J. Phys. Chem. 1982, 86,1400-1406
1400
potential drop in the film occurs, and this increases the band bending in the n-Si and eliminates the recombination, as compared to highly resistive films. Higher photocurrents and fill factors and a linear dependence of the limiting photocurrent on light intensity similar to that found with the bare electrode before passivation are expected in such a system. The greater improvement in stability when a concentrated electrolyte is used can be attributed to a decrease in water activityls and a concomitant decrease in the rate of water penetration through the film.
behaves as a fairly stable photoanode, as previously reported. Excitation at higher light intensities (typical solar levels) leads to a rapid passivation by the reaction of the photogenerated holes and the small amounts of water that penetrate the film,even in the presence of a substance such as iodine which decreases the resistance of the film. Acknowledgment. The support of this research, which is a joint project with Professor A. B. P. Lever of York University, by the Office of Naval Research and the Solar Energy Research Institute is gratefully acknowledged. M.C.-A. gratefully acknowledges the support of his fellowship by the Programa de Superacion del Personal Academio of the Universidad Nacional Autonoma Mexico.
Conclusion At low light intensities, n-Si coated with HzPc or CuPc
Spectroscopic Studies of Dicyanohemin in Cationic Micelles M. J. Mlnch’ hparfment of Chemistry, Unlversliy of the Paclflc, Stockton, Callfornkr 952 1 1
and Gerd N. La Mar hpattment of Chemistry, Unlversky of Callfornkr at Davls, Davis, Califomkr 95616 (Received: August 25, 1981; In Flnal Form: November 9, 1981)
The incorporation of low-spin iron(II1) dicyanoprotoporphyrin IX (dicyanohemin) into micelles of hexadecyltrimethylammoniumbromide (CTABr)was studied by visible absorption spectroscopy, fluorescence,and ‘Hand 13C NMR spectroscopy. Dicyanohemin interacts strongly with CTABr, forming especially stable premicellar aggregates when the dicyanohemin and CTAl3r concentrations are roughly comparable, even if the CTABr concentration is 2 orders of magnitude more dilute than the critical micelle concentration. The spectroscopic properties of the premioellar aggregate indicate porphyrin-porphyrin interaction within the complex. A more conventional dicyanohemin-micelle aggregate is observed at higher CTABr-to-dicyanoheminratios. The spectroscopic properties of both types of aggregates can be rationalized in terms of possible structures.
The high-field NMR spectroscopy of paramagnetic metalloporphyrins in membranes or lipid bilayers has not been investigated. This is surprising in light of the importance of membrane-bound metalloporphyrins in electron transport and photosynthesis and the clear success of the use of NMR with paramagnetic pr0teins.l As in the case of proteins, a study of a model system containing many of the features of a metalloporphyrin-membrane complex is a necessary first step. We report here a study of paramagnetic low-spin iron(II1) porphyrin complexes in a model membrane system, micelles of hexadecyltrimethylammonium bromide (CTABr). The hydrophobic interior of a micelle has been often likened to that of a lipid bilayer? and considerable work on the interactions between small aromatic molecules and CTAJ3r micelles as models of interactions within membranes has been r e p ~ r t e d . ~ - ~ Some older work on the spectroscopic and chemical properties of hemin incorporated into micelles!.’ including (1) La Mar, G. N. In “BiologicalApplicationsof Magnetic Resonance“; Shulman, R. G., Ed.; Academic Press: New York, 1979; pp 305-43. (2) Fendler, J. H. “Membrane Mimetic Chemistry”; Wiley-Interscience: New York, 1981. (3) (a) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973.95.3262-72. (b) E r i h n . J. C.: Gdberp. G. Acta Chem. Scand. 1966,20,‘2019-27. (c) L k n , J. W.; M&d, L. KJ.Phys. Chem. 1974. 78. 834-9. (4)B b t o n , C. A.; Minch, M. J. J. Phys. Chem. 1974, 78, 1490-7. (5) Minch, M. J.; Giaccio, M.; Wolff, R. J. Am. Chem. Soc. 1976,97, 3766-72. Minch, M. J.; Shah, S. S. J. Org. Chem. 1979, 44, 3252-5.
