Oxidative Destruction of Vesicles of a Functionalized Surfactant

(bleach), ceric ammonium nitrate, hydrogen peroxide, and sodium ... hypochlorite and ceric ammonium nitrate, the giant vesicles were destroyed within ...
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Oxidative Destruction of Vesicles of a Functionalized Surfactant David A. Jaeger* and Xiaohui Zeng Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071-3838 Received June 23, 2003. In Final Form: July 22, 2003 Both giant and small unilamellar vesicles of functionalized surfactant 1 (potassium O,O′-didodecylphosphorodithioate) were subjected to oxidation, which converts 1 into disulfide-linked dimer 4 [bis(O,O′didodecylthiophosphoryl) disulfide], with concomitant destruction of the vesicles. Sodium hypochlorite (bleach), ceric ammonium nitrate, hydrogen peroxide, and sodium hydrogenperoxymonocarbonate were used as oxidizing agents. As determined by 31P NMR, the oxidations of small unilamellar vesicles of 1 to 4 with sodium hypochlorite and ceric ammonium nitrate were complete within 5 min, and they were slower with the other two oxidizing agents. The oxidations of giant vesicles with the same reagents, which were followed in real time by phase-contrast optical microscopy, displayed the same relative rates. With sodium hypochlorite and ceric ammonium nitrate, the giant vesicles were destroyed within ca. 16 and 50 s, respectively. The results suggest that surfactant 1’s vesicles have potential as storage and release devices.

Introduction Numerous studies of vesicle chemistry have been reported. Most of these studies1 have involved submicroscopic vesicles (diameter ca. 20-200 nm),2 and several studies3,4,5 have involved giant vesicles (GVs, diameter ca. 10-200 µm).6 A notable feature available to GV but not to submicroscopic vesicle chemistry is the ability to observe the effects of chemical processes on the vesicles themselves in real time by optical microscopy. On the other (1) For examples, see: (a) Menger, F. M.; Caran, K. L.; Seredyuk, V. A. Angew. Chem., Int. Ed. Engl. 2001, 40, 3905. (b) Menger, F. M.; Azov, V. A. J. Am. Chem. Soc. 2000, 122, 6492. (c) Moss, R. A.; Jiang, W. Langmuir 1997, 13, 4498. (d) Moss, R. A.; Okumura, Y. J. Am. Chem. Soc. 1992, 114, 1750. (e) Ghirlanda, G.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1993, 58, 3025 and references therein. (2) Fendler, J. H. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982; Chapter 6. (3) (a) Jaeger, D. A.; Schilling, C. L., III; Zelenin, A. K.; Li, B.; KubiczLoring, E. Langmuir 1999, 15, 7180. (b) Jaeger, D. A.; Clark, T., Jr. Langmuir 2002, 18, 3495. (c) Jaeger, D. A.; Li, B.; Clark, T., Jr. Langmuir 1996, 12, 4314. (4) For recent examples, see: (a) Menger, F. M.; Seredyuk, V. A.; Kitaeva, M. V.; Yaroslavov, A. A.; Melik-Nubarov, N. S. J. Am. Chem. Soc. 2003, 125, 2846. (b) Do¨bereiner, H.-G.; Petrov, P. G.; Riske, K. A. J. Phys. Condens. Matter 2003, 15, S303. (c) Menger, F. M.; Seredyuk, V. A.; Yaroslavov, A. A. Angew. Chem., Int. Ed. Engl. 2002, 41, 1350. (d) Nurminen, T. A.; Holopainen, J. M.; Zhao, H.; Kinnunen, P. K. J. J. Am. Chem. Soc. 2002, 124, 12129. (e) Takakura, K.; Toyota, T.; Yamada, K.; Ishimaru, I.; Yasuda, K.; Sugawara, T. Chem. Lett. 2002, 404. (f) Luisi, P. L. Anat. Rec. 2002, 268, 208. (g) Dol, G. C.; Tsuda, K.; Weener, J.-W.; Bartels, M. J.; Asavei, T.; Gensch, T.; Hofkens, J.; Latterini, L.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Angew. Chem., Int. Ed. Engl. 2001, 40, 1710. (h) Tsumoto, K.; Nomura, S. M.; Nakatani, Y.; Yoshikawa, K. Langmuir 2001, 17, 7225. (i) Walde, P.; Ichikawa, S. Biomol. Eng. 2001, 18, 143. (j) Ghosh, S.; Lee, S. J.; Nakatani, Y.; Ourisson, G.; Ito, K.; Akiyoshi, K.; Sunamoto, J. J. Chem. Soc., Chem. Commun. 2000, 267. (k) Veronese, A.; Luisi, P. L. J. Am. Chem. Soc. 1998, 120, 2662 and references therein. (5) For examples, see: (a) Jeon, Y. J.; Bharadwaj, P. K.; Choi, S.; Lee, J. W.; Kim, K. Angew. Chem., Int. Ed. Engl. 2002, 41, 4474. (b) Zepick, H. H.; Blo¨chliger, E.; Luisi, P. L. Angew. Chem., Int. Ed. Engl. 2001, 40, 199. (c) Holopainen, J. M.; Angelova, M. I.; Soderlund, T.; Kinnunen, P. K. J. Biophys. J. 2002, 83, 932. (d) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. Engl. 1998, 37, 3433. (e) Menger, F. M.; Lee, S. J. Langmuir 1995, 11, 3685. (f) Menger, F. M.; Gabrielson, K. J. Am. Chem. Soc. 1994, 116, 1567. (g) Menger, F. M.; Balachander, N. J. Am. Chem. Soc. 1992, 114, 5862. (6) For overviews of GVs, see: (a) Giant Vesicles; Luisi, P. L., Walde, P., Eds.; Wiley: New York, 1999. (b) Menger, F. M.; Keiper, J. S. Curr. Opin. Chem. Biol. 1998, 2, 726. (c) Menger, F. M.; Angelova, M. I. Acc. Chem. Res. 1998, 31, 789. (d) Menger, F. M.; Gabrielson, K. D. Angew. Chem., Int. Ed. Engl. 1995, 34, 2091.

