J. Phys. Chem. 1991, 95, 6027-6034 a oblate ellipsoid. For 20 mol % diC,PC, the major radius of the ellipsoid will be equal to 113 A when the minor radius is taken as T/2(=22.5 A) and the volume is set equal to r(92.3)2(45) A3. If the aggregates grow larger as more DPPC molecules are added to the system, the center portion of the aggregate will resemble plain bilayer structure. Differential scanning calorimetry (DSC) analyses indicate the DPPC molecules in these binary PC aggregates have phase behavior similar to small unilamellar vesicles of pure DPPC. The model provided by SANS is consistent with those o b s e r v a t i ~ n s . ~A~ similar J~ model was used to explain the formation of mixed discoid micelles of egg lecithin/taurochenodesoxycholate (TCDC).33 The concepts discussed above
6027
should be applicable to other similar mixed systems. Acknowledgment. We thank Drs. Rex P. Hjelm, Jr., and Phil A. Seeger of the Los Alamos National Laboratory for their help with the SANS measurements, and the Los Alamos Neutron Scattering Center for allowing use of the Low-Q Diffractometer. T.-L.L. thanks the National Science Council, R.O.C., Grants NSC-79-0208-M007-33 and NSC-80-0208-M007-61, for support. M.F.R. acknowledges the support from NIH G M 26762. (33) Fromherz, P.; RBcker, C.; Riippel, D. Faraday Discuss. Chem. Soc. 1986. 81, 39.
Photoinduced Charge Separation by Chromophores Encapsulated in the Hydrophobic Compartment of Amphiphiiic Polyelectrolytes with Various Aliphatic Hydrocarbons Yotaro Morishima,* Yukio Tominaga, Mikiharu Kamachi,* Department of Macromolecular Science, Faculty of Science, Osaka University. Toyonaka, Osaka 560, Japan
Tadashi Okada,* Yoshinori Hirata, and Noboru Mataga Department of Chemistry, Faculty of Engineering Science, and Research Center for Extreme Materials, Osaka University, Toyonaka, Osaka 560, Japan (Received: December 28, 1990) Terpolymers of 50 mol % sodium 2-(acrylamido)-2-methylpropanesulfonate(AMPS), 1 mol % I-(pyreny1methyl)methacryhmide (PyMAm), and 49 mol % lauryl- or cyclododecyl- or adamantylmethacrylamide units were synthesized. A copolymer of 99 mol % AMPS and 1 mol IPyMAm units was also prepared as a reference polymer. Charge-transfer (CT) complexation, fluorescence quenching, and photoinduced electron transfer (ET) were studied, using methylviologen (MV2+)as an acceptor. In the terpolymers, the polymer-bound pyrene (Py) chromophores are compartmentalized in hydrophobic microdomains of a micellelike microphase structure formed by the terpolymers in aqueous solution, while the chromophores are exposed to the aqueous phase in the reference copolymer. In the compartmentalized systems, the CT complexation of Py with MV2+ was suppressed, but the fluorescence quenching was enhanced. Charge recombination (CR) of the primary ion pair generated by laser excitation was slowed by an order of magnitude as compared to that in the reference copolymer system, while very fast photoinduced forward ET occurred (within ca. 20 ps). Thus, the terpolymer systems showed charge separation that persisted for hundreds of microseconds. These findings were qualitatively interpreted in terms of sterical protection of the Py chromophore from a close contact with MV2+,although MV2+is highly concentrated on the surface of the hydrophobic microdomain of the terpolymers. A sterically hindered primary ion pair of a looser structure with a longer lifetime may be formed in the compartmentalized systems. The lauryl group was significantly less effective in protecting Py than were the cyclododecyl and adamantyl groups. A judicious choice of the hydrophobic group is suggested to achieve an optimal compartmentalization of Py, which will lead to an optimal efficiency for charge separation.
Introduction Molecular understanding of the mechanism of primary photoinduced electron transfer (ET) and subsequent charge separation in natural photosynthetic systems and design of relevant model systems have been the subjects of extensive studies for decades. In recent years, there is a growing interest in exploiting efficient photodriven charge-separation systems in applications such as artificial light energy conversion into chemical potential, information processing, and organic photoimaging systems. For an efficient photoinduced charge separation to be achieved, it is critically important to prevent charge recombination (CR) of the geminate ion pair generated by photoexcitation. In general, ET rate is determined by several essential parameters such as reaction exothermicity or free energy gap (-AGO), separation and orientation of donor (D) and acceptor (A), internal and solvent reorganization energy, and solvent polarity.I4 The ET rate (1) Strauch, S.;McLendon, G.; McGuire, M.; Guarr, T. J. Phys. Chem. 1983, 87, 3579. (2) (a) Joran, A. D.; Leland, B. A.; Geller, 0.0.;Hopfield. J. J.; Dervan, P. B. J . Am. Chem. Soc. 1984, 106,6090. (b) Leland, B. A.; Joran, A. D.; Felker, P. M.; Hopfield, J. J.; Zewail, A. H.; Dcrvan, P. B. J . Phys. Chem. 1985,89, 5571. (c) Joran, A. D.; Leland, B. A,; Felker, P. M.; Zewail, A. H.; Hopfield, J. J.; Dervan, P. B. Nature 1987, 327, 508. (3) (a) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem. Soc. 1984,106, 5057. (b) McLendon, G.;Miller, J. R. J . Am. Chem. Soc. 1985, 107, 78 1 1.