one 60-MHz ‘HNMR study? has also been reported, but these earlier studies were handicapped by an inability to define the local environment of the hemin except in quite general terms. In paramagnetic heme complexes, the unpaired spin of the iron perturb the ‘H and 13Cresonances of groups near the iron permitting the resolution of many of these peaks outside the often poorly resolved diamagnetic spectral region.’ These hyperfine shifted resonances are exceptionally sensitive to the heme micr0environment.l The direct interaction of the metalloporphyrin with surfactant molecules alters the balance of forces within a micelle, provoking changes in micelle structure and properties. Hence, changes in the chemical shifts and the line widths of both heme and surfactant resonances as a function of heme-to-surfactant concentration ratio can be interpreted in terms of changes in heme ligation, solvation, and orientation in the micelle. Electronic spectra taken over the same concentration range further characterize the system. A detailed understanding of how guest molecules alter surfactant aggregation is necessary to the design of micelles (6) (a) Yanagi, Y.; Sekuzu, I.; Orii, Y.; Okunuki, K. J.Biochem. (Tokyo) 1972, 71,47-56. (b) H h , w.; Fendler, J. H. J. Chem. SOC.,Dalton Tram 1974,238-44. (c) Simplicio, J.Biochem. (Tokyo) 1972,11,2525-8, 2529-34. (7) Simplicio, J.; Schwenzer, K.; Maenpa, F. J.Am. Chem. SOC.1975, 97, 7319-26. (8) Simplicio, J.; Schwenzer, K. Biochemistry 1973, 12, 1923-9.
0 1982 American Chemical Society
Dicyanohemin in Cationic Micelles
TABLE I: Effect of Tetra-n-ButylammoniumBromide on the Visible Spectrum of Dicyanohemina lo4[Bu,NBr], absorbance M h = 419 nm h = 460 nm 1.020 0.235 0.1 0.970 0.249 2.0 0.910 0.260 28.0 0.910 0.242 Optical densities (absorbances) at 419 and 460 nm for 1.5 X 10.' M hemin in 0.002 M NaCN a t pH 9.7 with various concentrations of tetra-n-butylammonium bromide.
and vesicles that mimic the specialized functions of biological membranes such as artificial photosynthesis? The results reported here reveal how large, planar, aromatic species, especially metalloporphyrins, are situated in cationic micelles and, by analogy, in lipid bilayers.
Experimental Section Hexadecyltrimethylammoniumbromide (CTABr) was recrystallized repeatedly by the method of Grunwald.lo Purified CTABr gave a flat concentration-surface tension profile above the cmc, in agreement with previous work.& Purchased samples of hexadecyltrimethylammonium chloride (CTACl), hemin chloride, and protoporphyrin IX (sodium salt) were shown by NMR to be free from impurities and were used without additional purification. All inorganic compounds were of analytical reagent quality. Optical spectra were recorded on Cary 17 or Cary 218 recording W-vis spectrometers with l-cm quartz cuvettes and thermostated cell compartments. All solutions were prepared within 1h of use and stirred just before use to ensure uniform mixing. The concentrations of ligands and solution pH values were selected to ensure complete coordination of the hemin iron in the presence of CTABr. The emission spectrum of 6.5 X lo4 M protoporphyrin IX dianion in 3.3 X 10-3M KOH was determined as a function of CTACl concentration. A recording fluorescence spectrometer operating at an excitation wavelength of 395 nm was used. No effort was made to exclude oxygen from the solutions. 'H NMR spectra were obtained on a Nicolet 360-MHz pulsed FT NMR spectrometer using up to 16K data points over a 10-KHz bandwidth. Chemical shifts were measured vs. the residual water line or 0.5% dioxane. The residual water line chemical shift was calibrated as a function of temperature and pH to DSS,2,2-dimethyl-2-silapentane5-sulfonate, in the presence of CTABr. 13CNMR spectra were obtained on a Nicolet 200-MHz spectrometer operating at 50.3 MHz. Proton noise decoupling was interrupted between acquisitions to minimize sample heating. Spin-lattice relaxation times, Tl, were measured by using a 180°-~-90' inversion recovery technique where 7 is the time interval between the 180' and 90' pulses.'l The delay between successive accumulations was at least 10Tl. 13C chemical shift values were measured vs. internal dioxane (67.86 ppm). Results Visible Spectra. Hemin is normally dimeric in alkaline aqueous solution12but yields a monomer, presumably with (9) Tunuli, M. S.;Fendler, J. H. J. Am. Chem.Soc. 1981,103,2507-13. (10) Duynstee, E.F.;Grunwald, E. G. J. Am. Chem. SOC.1959, 81, 4540-2. (11) Martin, M. L.; Martin, G. J.; Delpuech, J. J. "Practical NMR Spectroscopy"; Heyden Press: Philadelphia, PA, 1980; p 257. (12) Goff, H.; Morgan, L. 0. Znorg. Chem. 1976,15, 2062.