hand, the course of submicroscopic vesicle but not that of GV processes can often be followed by spectroscopic methods. Among the studies of GV chemistry, some have involved processes that resulted in GV destruction/damage.3,5 In particular, we have reported3a the synthesis of functionalized surfactant 1 (potassium O,O′-didodecylphosphorodithioate) and the reactions of both its GVs and small unilamellar vesicles (SUVs)2 with 2 (2-chloroethyl phenyl sulfide) to give 3 at 25 °C (eq 1). The conversion of vesicular

1 into 3 resulted in the destruction of the vesicles, since neutral 3 cannot support vesicle formation. In the SUV reactions, the conversion of 1 to 3 was ca. 70% complete after 19 h, and in the GV reactions, most of the GVs were destroyed during the period from 10 to 17 h.3a Herein we report a study of another chemical reaction of 1’s GVs and SUVs, namely, their oxidation to disulfidelinked dimer 4 (eq 2), which results in vesicle destruction,

and in particular, observable GV destruction during periods as short as ca. 16 s. The oxidation has been effected with several oxidizing agents, including sodium hypochlorite (bleach), ceric ammonium nitrate (CAN), hydrogen peroxide, and sodium hydrogenperoxymonocarbonate (HOOCO2Na). This reaction necessarily destroys 1’s vesicles, because neutral 4 cannot support vesicle formation. Results and Dicussion In both water and a pH 8.0 (0.052 M) phosphate buffer, SUVs were prepared by sonication of a dispersion of 1 and

10.1021/la0351109 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/18/2003

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Table 1. Oxidations of 1’s SUVs and GVs to 4 at 23 °C SUVs small scalea oxidizing agent

solvent

NaOCl (NH4)2Ce(NO2)6 H2O2 HOOCO2Na

pH 8.0 buffer water pH 8.0 buffer pH 8.0 buffer

reaction

timec

SUVs preparative scalea

% conversion of

5 min 5 min 3h 24 h

100 100 ca. 90 ca. 85

1d

reaction time 10 min 10 min 4h 24 h

% yield of 98 ( 3 88 ( 1 82 ( 2 82 ( 4

GVsb 4e

reaction timef ca. 16 s ca. 50 s ca. 260 min ca. 9 h

a [1] ) 0.010 M, and the ratio of equivalent amounts of oxidizing agent and 1 was 2:1, respectively; see the text for complete details. See the text for experimental details. c With NaOCl and (NH4)2Ce(NO2)6, the conversion of 1 to 4 was almost certainly 100% at times less than 5 min; see the text. d Determined by 31P NMR analysis of duplicate reaction mixtures. e Average values for duplicate reactions. f Time required for the destruction of all GVs, as observed by phase-contrast optical microscopy. b