0022-3654/9 1 /2095-6027$02.50/0
increases with an increase in -AGO (normal region) and reaches a maximum when -AGO matches the total reorganivttion energy.I0 However, it decreases at greater -AGO owing to an increased mismatch in the Franckxondon factor (inverted region). Thus, (4) Gould, I. R.; Ege, D.; Mattes, S.L.; Farid, S.J . Am. Chem. Soc. 1987, 109, 3194. (5) Hwang, J.-K.; Warshel, A. J . Am. Chem. Soc. 1987, 109, 715. (6) (a) Karas, J. L.; Lieber, C. M.; Gray, H. B. J. Am. Chem. Soc. 1988, 110, 599. (b) Axup, A. W.; Albin, M.; Mayo, S.L.; Grutchley, R. J.; Gray, H. B. J . Am. Chem. Soc. 1988,110,435. (7) Isied. S.S.;Vassilian, A.; Wishart, J. F.; Creutz, C.; Schwarz, H. A.; Sutin, N. J . Am. Chem. Soc. 1988, I IO, 635. (8) (a) Kakitani, T.; Mataga, N. Chem. Phys. 1985, 381. (b) Kakitani, T.; Mataga, N. J . Phys. Chem. 1985,89,4752. (c) Kakitani, T.; Mataga, N. J . Phys. Chem. 1986,90,993. (d) Yoshimori, A,; Kakitani, T.; Enomoto, Y.; Mataga, N. J . Phys. Chem. 1989,93,8316. (e) Kakitani, T.; Yoshimori, A.; Mataga. N. Electron Transfer in Organic, Inorganic, and Biological Systems. Adu. Chem. Ser., in press. (9) (a) Mataga, N.; Kanda, Y.; Okada, T. J . Phys. Chrm. 1986,90,3880. (b) Mataga, N.; Asahi, T.; Kanda, Y.; Okada, T.; Kakitani, T. Chcm. Phys. 1988, 127, 249. (c) Asahi, T.; Mataga, N. J . Phys. Chem. 1989, 93, 6575. (d) Ohno, T.; Yoshimura, A,; Mataga, N. J . Phys. Chrm. 1990, 91, 4871. (IO) (a) Marcus, R. A. Discuss. Faraday Soc. 1960,29,21. (b) Marcus, R. A. J . Chem. Phys. 1965,43,2654. (c) Levich, V. 0.Ado. Electrochem. Electrochem. Engl. 1966, 4, 249. (d) Kestner, N. R.; Logan, J.; Jortner, J. J . Phys. Chem. 1974, 78, 2148. (e) Ulstrup, J.; Jortner, J. J. Chem. Phys. 1975,63,4358. (f) Redi, M.; Hopfield, J. J . Chem. Phys. 1980, 72,6651. (g) Marcus, R. A.; Siders, P. J . Phys. Chem. 1982, 86, 622.
0 199 1 American Chemical Society
6028 The Journal of Physical Chemistry, Vol. 95, No. 15, 1991
the ET rate shows a bell-shape dependence on energy gapelo Therefore, a logical approach to an efficient charge separation is to choose a combination of D and A such that back ET occurs in the inverted region with a large - A G O , and forward ET occurs in the normal region with an appropriate driving force. A number of studies have focused on D-A systems where D and A are embedded in a rigid matrix"-I3 or they are separated by a rigid spacer with covalent bondings."-" Miller et al.ls showed the first experimental evidence for the bell-shape energy gap dependence in charge shift type ET reactions. Subsequently, the bell-shape relation was also proved for CR of radical ion pairs? Many studies have been reported on the photoinduced ET across the interfaces of some organized assemblies such as surfactant micellesI8 and ve~icles,'~ wherein some particular D and A species are expected to be separated by a phase boundary. We have previously shown that microphase structure of an amphiphilic polyelectrolyte in aqueous solution provides a photoinduced ET system, characteristics of which are very different from those of the conventional molecular assembly systenwm The micellelike microphase structure formed by an amphiphilic polyelectrolyte is "static" in nature as compared to the dynamic nature of conventional surfactant micelles,21although the stability of the microphase structure depends on the size, shape, and number (mole fraction) of the hydrophobic residues in the amphiphilic polyelectrolyte.21cAn amphiphilic random terpolymer of sodium 2-(acrylamido)-2-methylpropanesulfonate (AMPS) (41 mol a), styrene (St) (55 mol %), and 9-vinylphenanthrene (9-VPh) (4 mol %) units forms a stable microphase structure in aqueous solution, and pendant phenanthrene (Phen) residues are tightly encapsulated in the *compartment" of the hydrophobic aggregate of the St residues.20 Methylviologen (MV2+)added to an aqueous solution of this amphiphilic polyelectrolyte was electrostatically concentrated in the polymer microphase, but a close contact of MV2+ with the Phen residues was sterically hindered owing to the compartmentalization. Very fast photoinduced ET occurred (within ca. 30 ps) from the compartmentalized Phen to bound MV2+,whereas back ET was considerably slowed.2obTo account for these findings we have previously discussed two possibilities: (1) the compartmentalized Phen may have at least one acceptor situated close enough to give such a fast photoinduced ET, although the compartmentalized Phen moiety is separated from most of the bound MV2+species, and (2) the r-electron system of the St residues surrounding the Phen moiety in the hydrophobic microdomain may play a mediating role in the photoinduced ET ( 1 1 ) (a) Miller, J. R. Science (Wushingron, D.C.)1975, 189, 221. (b) Beitz, J. V.; Miller, J. R. J . Phys. Chem. 1979, 71,4579. (c) Miller, J. R.; Beitz, J. V. J . Phys. Chem. 1981, 74, 6746. (d) Miller, J. R.; Peaplcs, J. A.; Schmitt, M. J.; Closs, G. L. J . Am. Chem. Soc. 1982, 104,6488. (e) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J . Am. Chem. Soc. 1984, 106, 5057. (12) (a) Strauch, S.; McLcndon, G.;McGuire, M.; Guarr, T. J . fhys. Chem. 1983,87,3579. (b) Guarr, T.;McGuire, M.;Strauch, S.;McLendon, G. J . Am. Chem. Soc. 1983, 105,616. (13) (a) Milosavljevic, B. H.; Thomas, J. K. Chem. Phys. Lerf. 1985,114, 133. (b) Milosavljevic, B. H.; Thomas, J. K. J . Am. Chem. Soc. 1986, 108, 2513. (14) (a) Stein, C. A.; h i s , N . A,; Scitz, G . J . Am. Chem. Soc. 1982,101, 2596. (b) Pasman, P.; Koper, N . W.; Verhoeven, J. W. Red. Trav. Chim. fuys-Bas. 1982,101,363. (c) Mcs, G.F.; Van Ramcsdonk, H. J.; Verhoeven, J. W. J . Am. Chem. Soc. 1984, 106, 1335. (15) (a) Calcaterra, L. T.; Closs, G. L.; Miller, J. R. J . Am. Chem. SOC. 1983.105,670. (b) Miller, J. R.; Calcaterra, L. T.; Clm, G. L.J. Am. Chem. Soc. 1984, 106,3047. (16) (a) Wasielewski, M. R.; Niemczyk, M. P. J . Am. Chem. Soc. 1984, 106, 5043. (b) Wasielewski, M.R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985, 107, 1080. (17) Beratan, D. N . J . Am. Chem. SOC.1986, 108, 4321. (18) Thomas, J. K. The Chemistry of Excitation of Interjaces; ACS Monograph Series No. 181 : American Chemical Society, Washington, DC, 1984. (19) Fcndler, J . Membrane Mimeric Chemfsrry;Academic: New York, 1983. (20) (a) Morishima, Y.; Kobayashi, T.;Furui, T.;Nozakura, S.Mucromolecules 1981, 20, 1707. (b) Morishima, Y.; Furui, T.; Nozakura, s.; Okada, T.;Mataga, N. J . fhys. Chem. 1989. 93, 1643. (21) (a) Morishima, Y.; Itoh, Y.; Nozakura, S.Makromol. Chem. 1981, 182,3135. (b) Morishima, Y.; Kobayashi, T.;Nozakura, S.J . Phys. Chem. 1985.89,4081. (c) Morishima, Y.; Kobayashi, T.; Nozakura, S.f o / y m . J . 1989, 21. 267.