The Journal of Physical Chsmlstry, Vol. 86, No. 8, 1982 1401
structure 1, in the presence of micelle-forming concenCH=CH,
C
H
3
W
CH,
C
t
p
CH,
I
I
CH,
i
3
I
CH2
1,L = H,O,OH 2, L = CN
3,L=Im trations of CTABr.'t8 Reaction of hemin with cyanide or imidazole affords low-spin complexes 2 and 3, which are monomeric at low concentration M).13 Each has a visible spectrwn clearly distinct from that of the aquated dimer. In this study the visible spectra of the dimer in water (0.1 M phosphate buffer) and complexes 1-3 in high concentrations of CTABr were found to agree with those reported by Simplicio.8 The situation with lower CTABr concentrations proved to be more complicated. Dicyanohemin (2) interacts strongly with CTABr, forming two different types of aggregates with different visible spectra depending on the surfactant-to-porphyrin concentration ratio (Figure 1). Type I aggregate is characterized by a decrease in the intensity of the Soret band at 422 nm and the appearance of a broad absorption centered at 460 nm which is at maximum amplitude when the concentration of CTABr is only threefold greater than that of 2 (Figure 2). This aggregate is especially stable and forms even if the CTABr concentration is 2 orders of magnitude more dilute than the generally observed critical M CTABr.14 micelle concentration; cmc = 9.2 X Hydrophobic solutes have been shown to induce micellization before,15 but this is an especially pronounced example of premicellar aggregation. Type I aggregate is also formed at CTABr concentrations near or above its cmc if a nearly equivalent concentration of 2 is present. At higher CTABr-bdicyanohemin ratios, a second type of complex is formed where the band at 460 nm is replaced by a more intense one at 430 nm (Figure 1). For M 2, the absorbance at 430 nm increases with CTABr concentration over the range 10-4-10-3 M, but higher CTABr concentrations do not increase absorbance. This saturation effect is usually o b ~ ~ when e d dyes are taken into micelles and strongly suggests that the type I1 aggregate is a conventional micelle containing one metalloporphyrin molecule. At CTABr concentrations where the surfactant-toporphyrin ratio is greater than the average micelle aggregation number, N = 61,14each metalloporphyrin molecule is taken into a separate micelle so that additional surfactant does not change the environment of the heme molecule. The electrostatic interaction between 2 (-3 formal charge) and the surfactant quaternary ammonium head (13) La Mar, G. N.; Minch, M. J.; Frye, J. S. J.Am. Chem. SOC. 1981, 103, 5383-8. (14) Fendler, J. H.; Fendler, E. J. 'Catalysis in Micellar and Macromolecular Systems"; Academic Press: New York, 1975; p 20. (15) For other examples of this important but relatively unexplored phenomenon, see: Atik, S. S.; Singer, L. A. J. Am. Chem. SOC.1979,101, 6759-61. Reeves, R. L.;Harkaway, S. A. "Micellization, Solubilization and Microemulaions"; Mittal, K., Ed.;Plenum Press: New York, 1977; Vol. 2, pp 819-34.
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Minch and La Mar
The Journal of physical Chemktty, Vol. 86, No. 8, 1982
Figure 1. Visible spectra of 1.5 X 9.4 x 104; (c)1.8 x 10-6; (D) 9.4
M hemin wtth 0.02 M NaCN at pH 9.7 wtth various concentrations (M) of CTABr: (A) no CTABr; (B)
x 10-5; (E) 9.4 x 10-4. groups accounts for much of the stability of these aggregates, but changes in the visible spectra reveal the importance of hydrophobic interactions between the porphyrin ring and the adjacent surfactant hydrocarbon chains. The addition of a large excess of tetra-n-butylammonium bromide, a less hydrophobic salt that does not form micelles at these concentrations, to a solution of 2 causes an effect not nearly so pronounced as that observed with CTABr (Table I). Analogous changes in the &ret bands of other iron(II1) porphyrin complexes, including ones with neutral ligands, are caused by CTABr concentrations well below ita cmc (Table 11). In all cases there is a significant decrease in the extinction coefficient upon the addition of CTABr concentrations as low as 2 X lod M; a t higher concentrations the &ret band is again intense but shifted to longer wavelengths. Also the fluorescence of 6 X lo4 M protoporphyrin IX (as disodium salt) is quenched by premicellar concentrations (