multilamellar GVs3a,7 by hydration of a thin smear of solid 1. The GVs had diameters of ca. 25-100 µm in both solvents and were stable for g5 days at 23 °C. As we reported previously,3a the SUVs in water had hydrodynamic diameters of ca. 70 nm and were stable for g2 days, and the phase transition temperature of 1 vortexed in water is 16.3 °C. SUVs of 0.010 M 1 were oxidized to 4 in the pH 8.0 phosphate buffer or water at 23 °C with sodium hypochlorite, CAN, hydrogen peroxide, and HOOCO2Na, which was prepared from hydrogen peroxide and sodium bicarbonate (eq 3).8 In each reaction, the ratio of equivalent amounts of oxidizing agent and surfactant 1 was 2:1, respectively. In a set of small (NMR) scale reactions, the conversion of 1 into 4 was monitored by 31P NMR. The signal for 1 (δ 111.7) decreased as that for 4 (δ 85.3) increased, and no other signals were detected during the reaction times given in Table 1, other than that at δ 3.5 for phosphate of the buffer. The conversion of 1 to 4 was 100% complete within 5 min with sodium hypochlorite and CAN and almost certainly at much shorter reaction times, based on the results with GVs (see below). The conversion of 1 to 4 was ca. 90 and 85% complete after 3 and 24 h with hydrogen peroxide and HOOCO2Na, respectively. Thus the oxidation was much faster with sodium hypochlorite and CAN than with the other two oxidizing agents. In a set of preparative scale reactions, product 4 was isolated by column chromatography. The results of both sets of reactions are given in Table 1.

HOCO2- + H2O2 h HOOCO2- + H2O

(3)

GVs of 1 were oxidized to 4 at 23 °C with the same oxidizing agents, ratio of equivalent amounts, and solvents as used in the SUV oxidations. After the addition of the oxidizing agent, each reaction mixture was observed in real time by phase-contrast optical microscopy. The reaction times required for destruction of the GVs were ca. 16 s with sodium hypochlorite, ca. 50 s with CAN, ca. 260 min with hydrogen peroxide, and ca. 9 h and HOOCO2Na, as summarized in Table 1. Figure 1 contains phase-contrast photomicrographs of a GV of 1 that has been subjected to oxidation with sodium hypochlorite. Parts a and b of Figure 1 show the GV before and at 4 s after the addition of sodium hypochlorite, at which time it is evident that GV damage is under way. Parts c and d of Figure 1 show the remains of the GV at 11 and 16 s after the addition of the oxidant, documenting that the GV has been destroyed within 16 s. The same oxidation with sodium hypochlorite was performed in the pH 8.0 phosphate buffer prepared in D2O, and after 5 min (7) Menger, F. M.; Lee, S. J.; Keiper, J. S. Langmuir 1996, 12, 4479. (8) (a) Bennett, D. A.; Yao, H.; Richardson, D. E. Inorg. Chem. 2001, 40, 2996. (b) Richardson, D. E.; Yao, H.; Frank, K. M.; Bennett, D. A. J. Am. Chem. Soc. 2000, 122, 1729. (c) Yao, H.; Richardson, D. E. J. Am. Chem. Soc. 2003, 125, 6211.

Figure 1. Phase-contrast photomicrographs of a GV of surfactant 1 undergoing oxidation with sodium hypochlorite at 23 °C, recorded (a) before, and at (b) 4 s, (c) 11 s, and (d) 16 s after the addition of sodium hypochlorite; scale bar ) 50 µm.

