyama et al." proposed the formation of a charge-transfer complex, C40H56+-.I 2.0028 g = 2.0028 g > 2.0028 > 1% radicals < 1% radicals
2.0028, a value near that of the free electron (2.0023) and the expected value for a hydrocarbon cation radical. Table I also shows a shift in g value to a smaller value (-2.003) and a decrease in the EP,R intensity when the sample is warmed from 77 K to 300 K (cf. runs 1 and 2; 3 and 4; 8 and 9; 11 and 12). Similar EPR results are obtained by using CHzClzas solvent (runs 13-18). For instance, as the concentration of @-carotenedecreases from M (run 13) to 2 X M (run 14), the gvalue increases 1X from 2.0042 to 2.0062, except for solutions containing equal concentrations of @-caroteneand iodine (run 15), where the gvalue equals 2.0027. Also shown in Table I is the slight variation in gvalue between degassed and nondegassed samples (cf. runs 13, 14, and 15 with runs 16, 17, and 18). The EPR data clearly show that a radical species is formed when @-caroteneis mixed with highly concentrated iodine solutions and frozen at 77 K. The facts that iodine must be present in large excess and at a reaction temperature of 77 K explain why radicals have not been observed before by other workers, since earlier studies were conducted using dilute iodine solutions (approximately M) and at temperatures ranging only from 25 to -80 0C.14*1s The data at 77 K are consistent with a reaction mechanism of the type
evidence to support the notion that @-caroteneradical cations complex with polyiodides at low temperature. For instance, the triiodide ion, I 3 (Table I11 (a-c)). ( 3 ) The A,, remains constant at -1030 nm over the range of concentrations studied. This is consistent with the idea that @-carotenecations and cation radicals, which absorb in this region, are the predominant products in dilute solutions of @-carotene. Interaction of I2 with Carotenoids II and III. EPR and optical spectra were also recorded for the reaction of I, with P-apo-8'carotenal and with canthaxanthin in C2H4C12solvent; the results are summarized in Table VI. For comparison, the results obtained for the reaction of I2 with @-caroteneare also given. Table VI shows that the wavelength at which maximum optical absorption occurs varies depending on the carotenoid used. The (32) Lupinski, J. H.; Huggins, C. M. J . Phys. Chem. 1962, 66, 2221.
The Journal of Physical Chemistry, Vol. 92, No. 16, 1988 4605
Carotenoid Cation Radicals
TABLE VI. g Values and ,A,
for the Reaction of I2 with Carotenoids I,
11, and I11 in C2H4Cl2Solvent
102[carotenoid], 102[12], M g M &carotene 5 10 2.0043 5 10 2.0031 0.2 10 2.0066 0.04 10 2.0060 0.08 0.1 canthaxanthin 5 10 2.0031 5 10 2.0028 0.5 1 2.0031 0.05 2 2.0031 0.05 0.1 8-apo-8’5 10 2.0024 carotenal 5 10 2.0024 0.5 1 2.0023 0.02 2 2.0027 0.05 0.1
temp,, , ,A K nm 17.0 11.7 20.7 21.7 9.0 4.4 9.5 10.9
77
300 77 77 300 77 300 77
Ea, eV
DDQ 3.13c
TCNQ 2.8d
chloranil 2.76,’2.49’
950
(i) @-Apo-8’-carotenal 820 850 850 2.0023 2.0046 2.0031 9.8 4.7 8.2
930
, , ,A (300 K) g (77 K) AH,,, G
(ii) Canthaxanthin 900 869’ 930 2.0030 2.0031 4.8 9.5
850
, , ,A (300K) g (77 K) AH,,, G
(iii) p-Carotene 950 800 775, 860 790 2.0030 2.0026 2.0028 10.8 5.0 6.4
77
300 77 77 300
I2 3.32b
, , ,A, (300 K) g (77 K) AH,,, G
77
300 11.1 13.1 8.2 14.3
TABLE VII: Optical and EPR Spectra of Carotenoids I, 11, and 111 in C2HlC12 Solvent in the Presence of Various Electron Acceptors” Br2 2.368 860h
925h
925
-
779
‘Concentrations: absorption (i) [@-apo-8’-carotenal] = 5 X lo4 M, A- value is highest for @-caroteneand decreases as the carotenoid [acceptor] = 1 X lo-’ M; (ii) [canthaxanthin] = 8 X lo4 M, [accepis changed to canthaxanthin (111, Chart I) and then to @-apotor] = 1 X M; (iii) [B-carotene] = 8 X lo4 M, [acceptor] = 1 X 8’arotenal (11). As discussed earlier, the & values for various lo-’ M. EPR [carotenoid] = 5 X M; [acceptor] = 1 X M. carotenoid cations and cation radicals have been well characterized, From AHf(12(g),298.15K) = 14.923 k ~ a l / m o l and ~ ~ AHf(Iand all occur in the spectral region 800-1500 nm.15,30In hexane (g),298.15K) = 25.535 k ~ a l / m o l ’and ~ from E, of I(g) = 3.06134 eV solvent @-carotene (I, 11 double bonds) absorbs at 1040 nm and and AH(reaction) = -24.0 kcal/mol (estimated) for I-(g) + I,(g) canthaxanthin (111, 11 double bonds 2 C = O ) at 960 nm. The 13-(g),35for the reaction the 1.512 + e-(g) 13-(g). cEstimated from A, value for @-apo-8’-carotenal (11, 9 double bonds C=O) C T spectra, for the reaction DDQ(g) + e-(g) DDQ-(g).36 For the reaction TCNQ(g) + e-(g) TCNQ-(g).)’ CFor the reaction chloris known has not been previously determined, but since the A, anil(g) + e-(& ~hloranil-(g).