Langmuir 2001, 17, 5781-5790
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Reactivity of Benzophones in the Different Binding Sites of Sodium Cholate Aggregates O. Rinco, M. H. Kleinman, and C. Bohne* Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6 Received April 9, 2001. In Final Form: July 5, 2001
The reactivity of excited benzophenone (Bp) and 4,4′-dimethylbenzophenone (DMBp) in the presence of sodium cholate (NACh) aggregates was studied by following the kinetics of the excited triplet states and the ketyl radicals of both ketones. At low NACh concentrations only primary aggregates are present in solution. The ketyl radicals were formed from the reaction of the triplet ketones bound to the primary sites. The decay of the ketyl radicals occurred primarily by the reaction of these radicals in water. Some longlived triplets included in the primary aggregate were also observed. A different reactivity was observed at higher NACh concentrations where secondary aggregates are present. The binding process associated with the secondary binding sites is much faster than for the primary site. The hydrogen abstraction reaction in the secondary binding site is too slow to compete with the exit process, but self-quenching competes efficiently, leading to a shortening of the triplet lifetimes for both ketones. From the dynamic results it was concluded that only a small number of NACh molecules (8-13) are needed to define the primary and secondary binding sites.
Introduction Bile salt aggregates are intriguing supramolecular systems because of their complexity. Bile salts, such as sodium cholate (NACh, Scheme 1), are molecules that have hydrophobic methyl groups located on the concave side of the molecular framework and hydrophilic hydroxyl groups on the convex face. This distribution of substitutents gives bile salts their amphiphilic character that is responsible for their aggregation in water. Early experiments with bile salts were done1-6 at about the same time that the structure of micelles derived from linear surfactants, such as sodium dodecyl sulfate (SDS) or cetyltrimethylammonium halides, was established. For this reason, the structure of bile salt aggregates was rationalized with the micellar model where the supramolecular structure is viewed as a pseudo-phase that contains a continuum of binding regions with progressively changing properties. Our preliminary results7,8 indicate that bile salt aggregates do not fit this model. Despite the numerous thermodynamic and structural studies2,3,6,9-32 reported * Corresponding author: Phone 250-721-7151; Fax 250-721-7147; E-mail
[email protected]. (1) Carey, M. C.; Small, D. M. J. Colloid Interface Sci. 1969, 31, 382. (2) Small, D. M.; Penkett, S. A.; Chapman, D. Biochim. Biophys. Acta 1969, 176, 178. (3) Small, D. M. The Physical Chemistry of Cholanic Acids. In The Bile Salts; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971; Vol. 1, p 249. (4) Chen, M.; Gra¨tzel, M.; Thomas, J. K. Chem. Phys. Lett. 1974, 24, 65. (5) Chen, M.; Gra¨tzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1975, 97, 2052. (6) Mukerjee, P.; Cardinal, J. R. J. Pharm. Sci. 1976, 65, 882. (7) Ju, C.; Bohne, C. Photochem. Photobiol. 1996, 63, 60. (8) Ju, C.; Bohne, C. J. Phys. Chem. 1996, 100, 3847. (9) Campanelli, A. R.; Candeloro de Sanctis, S.; Chiessi, E.; D’Alagni, M.; Giglio, E.; Scaramuzza, L. J. Phys. Chem. 1989, 93, 1536. (10) Conte, G.; Di Blasi, R.; Giglio, E.; Parretta, A.; Pavel, N. V. J. Phys. Chem. 1984, 88, 5720. (11) Esposito, G.; Giglio, E.; Pavel, N. V.; Zanobi, A. J. Phys. Chem. 1987, 91, 356. (12) Hofmann, A. F.; Mysels, K. J. Colloids Surf. 1988, 30, 145. (13) Janich, M.; Lange, J.; Graener, H.; Neubert, R. J. Phys. Chem. B 1998, 102, 5957.