the reaction mixture was transferred to an NMR tube from the microscope slide assembly used for observation by optical microscopy (see Experimental Section). A 31P NMR spectrum of the reaction mixture contained only one singlet at δ 86.0, which corresponds to 4, in addition to the phosphate buffer signal (see above). Thus it is evident that GV destruction results from the conversion of surfactant 1 into disulfide-linked dimer 4, which, as noted above, cannot support vesicle formation. The oxidative destruction of 1’s GVs with CAN, as observed by phase-contrast optical microscopy, was comparable (not shown), occurring within ca. 50 s. As noted above, the destruction of 1’s GVs was slower with hydrogen peroxide and HOOCO2Na than with sodium hypochlorite and CAN. Figure 2 contains a photomicrograph of GVs undergoing oxidation, recorded at 42 min after the addition of hydrogen peroxide. Even though destruction of the GVs required about ca. 260 min with hydrogen peroxide (Table 1), it is evident that observable damage to the GVs begins at shorter reaction times. Analogous observations (not shown) were made in the GV oxidations with HOOCO2Na. In a GV oxidation with hydrogen peroxide, the reaction mixture was plunge-frozen into liquid N2 at 45 min after the addition of the oxidant, and then it was lyophilized. The residue was suspended in CDCl3 and filtered. By 31P NMR analysis of the filtrate, the conversion of 1 to 4 was 71%. In the oxidation of vesicular 1 to 4 by CAN in water, the actual oxidizing agent is almost certainly Ce4+. But the identities of the actual oxidizing agents in the pH 8 buffer with sodium hypochlorite, hydrogen peroxide, and HOOCO2Na cannot be stated with certainty. The possible oxidizing agents at pH 8 with sodium hypochlorite include

Oxidative Destruction of Vesicles

Figure 2. Phase-contrast photomicrograph of GVs of surfactant 1 undergoing oxidation with hydrogen peroxide at 23 °C, recorded at 42 min after the addition of hydrogen peroxide; scale bar ) 50 µm. -OCl itself as well as HOCl, Cl , Cl -, and Cl O,9 and with 2 3 2 hydrogen peroxide, they are H2O2 itself and perhaps OOH.10a With HOOCO2Na, the possible oxidizing agents include HOOCO2- itself and perhaps HOOCO2H,10b as well as H2O2 and -OOH, since HOOCO2- is in equilibrium with H2O2 (eq 3, K ) 0.32 M-1).8b In general, oxidation by an anionic species (e.g., -OCl, Cl3-, -OOH, and HOOCO2-) should be impeded by its electrostatic repulsion from the negatively charged interfaces of vesicular 1. On the other hand, oxidation by Ce4+ should be facilitated by its electrostatic attraction to the interfaces. The results of Table 1 show that the oxidations of vesicular 1 to 4 with HOOCO2Na are slower than those with hydrogen peroxide, in contrast to the greater reactivity of the former in the oxidations of aryl sulfides to sulfoxides,8a which proceed by nucleophilic attack of the sulfide at the electrophilic oxygen of HOOCO2-.8a Given the repulsion of HOOCO2- from the interfaces of vesicular 1 noted above, hydrogen peroxide, which is in equilibrium with HOOCO2- (eq 3), is likely the major oxidizing agent in reactions with HOOCO2Na. Note that all but one of the oxidations were performed in a pH 8 phosphate buffer. Therefore, given the formation of HOOCO2- from bicarbonate and hydrogen peroxide (eq 3), it is appropriate to address the likelihood of the formation of dihydrogen and monohydrogenperoxymonophosphate (H2PO5- and HPO52-, respectively)11 in the phosphate buffer. Since it is known that hydrogen peroxide does not convert H2PO4- or HPO42- into the corresponding peroxy anions, the possibility of these species as oxidizing agents can be discounted in reaction mixtures containing hydrogen peroxide.12,13 Also, H2PO5- and HPO52- are not formed in reaction mixtures containing sodium hypochlorite either, since it is known that H2PO5- oxidizes Clto Cl2.13 We have not studied the mechanism of the oxidation of vesicular 1 to disulfide-linked dimer 4. It is known that the mechanism of thiol (RSH) to disulfide (RSSR) oxidation

(9) For a discussion of the equilibria involving these species, see: Jaeger, D. A.; Clennan, M. W.; Janrozik, J. J. Am. Chem. Soc. 1990, 112, 1171 and references therein. (10) Relevant pKa values are as follows: (a) HOCl, 7.5; H2O2, 11.7 (Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley-Interscience: New York, 1988). (b) HOOCO2H, ca. 4; HOOCO2-, ca. 10.6 (ref 8a). (11) The pKa values of peroxymonophosphoric acid (H3PO5) are 1.1, 5.5, and ca. 13 (25 °C) (Battaglia, C. J.; Edwards, J. O. Inorg. Chem. 1965, 4, 552). (12) Husain, S.; Partington, J. R. Trans. Faraday Soc. 1928, 24, 235. (13) Peroxymonophosphoric acid can be formed by the reaction of hydrogen peroxide and phosphorus pentoxide (Mamantov, G.; Burns, J. H.; Hall, J. R.; Lake, D. B. Inorg. Chem. 1964, 3, 1043).