~*/Estimated from C T spectra.36 to decrease with the number of double bonds in the carotenoid ZFrom AHf(Br(g),298.15K) = 26.741 k ~ a l / m o l and ~ ~ AHf(Br2chain, we expect that this species should absorb in a region similar (g),298.15K) = 7.387 k ~ a l / m o l ’and ~ from E, of Br(g) = 3.364 eV,34 = 915 to septapreno-@-carotene (9 double bonds), which has A, for the reaction 0.5Br2(g) + e-(g) Br-(g). hCH2C12solvent. ‘Very nm. Thus Table VI shows that all carotenoids studied do form low intensity. ’Band shifts from 925 to 775 nm within a few minutes. “cationic” species (Le., cation radicals or cations) on addition of 12, and, furthermore, the decrease in A, as the carotenoid is of dehydro-@-carotenewith trifluoroacetic acid and suggested that changed from @-caroteneto canthaxanthin to @-apo-8’-carotenal the results were due to formation of two isomeric forms of the is a reflection of the decreasing bond length of the carotenoid chain. product. The EPR results obtained for the reactions of iodine with The reaction of Br2with all three carotenoids is in sharp contrast canthaxanthin and @-apo-8’-carotenal show that the g value is to their reactions with 12. For example, no new absorption bands almost independent of concentration and temperature. This beor EPR signals were observed for the reactions of canthaxanthin havior is in contrast to that observed with @-carotene. The reason and p-apo-8’-carotenal with Br2, while the reaction of @-carotene for this is not understood but seems to suggest that @-apo-8’with Br2 produced unstable species (Le., a new absorption band carotenal and canthaxanthin are less able to form complexes with is produced at 925 nm, which shifts in less than 5 min to 775 nm). the polyiodide anions. Indeed, the first oxidation potentials of This result can be explained by the fact that Iz and Br2 behave these two carotenoids are -0.15 V more positive than that of far differently in solution. For instance, we have already discussed @-carotene,29indicating they are poorer electron donors. that iodine readily undergoes complex equilibrium reactions to Interaction of Carotenoids I , II, and III with Other Electron form I< and higher order polyiodide anions, I;. Thus iodine forms Acceptors. The interaction of carotenoids I, 11, and 111 with the complexes stable for hours when the carotenoids react with these electron acceptor molecules DDQ, TCNQ, chloranil, and Br2 was polyiodide anions. On the other hand, Br2 does not easily form also studied, to investigate the effect of acceptor electron affinity, polybromide species in solution and reacts very readily as Brz.39 E,. The EPR and optical results, as well as the electron acceptor In fact, there is evidence in the literature that the reaction of Br2 ionization reactions and their respective electron a f f i n i t i e ~ , ~ ~ - ~with ~ @-carotenemerely results in the addition of Br2 to the carare given in Table VII. otenoid chain@and not in the formation of a cation species. Hence The results show that, in general, the acceptors react with the the E, values that must be compared in the present study are carotenoids according to the electron affinity of the acceptor Br-(g) (E, = 2.36 eV)34*3sand 1.512(g) 0.5Br2(g) + e-(g) molecule. For example, the electron acceptor molecules chloranil e-(g) 13-(g) (E, = 3.32 eV).3”36 The high reactivity of iodine, and TCNQ have much lower E , values than does I2 [2.7638and in relation to bromine, is therefore due to the fact that the E, of 2.8 eV,37for the reactions chloranil(g) + e-(g) chloranil-(g) Br2 (to form Br-) is very similar to the E, of I2 (to form I-), but and TCNQ(g) e-(g) TCNQ-(g), respectively, versus 3.32 the formation of I,- is stabilized by an extra I eV. Thus iodine 13-(g)], and when eV33-35for the reaction 1.512(g) e-(g) is a far better acceptor than bromine. chloranil was reacted with the carotenoids, either no absorption The production of an unstable species on reaction of Br2 with bands are observed or only bands of very low intensity are prop-carotene is further evidence of the highly reactive Br2 molecule. duced. The reaction of @-carotene(I) with TCNQ leads to an I n the study of I2/D-carotene mixtures Lupinski14 indicated the interesting observation, Le., the appearance of two new absorption necessity to perform all absorption experiments immediately bands. The reason for this is not clear; however, Ioffee et al.15 following mixing of the two reactants. In the present study it was have previously reported two absorption bands for the reaction found that if a significant delay ( hours) was allowed between mixing and the measurement, the ,A, of @-carotene-I2 products (33) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Halow, I.; Baily, S.M.; shifted to lower wavelengths. unstable situation has also been Schumm, R. H. National Bureau of Standards Technical Note 270-3; U. S . reported by Ioffee et al.,Is who found that the new absorption Government Printing Office: Washington, DC, 1968. bands that arise from interaction of anhydro-vitamin A and ax(34) Hotop, H.; Lineberger, W. C. J . Phys. Chem. Ref, Data 1975,4,539.