Morishima et ai. CHART I CII.
1
,J 1 :'="
:'=" NI I I
CI I,-C-CII, I
R=
-tCH2)
,,
CH3
1,J
:'="
NI I I
NII
13
CII,
1,
I I
p o l y ( A / L a / Py )
U
to MV2+ bound on the charged surface of the hydrophobic microdomain. In the present study, we further attempted to elucidate the origins of the compartmentalization effects on the photoinduced ET. We employed aliphatic hydrocarbons such as lauryl, cyclododecyl, and adamantyl groups to compartmentalize a hydrophobic chromophore to rule out the possibility of the r-electron mediating interaction. We presently chose pyrene (Py) as the chromophore. The amphiphilic polyelectrolytes prepared in the present study are illustrated in Chart I. The content of the hydrophobic monomer units in the terpolymers is 49 mol %, which assures the formation of the microphase structure while maintaining solubility in water.21c The content of the Py residue is limited to 1 mol % to avoid chromophore-chromophore interactions. For comparison, an amphiphilic copolymer poly(A/Py) was employed as a reference polymer, in which the Py residues are exposed to the water phase in aqueous solution.21b*c Recently, Delaire et a1.22"*b have reported an efficient charge separation in photoinduced ET from poly(methacry1ic acid) (PMA) bound diphenylanthracene (DPA) to MV2+ in an acidic aqueous solution. The significance of the hydrophobic protection of the chromophore by a compact coil of PMA in enhancing charge separation efficiency has been discussed by Webber and co-workers.22 These earlier studies suggest a promise of encapsulation of a chromophore in the hydrophobic compartment of an amphiphilic polyelectrolyte for design of an efficient photoinduced charge separation system. Experimental Section
I-(Pyrenylmethyl)methacrylamide (PyMAm). In a cooled autoclave, 5.09 g (22.1 mmol) of 1-pyrenecarboxaldehyde, 1.5 g of activated Raney nickel, 80 mL of cooled N,N-dimethylformamide (DMF), and 12 g (0.7 mol) of liquid ammonia were (22) (a) Delaire, J. A.; Rodgen, M. A. J.; Wcbber, S.E. Eur. folym. J . 1986,22.189. (b) Delaire, J. A,; Sanquer-Bamer, M.;Webbcr, S.E. J. Phys. Chem. 1988, 92, 1252. (c) Stramel, R. D.; Npuyen, C.; Webber, S. E.; Rodgen, M. A. J. J . fhys. Chem. 1988, 92, 2934. (d) Stramel, R. D.; Webber, S.E.; Rodgen, M. A. J. J . Phys. Chem. 1988, 92,6625.
Photoinduced Charge Separation placed. Hydrogen gas was introduced into the autoclave until the pressure reached 82 kg/cm3 at room temperature. Then the autoclave was heated at 75 "C, at which temperature the hydrogen pressure indicated about 95 kg/cm3,and the reaction mixture was magnetically stirred for 3 h. After the reaction, the mixture was filtered to remove the catalyst and was evaporated under reduced pressure. The residue was treated with 500 mL of aqueous 1 N HCl at ca. 90 "C to extract the product in the hydrochloride form. Upon cooling, 1-pyrenylmethylamine hydrochloride was crystallized from the extracting solution. The yield was 35.1% (2.08 g): NMR (DMSO-d,) (for the free amine form) 6 4.4 (s, 2 H), 7.9-8.6 (m, 9 H). Anal. Calcd. for C1,HI4NCl: C, 84.25; H, 5.72; N, 4.68. Found: C, 84.06; H, 5.72; N, 4.68. To a suspension of 2.37 g (8.85 "01) of 1-pyrenylmethylamine hydrochloride in 500 mL of benzene was added 34.0 g (0.177 mol) of triethylamine with vigorous stirring. The mixture was then cooled in an ice bath. To this stirred solution was added dropwise 9.20 g (0.104 mol) of methacryloyl chloride in 20 mL of benzene over a period of 30 min. After stirring for 7 h at room temperature the reaction mixture was filtered to remove triethylamine hydrochloride. The filtrate was washed successively with aqueous 1 N HCl, saturated NaCI, 1 M NaHC03, and again saturated NaCl aqueous solutions. The organic layer was dried over MgS0, overnight. The volatiles were removed under reduced pressure to give a crude product which was recrystallized from n-hexane. The crystals (PyMAm) obtained in 64.5% yield (1.71 g) were pale yellow needles (mp 146 "C): NMR (CDC13) 6 3.0 (s, 3 H), 5.2 (d, 2 H), 5.3 (s, 1 H), 5.7 (s, 1 H), 6.2 (s, 1 H), 7.9-8.4 (m, 9 H). Anal. Calcd. for C,,H,,NO: C, 84.25; H, 5.72; N, 4.60. Found: C, 84.25; H, 5.72; N, 4.68. MS, m / z 299. Cyclododecylmethacryhmide (CdMAm). To a solution of 25.0 g (0.136 mol) of cyclododecylaminein 1 L of benzene was added 18.1 g (0.179 mol) of triethylamine with vigorous stirring. To this stirred mixture was added dropwise 17.4 g (0.166 mol) of methacryloyl chloride in 30 mL of benzene over a period of 30 min under cooling in an ice bath. After stirring for 12 h at room temperature the reaction mixture was filtered to remove triethylamine hydrochloride. The filtrate was successively washed with aqueous 1 N HCl, saturated NaCl, 1 M NaHC03, and saturated NaCl solutions. The organic layer was dried over MgSO, overnight. The volatiles were removed under reduced pressure to give a crude product which was recrystallized from n-hexane. The product (CdMAm) obtained in 55.2% yield (18.9 g) was white needles (mp 157.5 "C): NMR (CDCI3) 6 1.1-1.8 (m, 23 H), 2.0 (s, 3 H), 4.1 (s, 1 H), 5.3 (s, 1 H), 5.6 (s, 1 H). Anal. Calcd. for C16H,N0, C, 76.31; H, 11.63; N, 5.71. Found: C, 76.31; H, 11.71; N, 5.61. Adamantyl"cryhmide (AdMAm). AdMAm is synthesized from adamantylamine and methacryloyl chloride in an analogous manner as CdMAm. The product obtained in 43.5% yield was white needles (mp 104.5 "C): NMR (CDC13) 6 1.6-2.0 (m, 15 H), 2.0 (s, 3 H), 5.2 (s, 1 H), 5.3 (s, 1 H), 5.6 (s, 1 H). Anal. Calcd. for C14H21NO:C, 76.67; H, 9.65; N, 6.39. Found: C, 76.76; H, 9.73; N, 6.46. hurylmethacrylamide (LaMAm). LaMAm was synthesized as reported previously.21c Co- and Terpolymers. A representative procedure for the terpolymerization is as follow. A glass ampule containing 1.76 g (8.35 mmol) (50 mol % on the basis of the total monomers) of AMPS, 2.06 g (8.18 mmol) (49 mol 7% on the basis of total monomers) of CdMAm, 0.06 g (0.2 mmol) (1 mol % on the basis of the total monomers) of PyMAm, 0.0146 g (0,0835 mmol) of AIBN, and 40 mL of DMF was outgassed on a vacuum line by five freeze-pumpthaw cycles and then sealed under vacuum. Polymerization was carried out at 60 OC for 8.5 h. The polymerization mixture was poured into a large excess of ether to precipitate the resulting polymer. The polymer was purified by reprecipitating from methanol into a large excess of ether. The polymer ..vas then dissolved in a dilute aqueous sodium hydroxide. The pH value was adjusted at 11 and the solution was dialyzed against pure water for a week and finally lyophilized. The conversion of the terpolymer was 19.5%.