Scheme 1
over the years, the structure of bile salt aggregates is still ill-defined at the molecular level, and competing models are proposed. The studies on the binding dynamics of guests to bile salt aggregates8 provided new structural information showing that dynamic studies are essential to characterize the supramolecular features of these aggregates. We established that bile salt aggregates have defined binding sites with distinct properties.7,8 Hydrophobic molecules, such as naphthalene, bind to a hydrophobic site to which (14) Jover, A.; Meijide, F.; Rodrı´guez Nu´n˜ez, E.; Va´squez Tato, J. Langmuir 1997, 13, 161. (15) Kawamura, H.; Murata, Y.; Yamaguchi, T.; Igimi, H.; Tanaka, M.; Sugihara, G.; Kratohvil, J. P. J. Phys. Chem. 1989, 93, 3321. (16) Kratohvil, J. P. Adv. Colloid Interface Sci. 1986, 26, 131. (17) Kratohvil, J. P.; Hsu, W. P.; Jacobs, M. A.; Aminabhavi, T. M.; Mukunoki, Y. Colloid Polym. Sci. 1983, 261, 781. (18) Kratohvil, J. P.; Hsu, W. P.; Kwok, D. I. Langmuir 1986, 2, 256. (19) Li, G.; McGown, L. B. J. Phys. Chem. 1993, 97, 6745. (20) Li, G.; McGown, L. B. J. Phys. Chem. 1994, 98, 13711. (21) Mazer, N. A.; Carey, M. C.; Kwasnick, R. F.; Benedek, G. B. Biochemistry 1979, 18, 3064. (22) Meyerhoffer, S. M.; McGown, L. B. Anal. Chem. 1991, 63, 2082. (23) Meyerhoffer, S. M.; McGown, L. B. J. Am. Chem. Soc. 1991, 113, 2146. (24) Meyerhoffer, S. M.; Wenzel, T. J.; McGown, L. B. J. Phys. Chem. 1992, 96, 1961. (25) Nithipatikom, K.; McGown, L. B. Anal. Chem. 1988, 60, 1043. (26) O’Connor, C. J.; Wallace, R. G. Adv. Colloid Interface Sci. 1985, 22, 1. (27) Oakenfull, D. G.; Fischer, L. R. J. Phys. Chem. 1977, 81, 1838. (28) Oakenfull, D. G.; Fischer, L. R. J. Phys. Chem. 1978, 82, 2443. (29) Zakrzewska, J.; Markovic, V.; Vucelic, D.; Feigin, L.; Dembo, A.; Mogilevsky, L. J. Phys. Chem. 1990, 94, 5078. (30) Zana, R. J. Phys. Chem. 1978, 82, 2440. (31) Zana, R.; Guveli, D. J. Phys. Chem. 1985, 89, 1687. (32) Hashimoto, S.; Thomas, J. K. J. Colloid Interface Sci. 1984, 102, 152.
10.1021/la010526c CCC: $20.00 © 2001 American Chemical Society Published on Web 08/24/2001
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Langmuir, Vol. 17, No. 19, 2001 Scheme 2
access of molecules from the aqueous phase is restricted. This binding site has the properties of host-guest complexes. More hydrophilic molecules, such as xanthone, bind primarily in a fairly hydrophilic environment within the aggregate from which exit is much faster than from the hydrophobic site. The dynamic properties of this hydrophilic site are similar to those observed for micelle binding. Therefore, bile salt aggregates have simultaneously very well-defined and restricted binding sites and generic binding environments. The presence of binding sites with discrete properties makes these aggregates more sophisticated self-organized structures than micelles. For example, conceptually one can imagine that sequential chemical reactions can be compartmentalized in different sites of the bile salt aggregate and reaction intermediates can migrate between the discrete sites without having to exit into the aqueous phase. In this work we describe the dynamic behavior of transients formed from the photochemistry of benzophenone (Bp) and 4,4′-dimethylbenzophenone (DMBp) when included in bile salt aggregates. Irradiation of Bp leads to n, π* excited triplet states that can abstract hydrogens from suitable donors (Scheme 2).33,34 The objective of the present work was to use reactive probe molecules (Bp and DMBp) that could be located in either of the binding sites and determine how binding to NACh aggregates affects their reactivities. In addition, we employed DMBp because it is expected to have a similar reactivity to Bp, but it is a more hydrophobic guest. Experimental Section NACh, (98%, Aldrich), sodium nitrite (NaNO2, 97%+, Aldrich), sodium chloride (NaCl, BDH), DMBp (Acros, purity checked by GC; g99%), methanol (Aldrich, spectrograde), 2-propanol (Aldrich, spectrograde), dichloromethane (Aldrich, reagent grade), nitrous oxide (Praxair, USP), and oxygen (Praxair) were used as received. Bp (Aldrich, 99%+) was recrystallized from methanol/ petroleum ether, and its purity was checked by GC (g99%). Water was distilled and deionized (Sybron-Barnstead). NACh solutions (10-40 mM) were prepared by dissolving the appropriate amounts of NACh in water. Aqueous Bp and DMBp solutions with and without NACh were prepared in the presence of 0.2 M NaCl, because the size of the bile salt aggregates depends on the ionic strength of the medium and the bile salt concentration.16,18,21,35 Bp and DMBp are not very soluble in water, and special care has to be taken to prepare the aqueous solutions for these ketones. A film of Bp was deposited on the sample vial by dissolving Bp in methanol and evaporating the solvent with a stream of air while manually rotating the flask. Water or NACh solutions were added to this sample vial, and the solutions were stirred overnight. In the case of DMBp, the solid was stirred in water or in solutions containing NACh. The excess ketone was removed by filtration for both preparation procedures. The (33) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; Blackwell Scientific Publications: Oxford, 1991. (34) Turro, N. J. Modern Molecular Photochemistry; Benjamin/ Cummings Publishing Co.: Menlo Park, CA, 1978. (35) Shurtenberger, P.; Mazer, N.; Ka¨nzig, W. J. Phys. Chem. 1983, 87, 308.
Rinco et al. amount of ketone solubilized depends on the NACh concentration, and the ketone concentrations were determined from absorption spectra (see below). The Bp and DMBp concentrations in water were
