Langmuir 2005, 21, 2721-2727
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Photooxidation of Diglycine in Confined Media. Application of the Microreactor Model for Spin-Correlated Radical Pairs in Reverse Micelles and Water-in-Oil Microemulsions Ryan C. White, Valery F. Tarasov, and Malcolm D. E. Forbes* Venable and Kenan Laboratories, Department of Chemistry, CB #3290, University of North Carolina, Chapel Hill, North Carolina 27599 Received October 24, 2004. In Final Form: January 6, 2005 Time-resolved electron paramagnetic resonance spectra (X-band) of correlated radical pairs created in AOT reverse micelles and microemulsions are presented, simulated, and discussed using the microreactor model. The radicals are formed inside the water pool using photooxidation of diglycine by the excited triplet states of two different anthraquinone sulfonate salts. Water pool size and temperature effects on the spectra are reported, and the simulations allow for extraction of the diffusion coefficient in the interior, which monotonically increases with water pool size. The data directly correlate with the diffusional properties of correlated radical pairs in regular aqueous micelle solutions studied previously by similar methods. Competition between H-atom abstraction and electron transfer is observed with anthraquinone sulfonate, but electron transfer is the only reaction pathway observed when anthraquinone disulfonate triplet state is the sensitizing species.
Introduction The oxidation of amino acid residues within proteins plays a key role in many types of cellular malfunction such as Alzheimer’s disease1-3 and in the initial stages of cataractogenesis.4 For example, there is strong evidence that one of the key steps in fibril and plaque formation in brain tissue involves the one-electron oxidation of the methionine-35 residue on the Rβ-amyloid protein. To better understand these reactions, there have been several recent studies of radicals and radical cations formed from amino acids and short peptides from our laboratory5-7 and others.2,8-10 The goal of this research is to better characterize the physical and chemical properties of these important reactive intermediates and to compare the reactivity of the small molecule models systems to the proteins themselves. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy is an effective technique for the study of amino acid derived radicals and radical cations formed by photooxidation and photoinitiated electron * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Rauk, A.; Armstrong, D. A.; Fairlie, D. P. J. Am. Chem. Soc. 2000, 122, 9761. (2) Schoneich, C.; Pogocki, D.; Wisniowski, P.; Hug, G. L.; Bobrowski, K. J. Am. Chem. Soc. 2000, 122, 10224. (3) Varadarajan, S.; Yatin, S.; Kanski, J.; Jahanshahi, F.; Butterfield, D. A. Brain Res. Bull. 1999, 50, 133. (4) Davies, M. J.; Truscott, R. J. W. J. Photochem. Photobiol., B 2001, 63, 114. (5) Burns, C. S.; Rochelle, L.; Forbes, M. D. E. Org. Lett. 2001, 3, 2197. (6) Clancy, C. M. R.; Forbes, M. D. E. Photochem. Photobiol. 1999, 69, 16. (7) Morozova, O. B.; Yurkovskaya, A. V.; Tsentalovich, Y. P.; Forbes, M. D. E.; Sagdeev, R. Z. J. Phys. Chem. B 2002, 106. (8) Tarabek, P.; Bonifacic, M.; Beckert, D. J. Phys. Chem. A 2004, 108, 3467. (9) Tarabek, P.; Bonifacic, M.; Naumov, S.; Beckert, D. J. Phys. Chem. A 2004, 108, 929. (10) Goez, M.; Rozwadowski, J.; Marciniak, B. Angew. Chem., Int. Ed. 1998, 37, 628.