The Journal of Physical Chemistry, Vol. 95, NO. 15. 1991 6029 Other co- and terpolymers were prepared and purified analogously. The copolymer compositions were determined by the N / C and S / C ratios. Measurements. Absorption spectra were recorded on a Shimadzu UV-2100 spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-O2A spectrofluorometer at room temperature. Fluorescence decay was measured by the correlated singlephoton-counting technique. The excitation source for the system was a synchronously pumped, cavity-dumped, mode-locked Nd:YAG dye laser. The time resolution of the instrument was 70 ps, and excitation was carried out at 355 nm. The fluorescence decay curves were analyzed by deconvoluting with the system response function and varying the parameters of the fitting function until the best least-squares agreement with experiments was o b tained. Nanosecond laser photolysis was performed by using a Qswitched Nd:YAG laser (Quantaray DCR-2) operated at third harmonic (150 mJ per flash at 355 nm with a 6-11s fwhm) as reported previously.2q Picosecond laser photolysis was performed by using a cresyl violet 670/DCM dye laser (Quantel PTL 10) pumped by the second harmonics of a mode-locked Nd:YAG laser (Quantel YG 501C). The output energy of the dye laser is about 1.5 mJ at 652 nm. The sample solutions were excited with the second harmonics of the dye laser. The picosecond continuum used to measure transient absorption spectra was generated by focusing the fundamental of the YAG laser into a D 2 0 cell. The pulse widths of the dye and YAG laser were about 13 and 20 ps, respectively. The monitoring light was detected by a multichannel photodiode array and digitized signals were transferred to a microcomputer. Typically, 20 shots of data were accumulated and then the transient absorbances were calculated. All the sample solutions for the spectroscopic measurements were deaerated by bubbling with Ar gas for 30 min.
Results Amphiphilic Co- and Terpolymers. The polymers prepared in the present study are characterized as "ideal copolymers", which means that the sequence distribution of the monomer units is completely random, and their composition is equal to the monomer feed composition. Therefore, the copolymer composition is directly determined by the feed ratio, which allowed us to prepare co-and terpolymers with well-defined composition. In aqueous solution, hydrophobic residues in an amphiphilic polyelectrolyte aggregate to form a micellelike microphase structure when hydrophobic interaction between the hydrophobic residues dominates electrostatic repulsion between charged segmentsa2' The balance of the hydrophobic (attractive) and electrostatic (repulsive) interactions depends on the content of the hydrophobic residues as well as their molecular size and hydrophobicity. We have previously ascertained that a copolymer of AMPS (56 mol %) and LaMAm (44mol %) forms the microphase structure in aqueous A content of 49 mol % lauryl groups in poly(A/La/Py) should be high enough to allow the polymer to form the microphase structure in aqueous solution. Since the numbers of aliphatic carbon atoms in the cyclododecyl and adamantyl groups are similar to that in the lauryl group, it is reasonable to assume that poly(A/Cd/F'y) and poly(A/Ad/Py) also form the microphase structure. The Py residues would be hydrophobically incorporated in the aggregates of the hydrophobic groups, thus giving rise to the compartmentalization of the Py chromophores in the hydrophobic microdomains. Spectroscopic Behavior. Fluorescence emitted from the Py residues provides useful information on the nature of their local environments. It is known that the ratio of the third to first vibrational fine structure, I J I , , of the pyrene fluorescence is dependent on local environmental polarity.23 (23) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chcm. Soc. 1977,99, 2039.
6030 The Journal of Physical Chemistry, Vol. 95, No. IS, 1991
Morishima et al. 10
m
2 4
2 -10 L; 10 Y
CT
t m
WFMLEffiTH Inin)
Figure 1. Fluorescence spectra for poly(A/Py) (-- -) and poly(A/Cd/Py) (T) in aqueous solution: Absorbances for the sample solutions were adjusted to 0.50 at 343 nm (excitation wavelength).