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varies with the oxidizing agent.14 Possibilities include sequences such as (a) oxidation of RSH/RS- to RS‚, followed by radical coupling to give RSSR,15 and (b) nucleophilic displacement by sulfur at the active oxygen of a peroxy species to give sulfenic acid (RSOH), followed by its reaction with RSH to afford RSSR.16 The pKa of (MeO)2PS2H is 1.55 in 93:7 water-ethanol,17 and that of (C12H25O)2PS2H in vesicular 1 should be somewhat higher, based on comparisons of the pKa values of nonaggregated and aggregated long-chain carboxylic acids.18 It is possible that within vesicular 1, the O,O′-didodecylphosphorodithioate anions are partially protonated to give (C12H25O)2PS2H in both water and the pH 8 buffer. The above results suggest that surfactant 1’s vesicles have potential as storage and release devices.19 Watersoluble compounds could be entrapped within their aqueous compartments and then released upon demand as 1 is oxidized to 4. As mentioned above, there have been previous reports of the destruction/damage of GVs effected by chemical processes,3,5 and two of them involve oxidation of the GV-forming surfactant.5a,b Also, note that in its oxidation to disulfide-linked dimer 4, surfactant 1 functions as a chemodegradable surfactant,20 since it is converted into a nonsurfactant product by a chemical reaction. Summary SUVs and GVs of functionalized surfactant 1 were subjected to oxidation to give disulfide-linked dimer 4, which results in their destruction. The oxidations were much faster with sodium hypochlorite and CAN than with hydrogen peroxide and HOOCO2Na. As observed by phasecontrast optical microscopy, the GVs were destroyed within periods as short as ca. 16 s with sodium hypochlorite and ca. 50 s with CAN. The results suggest that 1’s vesicles have potential as storage and release devices. Experimental Section General Procedures and Materials. 1H (400 MHz) and 13C (100.6 MHz) NMR spectra (23 °C) were recorded in CDCl3 with residual CHCl3 (δ 7.27) and CDCl3 (δ 77.00) as internal standards (relative to Me4Si), respectively. 31P NMR (162.1 MHz) spectra were recorded in CDCl3, the pH 8.0 phosphate buffer in D2O, and D2O with 85% H3PO4 as external standard. All J values are in hertz. Surfactant 1 was prepared as previously described.3a The pH 8.0 phosphate buffer (0.052 M) was prepared by mixing 100 mL of 0.10 M KH2PO4 and 93.4 mL of 0.10 M NaOH. The same buffer in D2O was prepared from 0.10 M KH2PO4 in D2O and 0.10 M NaOH in D2O. The H2O2 and NaOCl solutions were standardized iodometrically21 before use. Sonication was performed with a Branson 2200 (125 W) ultrasonic cleaner. Extracts were dried over Na2SO4. FAB high-resolution mass spectrometry (HRMS) was performed at the Washington University Resource for Biomedical and Bioorganic Mass Spectrometry, Department (14) Tarbell, D. S. In Organic Sulfur Compounds; Kharasch, N., Ed.; Pergamon: New York, 1961; Vol. 1, Chapter 10. (15) Capozzi, G.; Modena, G. In The Chemistry of the Thiol Group; Patai, S., Ed.; Wiley: New York, 1974, Chapter 17. (16) Evans, B. J.; Doi, J. T.; Musker, W. K. J. Org. Chem. 1990, 55, 2337. (17) Kabachnik, M. I.; Mastrukova, T. A.; Shipov, A. E.; Melentyeva, T. A. Tetrahedron 1960, 9, 10. (18) (a) Cistola, D. P.; Hamilton, J. A.; Jackson, D.; Small, D. M. Biochemistry 1988, 27, 1881. (b) Cistola, D. P.; Small, D. M.; Hamilton, J. A. J. Biol. Chem. 1987, 262, 10971. (19) For examples, see: (a) Spratt, T.; Bondurant, B.; O’Brien, D. F. Biochim. Biophys. Acta 2003, 1611, 35. (b) Sumida, Y.; Masuyama, A.; Takasu, M.; Kida, Y.; Nakatsuji, Y.; Ikeda, I.; Nojima, M. Langmuir 2001, 17, 609. (c) Thomas, J. L.; You, H.; Tirrell, D. A. J. Am. Chem. Soc. 1995, 117, 2949. (20) Jong, L. I.; Abbott, N. L. Langmuir 2000, 16, 5553. (21) Standard Methods of Chemical Analysis, 6th ed.; Welcher, F. J., Ed.; R. E. Krieger Pub. Co.: Malabar, FL, 1975; Vol. 2.