transfer.6,8,9,11 Previously we have used TREPR to study the photophysics of tyrosine anion,6 the structure of tyrosine radical in free solution,12 and the intramolecular electron-transfer reaction between tyrosine and an electron acceptor to make peptide biradicals.5 Also, chemically induced nuclear spin polarization methods have been employed to study intramolecular electron-transfer reactions in tyrosine-tryptophan dipeptides,7 and methionine photooxidation.10 The combination of high structural resolution from magnetic resonance spectroscopy and fast time resolution in kinetic measurements has led to a much better understanding of the structure and reactivity of several biologically important free radicals. An additional advantage of TREPR experiments comes from the observation of chemically induced electron spin polarization (CIDEP) phenomena, for which there are now four established mechanisms.13,14 These processes manifest themselves as different and easily distinguishable EPR spectral patterns. One of the most interesting CIDEP mechanisms is called the spin-correlated radical pair (SCRP) mechanism, which arises due to an observable spin exchange interaction between the unpaired electrons of two diffusionally restricted radical centers. Examples of SCRPs include covalently bound biradicals15,16 and micellized radical pairs.17-20 The most prominent feature (11) Lu, J.-M.; Wu, L. M.; Geimer, J.; Beckert, D. Phys. Chem. Chem. Phys 2001, 3, 2053. (12) Clancy, C. M. R.; Forbes, M. D. E. Submitted for publication in Org. Lett. (13) Harbron, E. J.; Forbes, M. D. E. Encyclopedia Chem. Phys. Phys. Chem. 2001, 2, 1389. (14) Forbes, M. D. E. Photochem. Photobiol. 1997, 65, 73. (15) Forbes, M. D. E. J. Phys. Chem. 1993, 97, 3396. (16) Forbes, M. D. E. J. Phys. Chem. 1993, 97, 3390. (17) Ishawata, N.; Murai, H.; Kuwata, K. Res. Chem. Int. 1993, 19, 59. (18) Closs, G. L.; Forbes, M. D. E.; Norris, J. R. J. Phys. Chem. 1987, 91, 3592. (19) Tarasov, V. F.; Bagranskaya, E. G.; Shkrob, I. A.; Avdievich, N. I.; Ghatlia, N. D.; Lukzen, N. N.; Turro, N. J.; Sagdeev, R. Z. J. Am. Chem. Soc. 1995, 117, 110.
10.1021/la047382x CCC: $30.25 © 2005 American Chemical Society Published on Web 03/04/2005
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Langmuir, Vol. 21, No. 7, 2005 Scheme 1
of SCRP spectra is the so-called “antiphase structure” (APS) where each individual hyperfine line of the free radicals is split into two components of opposite phase: emission (E) or absorption (A). Analysis of the APS line shape and its time dependence has been extensively investigated in several laboratories.21-23 There now exist models for correlated radical pair dynamics that allow for determination of the diffusion coefficients of the radicals in the microreactor. From such analyses we can estimate the internal microviscosity of the supramolecular structure and the intensity of the electron spin-spin Heisenberg exchange interaction between the radicals. In this paper we report the results of an extensive investigation of the confined radical pair formed by photooxidation of diglycine by water-soluble quinone acceptors in reverse micelles and microemulsions. This work represents the convergence of the two areas of research described above, photooxidation of peptides and diffusion of radical pairs, and is the first application of the microreactor model to reverse micelles, which are better mimics of cellular conditions (hydrophobic walls enclosing an aqueous interior) than ordinary aqueous micelles. Background Reverse micelles and water-in-oil microemulsions are microscopic spherical pools of water surrounded by a monolayer of surfactant separating the water pool from a hydrophobic bulk solution (Scheme 1). The surfactant bis(2-ethylhexyl) sulfosuccinate sodium salt (referred to as AOT) is commonly used because of its tendency to form uniform spherical reverse micelles, the sizes of which are controlled by the [H2O]/[AOT] ratio (W0). AOT reverse micelles and water-in-oil microemulsions can be used as a spherical “container” to encapsulate a water-soluble radical pair. There have been two previous reports of radical pairs created and contained within AOT reverse micelles. Both of these studies used water-soluble anthraquinone sulfonate salts as the photosensitizer, and both organic and inorganic substrates within AOT reverse micelles were used as donors. Turro et al.24 observed radical pairs in AOT reverse micelles through the oneelectron photooxidation of sodium sulfite. Akiyama et al.25 have also used anthraquinone derivatives to photooxidize hydroquinone substrates within AOT reverse micelles to (20) Weis, V.; Van Willigen, H. J. Porphyrins Phthalocyanines 1998, 2, 353. (21) Tarasov, V. F.; Forbes, M. D. E. Spectrochim. Acta, Part A 2000, 56, 245. (22) (a) Shushin, A. I. Chem. Phys. Lett. 1995, 245, 183. (b) Chem. Phys. Lett. 1997, 275, 137. (c) Chem. Phys. Lett. 1998, 282, 413. (23) Neufeld A. A.; Pedersen, J. B. J. Chem. Phys. 1998, 109, 8743. (24) Turro, N. J.; Khudyakov, I. V. J. Phys. Chem. 1995, 99, 7654. (25) Akiyama, K.; Tero-Kubota, S. J. Phys. Chem. B 2002, 106, 2398.