t-
r = ! 0 V C
TABLE I: Spectroscopic Data for the Py Residues in the Amphiphilic Co- and T ~ y w r ins Aqueous Solution polymer code polY(A1PY) poly(AILa/Py) polY(A1CdlPy)
relative fluorescence
X,,./nm4
IJIlb
intensitv
343 345 346 345
0.59 0.80 0.83 0.16
1.o 1.18 1.27 1.09
polY(A/Ad/Py) "Absorption maximum. bIntensity ratio of peak 3 to peak 1 in the fluorescence spectra. e Relative integrated emission intensity. Figure 1 compares fluorescence spectra of poly(A/Py) and poly(A/Cd/Py). In general, Py groups covalently bound to a polyelectrolyte tend to form excimer in aqueous solution even though the Py content in the polymer is very low. For example, Turro and Arora2' reported that poly(acry1ic acid) tagged with 1.5 mol '% Py group showed excimer emission in aqueous solution even when the polymer was ionized. The intensity ratio of monomer to excimer fluorescence, IM/Ie, was 4.9 at pH 8.*lc Since the Py content in the polymers is as low as 1 mol '% in the present study, the fluorescence spectra showed virtually monomer fluorescence in aqueous solution. Table I summarizes spectroscopic data. The I J I , ratio for poly(A/Py) is close to that reported for pyrene in aqueous solut i o ~ ~implying ?~ that the Py residues in the reference copolymer are exposed to the aqueous phase. On the other hand, the 13/1, ratios for the terpolymers are significantly higher than that for the reference copolymer, suggesting that the Py residues are located in less polar microenvironments. The hydrophobic compartmentalization was also reflected in fluorescence intensity; Le., the fluorescence intensity for the compartmentalized Py moieties was significantly higher than that for poly(A/Py). Fluorescence quantum yield of pyrene is known to increase as solvent polarity de~reases.2~ The absorption maximum for the compartmentalized Py moieties showed a 2-3-nm red shift as compared to that for poly(A/Py), suggesting also that the microenvironments around the Py groups are hydrophobic. There seems to be a significant difference in the apparent local polarity of the hydrophobic aggregate of different hydrophobic groups, the polarity decreasing in order of adamantyl > lauryl > cyclododecyl groups. A plausible explanation for this difference may be that the cyclododecyl groups closely surround the Py moiety in the hydrophobic microdomain, leading to an effective protection of the Py moiety from the aqueous phase. On the other hand, the adamantyl groups may not be able to sterically fit well with the Py moiety because they are rigid and have a three-dimensionally bulky structure. (24) (a) Turro, N.J.; Arora, K.S.Polymer 1986,27,783. (b) Arora, K. S.;Hwang, K.-C.;Turro, N . J. Macromolecules 1986, 19,2806. (c) Arora, K.S.;Turro,N . J. 1. folym. Scf.,folym. Phys. Ed. 1987, 25,243. (d) Arora,
K.S.; Turro, N.J. J. Polym. Sci., folym. Chem. Ed. 1987, 25, 259.
0
100
200
300
400
500
CHANNEL
Figure 2. Fluorescence decay profiles monitored at 400 nm for poly(A/Py) (a) and poly(A/Cd/Py) (b) in aqueous solution; excitation at 355 nm; 1 channel = 1.15 ns. TABLE 11: Double-Exponentirl Fits to Fluorescence D e u y for the Py Chromophore in the Amphiphilic Co- and Terpolymers Monitored at 400 nm in AQW Solution" polymer code i 1(nslla! (7): ns 148/0.916 136 polY(A1PY) 2910.084 201/0.142 168 poly(A/La/Py) 5510.258 23810.716 187 poly(A/Cd/Py) 5110.284 41/0.336 21910.664 159 poly(A/Ad/Py) "Excitation wavelength; 355 nm. bFitting function: I ( t ) = x u , exp(-t/i,). ?Averagelifetime defined by ( 7 ) = xa,i,. The effect of the microenvironment around the Py chromophore was also reflected in fluorescence decay data. As an example, a fluorescence decay profile for poly(A/Cd/Py) in aqueous solution is compared with that for poly(A/Py) in Figure 2. The decay for the Py chromophore in poly(A/Cd/Py) was considerably slowed as compared to that for the reference copolymer. Similar results were also observed for the terpolymers with the lauryl and adamantyl groups. The decay curves for the present polymers were not single exponential but best fitted to a double-exponential function. The values of the relative weight of the preexponential factor af and the lifetimes 7 , are listed in Table 11. The presence of shorter life component suggests that the microenvironments around the Py chromophore are not exactly uniform. The lifetimes for the longer life component, which is the major component, imply that the Py chromophore is compartmentalized in the hydrophobic microenvironment in the terpolymer systems and the local hydrophobicity of the microdomain decreases in order of cyclododecyl > lauryl > adamantyl groups. Charge-Transfer Complexation and Fluorescence Quenching. Pyrene is known to form a ground-state charge-transfer (CT) complex with methylviologen (MV2+) in aqueous solution.25 Figure 3 compares the absorption spectra of the terpolymers and the reference copolymer in aqueous solution in the presence of MV2+. For poly(A/Py) a strong C T band at 470 nm was observed. However, CT complexation was suppressed owing to the compartmentalization of the Py residue in the terpolymers. The (25) White, B. G. Trans. Faraday Soc. 1%9,65, 2000.
The Journal of Physical Chemistry, Vol. 95, No. 15, 1991 6031
Photoinduced Charge Separation
TABLE III: Fluorescence Quenching Constants and Absorbances at the Band for the AmpMpbWc Co- and Terpolymers with MV+in Aqueous Solution absorbance at 470 nmb
KW:
wlvmer code Poly(A/PY) poly(A/La/Py) PoMA/Cd/Py) PolY(A/Ad/PY)
104 M-I 4.4
0.082 0.047 0.038 0.034
10 6.7 8.1
'Apparent Stern-Volmer constant estimated from the initial slope of the plots in Figure 4. bDue to the CT band; [Py(residue)] = 0.25 mM, [MV2+] = 10 mM.
Wavelengthlnm
Figure 3. CT absorption bands for poly(A/Py) (l), poly(A/La/Py) (2), poly(A/Cd/Py) (3), and poly(A/Ad/Py) (4) in the presence of 10 mM MV2+ in aqueous solution; [Py(residue)] = 0.25 mM.
-0 0
X
-
1
Y
V
L
a
m
g o v)
m
a 1
Ch@I/pM
Figure 4. Stern-Volmer plots for the fluorescence quenching of poly(A/Py) (01, poMA/La/Py) (@, poly(A/Cd/Py) (01, and poIy(A/ Ad/Py) (A)by MV2+in aqueous solution: Absorbances for the sample solutions were adjusted to 0.50 at 343 nm (excitation wavelength).
extent of the CT suppression increased in the order of lauryl < cyclododecyl < adamantyl groups in the terpolymers. This order is not consistent with that of the local hydrophobicity around the Py moiety discussed above, but seems to be related to the structural rigidity of the hydrophobic groups. An important observation is that, despite the compartmentalization of the chromophore, the fluorescence from the Py residue in the terpolymers is more effectively quenched by MV2+ than that in the reference system (Figure 4). This trend is opposite to that observed for the CT formation; i.e., the compartmentalization led to a suppression of CT formation, while it led to an enhancement of fluorescence quenching. The fluorescence quenching is known to be due to photoinduced ET from Py to MV2+.26 Geometrical requirements such as distance and orientation for the CT formation are much more rigorous than those for ET. For the CT complexation D and A should come into ~ ~ ET can occur over contact in a face-to-face ~ r i e n t a t i o n ,while a range of D-A separations and orientation^.'^-'^ Being a hydrophobic dication, MV2+tends to be bound to the microphase of the terpolymer through electrostatic and also hydrophobic interactions. Therefore, the local concentration of MV2+ around the Py site should be very high, although a close contact with the Py moiety is sterically hindered. Therefore, it is reasonable to consider the efficient photoinduced ET can occur from the compartmentalized Py group to MV2+situated most closely ~
_
_
(26) Tazuke, S.; Kitamura, N.;Kawanishi, Y. J . Photochem. 1985, 29,
123
(27) Fkter, R. Organlc Charge-Transfer Complexes; Academic Press: London, 1970.