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of Chemistry, Washington University. Elemental analyses were performed by Atlantic Microlab, Norcross, GA. Oxidation of Vesicular 1. Oxidation experiments were performed as described below with sodium hypochlorite, ceric ammonium nitrate, hydrogen peroxide, and sodium hydrogenperoxymonocarbonate. Each experiment was at least duplicated and gave comparable results from run to run. Small-Scale Oxidation of 1’s SUVs with Sodium Hypochlorite in the pH 8.0 Phosphate Buffer. A mixture of 4.9 mg (0.0097 mmol) of 1 and 1.00 mL of the pH 8.0 phosphate buffer in D2O was sonicated in a 5-mm NMR tube at 55 °C for 15 min to give a translucent solution. The 31P NMR spectrum of this solution contained one singlet at δ 111.7, corresponding to 1,3a in addition to the phosphate buffer singlet at δ 3.5. Then 14.5 µL (0.00980 mmol) of 0.676 M NaOCl (Clorox bleach) was added, and the NMR tube was shaken vigorously to give a cloudy mixture, which was held at 23 °C. A 31P NMR spectrum of the reaction mixture recorded at 5 min after the addition of the bleach solution contained only one singlet at δ 85.3, corresponding to 4. Preparative-Scale Oxidation of 1’s SUVs with Sodium Hypochlorite in the pH 8.0 Phosphate Buffer. A mixture of 0.505 g (1.00 mmol) of 1 and 99.0 mL of the pH 8.0 phosphate buffer was sonicated at 55 °C for 15 min to give a translucent solution. Then 1.45 mL (0.980 mmol) of 0.676 M NaOCl was added in one portion to the solution with stirring at 23 °C. After the reaction mixture was stirred at 23 °C for 10 min, it was extracted with four 50-mL portions of EtOAc. The combined extracts were washed with 50 mL of H2O, dried, and rotary evaporated to give 0.511 g of an oil. This oil was chromatographed on a 20 cm × 2 cm (i.d.) column of silica gel (ICN 02776, 60 Å, 32-63 µm) packed in hexane and eluted with 1:75 (v/ v) EtOAchexane to give 0.465 g (100%) of 4 as a yellow oil, which was pure by 1H NMR (CDCl3). Another reaction performed identically gave a 95% yield of 4: 1H NMR (CDCl3) δ 4.22 (m, 4 H, 2 CH2O), 4.11 (m, 4 H, 2 CH2O), 1.75 (p, J ) 7.0, 8 H, 4 CH2CH2O), 1.27 (m, 72 H, 4 (CH2)9), 0.89 (t, J ) 6.8, 12 H, 4 CH3); 13C NMR (CDCl3) δ 68.89, 68.83, 31.92, 29.96, 29.87, 29.65, 29.60, 29.53, 29.36, 29.20, 25.52, 22.69, 14.12; 31P NMR (CDCl3) δ 86.0 (s). Anal. Calcd for C48H100O4P2S4: C, 61.89; H, 10.82. Found: C, 62.00; H, 10.83. FAB HRMS (2-nitrophenyl octyl ether matrix) calcd for C48H101O4P2S4 (M + H) 931.6058, found 931.6118. Small-Scale Oxidation of 1’s SUVs with Ceric Ammonium Nitrate in Water. This reaction was performed as for the above small-scale oxidation with sodium hypochlorite, using a mixture of 5.0 mg (0.0099 mmol) of 1 and 1.00 mL of D2O, to which was added 39.2 µL (0.020 mmol) of 0.51 M ceric ammonium nitrate in D2O. A 31P NMR spectrum of the reaction mixture recorded at 5 min after the addition of the oxidant contained only one singlet at δ 85.6, corresponding to 4. Preparative-Scale Oxidation of 1’s SUVs with Ceric Ammonium Nitrate in Water. This reaction was performed as for the above preparative scale oxidation with sodium hypochlorite, using 0.505 g (1.00 mmol) of 1 and 96.0 mL of H2O. To the resultant solution after sonication, 3.90 mL (1.99 mmol) of 0.51 M ceric ammonium nitrate in H2O was added dropwise over 5 min. After the reaction mixture was stirred for 10 min, it was worked up as above to give 0.459 g of an oil that upon chromatography gave 0.416 g (89%) of pure 4. Another reaction performed identically gave an 88% yield of 4. Small-Scale Oxidation of 1’s SUVs with Hydrogen Peroxide in the pH 8.0 Phosphate Buffer. This reaction was performed as above, using a mixture of 5.1 mg (0.010 mmol) of 1 and 1.00 mL of the pH 8.0 phosphate buffer in D2O, to which was added 1.2 µL (0.010 mmol) of 8.43 M H2O2 (Aldrich). The 31P NMR singlet for 1 (δ 111.7) decreased with time as that for 4 (δ 85.8) increased; the intensity of the singlet for 1 was ca. 10% of its original intensity after 3 h. Preparative-Scale Oxidation of 1’s SUVs with Hydrogen Peroxide in the pH 8.0 Phosphate Buffer. This reaction was performed as above, using 0.505 g (1.00 mmol) of 1 and 99.0 mL of the pH 8.0 phosphate buffer. To the resultant solution after sonication, 0.120 mL (1.01 mmol) of 8.43 M H2O2 was added dropwise over 5 min. After the reaction mixture was stirred for 4 h, it was worked up as above to give 0.474 g of an oil that upon chromatography gave 0.390 g (84%) of pure 4. Another reaction performed identically gave an 80% yield of 4.