+
+
--
- -
-
-
+
-
+
-
-
-
-
+
-
( 3 5 ) Topol, L. E. Inorg. Chem. 1971, 10, 736. (36) Chen, E. C. M.; Wentworth, W. E. J . Chem. Phys. 1975,63, 3183. (37) Compton, R. N.; Cooper, C. D. J . Chem. Phys. 1977, 66, 4325. (38) Cooper, C. D.; Freyh, W. F.; Compton, R. N. J . Chem. Phys. 1978, 69, 2367.
(39) Popov, A. I. In Halogen Chemistry; Gutmann, V., Ed.; Academic: New York, 1967; pp 236, 248. (40) MacBeath, M. E.; Richardson, A. L. J. Chem. Educ. 1986,63,1017.
J . Phys. Chem. 1988, 92, 4606-4610
4606
erophthene with iodine and with trifluoroacetic acid change rapidly with time. However, the iodine products are reasonably stable during the measurement if there is no delay following mixing. On the other hand, it appears that Br2 addition across the conjugated bonds is more rapid, and optical changes occur on the measurement time scale; quantitative data are therefore difficult to obtain. We conclude then that Br2 reacts quickly with the carotenoids to produce highly unstable Br2 addition species. In contrast, I, forms complex polyiodide anions in solution, which react on a slower time scale to generate relatively stable carotenoid cation/radical cation species. The lack of reaction of carotenoids with DDQ, despite its high E, value,36 is surprising. It is important to note, however, that even though DDQ reagent was rigorously purified, an intense background impurity EPR signal, g 2.0052 and AHpp 5 G, was always obtained. Thus, reaction of DDQ with the carotenoids was difficult to determine. The reaction of DDQ with canthaxanthin produced no changes in the DDQ EPR signal; the intensity, g value, line shape, and line width remained unchanged. Reaction of DDQ with @-caroteneresulted in loss of the DDQ EPR signal, and the appearance of a very small EPR line, with g = 2.0026, and AHpp= 5.0 G. The interaction of DDQ and @-apo-V-carotenal resulted in the observation of an EPR line of decreased intensity and complex in shape (possibly due to two superimposed radical signals). The resultant signal was determined to have a g value of 2.0046, with a line width twice that of the DDQ species alone, Le., AHpp= 9.8. For those reactions in which no electronic absorption changes were observed by using C2H4C12solvent, the experiments were repeated by using CH2C12. In the case of chloranil, the change of solvent had no effect. For Br2, however, new absorption bands were produced in CH2CI2solvent. This is in general agreement with the results of an earlier electrochemical study undertaken in our labor at or^,^^ using carotenoids I, 11, and 111, in which it
-
Chemiluminescent Reaction of As("")
-
was found that the radicals formed on oxidation of the carotenoids were more stable in CH2C12than in C2H4C12solvent. Table VI1 also shows that the absorption peak maximum, A,, for a particular carotenoid, does indeed vary with the acceptor used, in agreement with the results of Mallik et al.18J9for the reaction of @-carotenefilms with various acceptor vapors. ConcIusions @-Carotenecation radical monomers/dimers are produced (in either CH2Clzor C2H4C12solvents) when @-carotene(I) is reacted with excess iodine. In dilute solutions and at room temperature, the radicals are stabilized by I; ions, while at 77 K and in highly concentrated iodine solutions (0.1 M), higher order polyiodide anions are formed. The formation of radical cation-polyiodide anion "adducts" is responsible for the large g values observed in solutions where the ratio 12/@-caroteneis large. EPR measurements show that approximately 3% of the concentration at 0.4 m M (in 0.1 M I2 solution) actually forms radicals. When the concentration of @-caroteneis small (