550
650
750
WAVELENGTH (nm)
Figure 5. Nano- and microsecond time-resolved transient absorption spectra for poly(A/Ad/Py) (a), poly(A/Cd/Py) (b), poly(A/La/Py) (c), and poly(A/Py) (d) in aqueous solution in the presence of 10 mM MV". Absorbances for the sample solutions were adjusted to 1.0 at 355 nm (excitation wavelength in the laser photolysis).
to it, despite the face-to-face contact of the Py group with MV2+ to give CT complex is suppressed. Table 111 lists the absorbances of the CT band and the apparent Stern-Volmer constants estimated from the initial slopes of the Stern-Volmer plots shown in Figure 4. The quenching efficiency did not parallel with the CT-band intensity. Among the hydrophobic group employed, the CT complexation was most effectively suppressed by the adamantyl group, while, importantly, efficient fluorescence quenching occurred. It appears that MV2+ can penetrate into a hydrophobic microdomain to some extent, depending on the rigidity or tightness of the hydrophobic aggregate. Furthermore, whether the Py moiety can come into face-to-face contact with the MV2+ species seems to depend on the steric hindrance of the hydrophobic residues in the aggregate. In the poly(A/Ad/Py) system, the close contact of MV2+ with the Py moiety to attain an optimal geometry for the CT complexation is most effectively hindered, but its approach to allow efficient fluorescence quenching is possible to occur. Photoinduced ET. Figure 5 compares time-resolved absorption spectra for the terpolymers and the reference copolymer in the presence of MV2+in aqueous solution. These data were obtained ~ in the 50 ns to 15 ps time regime on a Q-switched nanosecond laser photolysis system. Transient absorption peaks due to the cation radical of the Py residue (Pya+) and the methylviologen radical cation (MV'+) were observed at 460 and 600 nm, re-
Morishima et al.
6032 The Journal of Physical Chemistry. Vol. 95, No. 15, 1991 (a)
w V
I
U
m
0.023
p:
0.025
0 v)
m U
400
500
600
700
800
900
500
400
600
700
BOO
900
WAVELENGTH (nm) Figure 6. Picosecond time-resolved transient absorption spectra for poly(A/Cd/Py) (a) and for poly(A/Py) (b) in aqueous solution in the presence of IO mM MVZt. Transient absorbances at the maks are indicated in the figure. Absorbances for the sample solutions were adjusted to 1.0 at 325 nm (excitation wavelength in the picosecond laskr photolysis).
spectively. The yields of the transient cation radicals for the compartmentalized systems were much higher than that for the reference system. Furthermore, the terpolymers with cyclododecyl and adamantyl groups exhibited higher transient yields than did the lauryl terpolymer in this time region. The yields of the Py*+and MV'+ transients observed on the microsecond time scale essentially depend on the events in the picosecond and subnanosecond time regions. As a representative example for the terpolymers the picosecond data for poly(A/ Cd/Py) were compared with those for the poly(A/Py) reference system in Figure 6. The buildups of the 460- and 600-nm bands were completed 20 ps after the pulse excitation, indicating that extremely fast ET from the singlet-excited state of the Py moiety to MVZ+occurred on a time scale comparable to or shorter than the duration of the laser pulse (ca. 20 ps). For the poly(A/ Py)-MV2+ system the transient absorbance decayed very rapidly over the picosecond regime, while, in contrast, the poly(A/Cd/ Py)-MV2+ system showed a much slower decay. It should be noted here that in the initial 100-ps time region the transient at 460 nm decayed more rapidly than did the transient at 600 nm as can be seen from Figure 6. After about 100 ps, however, the transients at 460 and 600 nm decayed in parallel. This tendency for the parallel decay persisted until both the transients completely decayed in several hundred microseconds. We have no explanation for this rather anomalous initial decay behavior at the present stage, and this remains an open question. We are attempting to clarify this point in our continuing work. Since no laser power dependence was found in the picosecond transient spectra, we can rule out the possibility of biphotonic photoionization of the Py residue to generate hydrated electrons. The time profiles of the absorbance due to MV'+ at 600 nm for the poly(A/Cd/Py)-MVZ+ and poly(A/Py)-MV2+ systems are illustrated in Figure 7. The decays are biphasic and can be fitted to the double-exponential function A ( t ) = A(O)[a exp(-t/T,) + (1 - CY) exp(-r/r2)] (1)
@
I
0
I
1m
am
%m
v3D
DD
T i m e I ps Figure 7. Time profiles of transient absorbances at 600 nm (due to MV") for poly(A/Cd/Py)-MV2+ (a) and poly(A/Py)-MVZt (b) systems: The solid lines represent the best-fit curves from use of q 1 with the parameters listed in Table IV.
where A ( t ) and A(0) are the 600-nm absorbances at time f and t = 0, respectively. The fit of the absorbance decay data to eq 1 is illustrated in Figure 7. Similar results were obtained for other terpolymer systems. The best-fit parameters for eq 1 are listed in Table IV. The decay parameters for the initial fast decay component ( l / r , ) for the cyclcdcdecyl and adamantyl polymers are of the order of IO9 S-I, which is 1 order of magnitude smaller than that for the reference polymer. The fractions of the fast decay component (a)for the compartmentalized systems (a= 0.5-0.6)
Photoinduced Charge Separation TABLE I V Relative Weinht of the Raxpoacatld Factors and the
The Journal of Physical Chemistry, Vol. 95, No. 15. 1991 6033 the reference copolymer system khl is almost 1 order of magnitude greater than kd.