White et al. Scheme 2
form semiquinone radicals. However, radical pair diffusion behavior was not modeled in these studies. Our photochemical system is shown in Scheme 2 and involves the photooxidation of glycyl-glycine (diglycine, Gly-Gly, or GG) which is a water-soluble, biologically relevant substrate.8 Two different water-soluble anthraquinone derivatives were chosen as sensitizers: 2-anthraquinone sulfonate (AQS), and anthraquinone-2,6-disulfonate (AQDS). Photoexcitation followed by fast intersystem crossing converts these anthraquinone analogues to their first excited triplet states. At neutral pH, the AQ(D)S triplet state reacts with the carboxylate terminus of the diglycine in a Kolbe-type26 one-electron oxidation. After subsequent decarboxylation, which is fast on the TREPR time scale, radicals 1 and 2 are formed. The R-amidomethylene radical, 1, and the reduced anthraquinone, 2, make up the confined radical pair in the interior of the AOT reverse micelle. When a radical pair is created in a reverse micelle, it remains in the interior although it is still highly mobile. This is advantageous for observation of sharp EPR transitions. Subsequent diffusion of the radical partners, coupled with the time evolution of the individual electron spin wave functions, gives rise to polarization of the electron paramagnetic resonance (EPR) transitions in a predictable way. Simulation of the APS polarization pattern and time dependence is described in detail in a previous publication.21 The model, which is summarized briefly below, can yield information about the relative diffusion coefficient, the interaction of the radicals with the surfactant walls, and the ultimate fate of the reactants (spin relaxation, chemical reaction, or escape processes). It is important to recognize that the APS pattern arises because the radical pair remains within a close enough proximity to allow for electron spin exchange (J) and that the magnitude of this spin exchange interaction is dependent on, and very sensitive to, the rate of encounters between the members of the radical pair. The first explanation of the APS spectral pattern was proposed by Closs, Forbes, and Norris,18 and also Buckley (26) Kraeutler, B.; Jaeger, C. D.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 4903.
Spin-Correlated Radical Pairs in AOT Reverse Micelles
et al.27 in 1987 and is generally called the CFN model. It assumes a completely time- and space-averaged spin exchange interaction, J, which can then be extracted directly from the EPR spectrum by simulation using an average J value as a coupling constant, much like simulating an NMR spectrum. This model essentially ignores diffusion by assuming it is fast compared to the magnitude of J expressed in inverse frequency units. However, there are several experimental conditions for diffusionally restricted radical pairs that lead to TREPR spectra that cannot be simulated using CFN. For example, studies on modified surfactant molecules synthesized in our laboratory have shown that SCRP polarization can evolve on the same time scale as the experiment (0.1-2 µs) and give rise to asymmetric line shapes.28 To simulate such spectra, a new model was proposed by Tarasov21 which explicitly takes into account radical diffusion and subsequent modulation of the exchange interaction. This is called the microreactor model, and we use it here to obtain diffusion, exchange, and relaxation parameters of hydrophilic radicals within the water pool of reverse micelles and water-in-oil microemulsions. Results and Discussion Reverse Micelle Parameters. Isooctane (2,2,4-trimethylpentane) was chosen as the hydrophobic bulk solution for our experiments. The total radius of a reverse micelle or water-in-oil microemulsion is the sum of the lamellar layer thickness (approximately 11 Å) and the water pool radius Rc. The value of Rc is dependent on W0 and can be estimated in a straightforward fashion using eq 129
Rc (Å) ) 36.65ν/g
(1)
where ν and g are the weight percentages of water and AOT, respectively. Our experiments were carried out with a majority of the reverse micelles containing only one radical pair. The Poisson distribution30 was used to determine the concentration of AQ(D)S needed to achieve this condition. In our experiments, 66-75% contained no photosensitizers, 22-28% contained one photosensitizer, 3-6% contained two photosensitizers, and