Discussion The effect of the compartmentalization on the CR of the primary ion pair is significantly different for different hydrophobic groups employed in the present study. The cyclododecyl and adamantyl groups are much more effective to reduce the CR rate than is the lauryl group (Table V). A plausible explanation for the reduced effectiveness of the lauryl group in protecting the Py chromophore is that the hydrophobic aggregate of the flexible poly (A/PY) 50 7.1 9.2 0.12 aliphatic linear chains has some fluidity owing to their confor8.3 6.0 16 0.40 poly(A/La/Py) mational freedom, which may allow the Py moiety to move 0.8 0.43 polY(A/Cd/Py) 1.1 0.9 somewhat freely and MV2+ to penetrate into the Py site in the 1.3 1.3 8.6 0.50 polY(A/Ad/Py) lauryl aggregate. This also explains the reduced effectiveness of #Charge escape yield. the lauryl group in suppressing the CT complexation and enhanced quenching of Py fluorescence in the lauryl terpolymer system are smaller than that for the reference system (a = 0.88), indi(Table 111). cating that the compartmentalized systems have a tendency for The lifetime of a geminate ion-pair state depends on the distance accumulation of MV'+. and orientation of D and A in an ion pair, in other words, on We have previously proposed a kinetic model to explain the whether the ion pair is "tight" or "loo~e".~A loose ion pair has biphasic decay of the transients observed in the picosecond and a longer lifetime. The primary ion pairs, Py'+//MV'+(MV2+), subnanosecond time region.20b According to the kinetic model, for the poly(A/Cd/Py)-MV2+ and poly(A/Ad/Py)-MV2+ systhe following reaction steps are taken into account: tems may have a looser structure with a longer separation distance because of the more restricted accessibility of MV2+ to the Py Py/ /MV2+( MV2+) 2 Py*/ /MV2+(MV2+) (2) site than for the poly(A/La/Py)-MV2+ system. In the poly(A/Py)-MV2+ system, a tight ion pair can be formed because a free access of MV2+ to the Py site allows the ion pair to attain Py*/ /MV2+( MV2+) -% Py'+//MV'+(MV2+) (3) an optimal distance and orientation, thus giving rise to a shorter lived geminate ion pair. Py'+//MV'+(MV2+) Py//MV2+(MV2+) (4) Webber et a1.22Chave reported an analogous result as to the photoinduced ET from a poly(methacry1ic acid) (PMA) bound Py'+//MV'+(MV2+) -!!Py'+//MV2+(MV'+) . (5) pyrene (PMA-Py) to MV2+. They found that no charge separation occurred on a nanosecond time scale, although efficient Py'+//MV2+(MV'+) Py//MV2+(MV2+) (6) "static" quenching of pyrene fluorescence occurred. On the other hand, they observed efficient charge separation in the photoinduced Here, Py//MV2+ represents a pair of a compartmentalized Py ET of 9,lO-diphenylanthracene (DPA) and perylene (PER) bound moiety and the nearest situated MV2+species, and MV2+desigto PMA;22b*d i.e., charge-pair photoproducts persisted for hundreds nated in parentheses represents a MV2+ species located most of microseconds for the PMA-DPA-MV2+ and PMA-PERclosely to the MV2+species in the Py//MV2+ pair. Photoinduced MV2+systems. They have discussed their results in terms of the ET would occur from a singlet-excited Py moiety (Py*) to the "loose or tight" geminate ion pair;9 Le., if chromophore-MV2+ nearest situated MV2+ (step 3). A primary ion-pair state thus pairs are in intimate contact, as speculated for the PMA-Py-MV2+ produced may undergo a fast CR (step 4) or a charge escape (step system, photoinduced ET would lead to static quenching and not 5 ) . The kinetic model assumes that the charge escape occurs via to charge separation. They have pointed out that the sterical an electron exchange between M V and MV2+bound sideby-side on the surface of the hydrophobic m i c r o d ~ m a i n . ~ ~ +If~the * $ ~ ~ properties of the chromophore itself may also play a significant role of whether or not the approach of the quencher to attain a charge escape takes place via step 5 , back ET via step 6 would tight ion-pair state is hindered. They have implied that, as combe slowed depending on the separation between Py'+ and the pared to DPA, pyrene should be completely protected to attain (MV") sites. The rate constants for steps 4, 5 , and 6 can be efficient charge separation. The results of the present study are related to the parameters in eq 1 as follows:Mb qualitatively in agreement with their implication. kb.1 = a(1/71) + (1 - a)(l/TZ) (7) In the present system, reaction exothermicity for CR (-AGOCR) in polar media is estimated to be 1.84 V by using electrochemical kb,2 /72 (8) data (E, 2(Py'+/Py) = 1.16 V vs SCE in acetonitrilem and Eo(MV2+/hV") = -0.68 V vs SCE in water20). Energy gap for (9) kd (1 - a)[(1/71) - (1/72)1 charge separation (-AGOcs) is estimated to be 1.49 V by use of Charge escape yield tl is given by the lowest excited singlet energy (AEW = 3.33 eV) for pyrene.)' These values suggest that very rapid forward ET occurs but the = kd/(kd + k b , l ) (10) CR is also very rapid in polar media. When the Py chromophore The rate constants and the charge escape yields thus estimated is confined to a hydrophobic compartment, the primary charge are listed in Table V. The CR rate is slowed and the charge separation and CR processes would occur in a less polar microescape yield is increased owing to the compartmentalization of environment. It should be noted that in less polar media, -AGOCR the Py midue. The CR rate constants for the terpolymer systems becomes larger, while -AGOcs becomes smaller. In addition, are of the order of 1O1O s-I and are smaller than that for the reorganization energy becomes smaller in less polar media. Thus, reference copolymer system by 1 order of magnitude. For the for the pyrene-MV2+ET system in less polar media, the CR rate terpolymer systems kb,, is almost comparable to kdrwhereas for may become slower as a result of an increased inverted effect. Since both -AGOcs and reorganization energy become smaller in less polar media, the forward ET rate is less dependent on the (28)(a) Gauliello, J. G.; Ghosh. P. K.; Bard, A. J. J . Am. Chcm. Soc. polarity than is the CR rate. 1985,107,M27.(b) Atherton, S.J.; Tsuka, K.; Wilkins, R. G. J . Am. Chcm.
2 2
Soc. 1986, 108,3380.
(29)(a) Takuma, K.;Sakamoto, T.; Nagamura, T.; Matsuo, T. J . Phys. Chcm. 1981.85.619. (b) Matsuo, T.; Sakamoto, T.; Takuma. K.; Sakura, K.; Ohraka, T. J . Phys. Chcm. 1981,85, 1277. (c) Matsuo, T.Pure Appl. Chcm. 1982,54* 1693.
(30) Pysh. E. S.;Yang, N. C. J . Am. Chcm. Soc. 1963, 32, 2124. (31)Murov, S. L. Hundbook ofPhofochcmisfry; Marcel Dekker: New York, 1976.
J . Phys. Chem. 1991, 95,6034-6040
6034
Therefore, the reduced kh, for the compartmentalized systems may be qualitatively explained in terms of a combined effect of the microenvironmental polarity and the sterically hindered loose ion-pair state. The values of kd should depend on the local concentration of bound MVZ+around the Py site. At a higher local concentration of MV2+ the rate of an electron relay from MV'+ to the nearest MV2+ dication is faster.29 Among the hydrophobic groups employed, the lauryl group exhibited the largest kd,suggesting that MV2+ can penetrate into the lauryl aggregate so as to increase the local concentration of MV2+ around the Py site. It is reasonable to consider that kb,2is mainly determined by and (MV'+) formed as a result of the the distance between electron relay (step 6 ) . Thus, kb,2may also depend on the local concentration of MV2+. At a lower acceptor concentration a single-step electron relay from MV'+ to (MV2+) shown in step 5 will create a longer separation between and (MV'+), leading to a reduced kb,2.A larger kb,2 observed for the lauryl terpolymer suggests a higher MV2+ local density around the Py site.
w+
w+
Conclusion The Py chromophore was compartmentalized in the hydrophobic aggregate of the amphiphilic polyelectrolytes (Chart I)
in aqueous solution. The added MV2+ was concentrated on the surface of the hydrophobic aggregate. However, the compartmentalized Py moiety was sterically protected from a close contact with bound MV2+. The rate of CR of the primary ion pair was slowed by an order of magnitude, while very fast photoinduced forward ET occurred (kET> 10" s-l), thus giving rise to charge separation that persisted for hundreds of microseconds. In sharp contrast, the reference polymer, in which the Py residue is not compartmentalized, showed a rapid CR with a rate constant of the order of 10" s-I. The reduced CR rate for the compartmentalized system was qualitatively explained in terms of a sterically hindered loose structure for the primary ion pair. The lauryl group was significantly less effective in compartmentalizing the pyrene moiety than the cyclododecyl and adamantyl groups. An optimal compartmentalization of the chromophore may be achieved by a judicious choice of the hydrophobic group, which will lead to an optimal efficiency for charge separation. Registry No. (A)(La)(Py) (copolymer), 134131-50-3; (A)(Cd)(Py) (copolymer), 134110-08-0 (A)(Ad)(Py) (copolymer), 13411049-1; PyMAm, 134110-06-8;CdMAm, 13675-35-9; AdMAm, 134110-07-9; 1pyrenecarboxaldehyde, 3029- 19-4; 1-pyrenylmethylamine hydrochloride, 93324-65-3; 2-adamantylamine, 13074-39-0; methylviologen, 1910-42-5; ammonia, 7664-41-7; methacryloyl chloride, 920-46-7; cyclododecylamine, 1502-03-0.
Electrochemical Study of Kinetics of Electron Transfer between Synthetic Electron Acceptors and Reduced Moiybdoheme Protein Sulfite Oxidase L. A. Coury, Jr.; Royce W. Murray,* Kenan Laboratories of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290
J. L. Johnson, and K. V. Rajagopalan Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 2771 0 (Received: February 1 1 , 1991)
The electrocatalytic oxidation of sulfite using transition-metal complex or cytochrome c mediators and the molybdoheme protein sulfite oxidase (SO) as catalysts is used to measure the rate constant (k12)for cross electron transfer between mediator and enzyme. The enzyme heme is the reaction site for cobalt phenanthroline complex mediators; the Mo fragment obtained by tryptic cleavage of native enzyme is unable to reduce the Co species. Within a structurally similar set of cobalt complexes, when (mediator SO) A E O ' < 0.2 V, k I 2varies with reaction free energy according to relative Marcus-Hush theory; we find k l l = 1 X lo8 M-I s-I for the apparent electron self-exchange rate constant of the enzyme heme site. Attempts to achieve faster heme turnover rates result in a shift of rate control to another process, possibly the internal Mo Fe electron transfer. Ionic strength effects indicate a negative charge at the heme site and substantial electrostatic rate effects.
-
Electron self-exchange rate constants (kll) for redox proteins can be derived from NMR and EPR measurements' on mixtures of oxidized and reduced Drotein, and from ~ross-electron-transfer2~ reaction rates with small-moleculereagents! These studies include "blue" cop r proteins,' flavodoxin,5 high-potential iron proteins (HiPIPs), p"and several cytochromes c . ~ , ~The ~ . relationships ~ between reaction free energy and intramolecular electron-transfer rates between protein redox sites and protein-attached synthetic redox sites,' and in protein/protein complexes? have also been intensively investigated in recent years. In the case of the molybdoheme protein sulfite oxidase (SO), prior reports9J0 contain no information on reaction free energy effects or enzyme self-exchange dynamics. Elucidation of the electron-transfer dynamics of biomolecules like SO is relevant to *Corresponding author. 'Present address: Department of Chemistry, Duke University, Durham, NC 27706. 0022-3654/91/2095-6034$02.50/0
understanding the complexities of metabolic redox pathways;" appreciating the reactivity of enzymes like SO is also of interest (1) (a) Dixon, D. W.; Hong, X.;Wochler, S.E.;Mauk, A. G.; Sishta, B. P.J . Am. Chem. Soc. 1990,112,1082-1088, and referenccs cited thmin. (b) Grocntveld, C. M.;Canters, G. W. J. Biol. Chem. 190,263, 167-173. (c) Dahlin, S.;Reinhammar, B.; Wilson, M.T. Blochem. J . 1984,218,609-614. (2) (a) Marcus, R. A. Annu. Reo. Phys. Chem. 1964,15, 155-196. (b) Marcus, R. A.; Sutin, N.Biochim. Biophys. Acta 1985,811,265-322. (3) Wherland, S.; Gray, H. B. In Biological Aspecrs ofInorgank Chemistry; Addison, A. W.,Cullen, W. R.,Dolphin, D., James, B. R., Eds.; Wiley: New York, 1977; pp 289-368. (4) (a) McArdle, J. V.; Coylc, C. L.; Gray, H. B.; Yoneda, G. S.;Holwerda, R. A. J . Am. Chem. Soc. 1977,99,2483-2489. (b) McArdle, J. V.; Gray, H. B.; Crcutz, C.; Sutin, N. J . Am. Chem. Soc. 1974,96,5737-5741. ( 5 ) (a) Simondsen, R.; Tollin, G. Biochemistry 1983,22. 3008-3016. (b) Meyer, T. E.;Rzysiecki, C. T.; Watkins, J. A,; Bhattacharyya, A,; Simondscn, R. P.; Cusanovich, M. A.; Tollin, G. Proc. Natl. Acad. Sci. 1983, 80, 6740-6744. (c) Cusanovich, M.A.; Meyer, T. E.;Tollin, G . Ado. Inorg.
Biochem. 1988, 7, 37-91.
0 1991 American Chemical Society