Jaeger and Zeng Small-Scale Oxidation of 1’s SUVs with Sodium Hydrogenperoxymonocarbonate in the pH 8.0 Phosphate Buffer. This reaction was performed as above, using a mixture of 5.1 mg (0.010 mmol) of 1 and 1.00 mL of the pH 8.0 phosphate buffer in D2O, to which was added 20.5 µL (0.010 mmol total HOOCO2Na and H2O2) of a sodium hydrogenperoxymonocarbonate solution prepared below. The 31P NMR singlet for 1 (δ 111.2) decreased with time as that for 4 (δ 85.5) increased; the intensity of the singlet for 1 was ca. 15% of its original intensity after 24 h. The solution of sodium hydrogenperoxymonocarbonate was prepared as follows, according to the literature procedure.8a To 1.00 mL of H2O containing 0.21 g (2.5 mmol) of NaHCO3 was added 59 µL (0.50 mmol) of 8.43 M H2O2. The resultant solution was allowed to equilibrate for 20 min at 23 °C prior to use. Preparative-Scale Oxidation of 1’s SUVs with Sodium Hydrogenperoxymonocarbonate in the pH 8.0 Phosphate Buffer. This reaction was performed as above, using 0.505 g (1.00 mmol) of 1 and 98 mL of the pH 8.0 phosphate buffer. To the resultant solution after sonication, 2.10 mL (1.1 mmol total HOOCO2Na and H2O2) of the sodium hydrogenperoxymonocarbonate solution (see above) was added dropwise over 10 min. After the reaction mixture was stirred for 24 h, it was worked up as above to give 0.480 g of an oil that upon chromatography gave 0.361 g (78%) of pure 4. Another reaction performed identically gave an 85% yield of 4. Formation of 1’s GVs. GVs were prepared at 23 °C in the pH 8.0 phosphate buffer or water. To one side of the bottom of the well of a microscope slide sample assembly,3a 0.20 mg (0.00040 mmol) of 1 was smeared over an area of ca. 8 mm × 2 mm. Then 0.50 mL of the solvent was added, and the sample assembly was allowed to stand undisturbed for 2 h at 23 °C in a chamber containing an aliquot of the pH 8.0 phosphate buffer. In controls, GVs were observed by phase-contrast optical microscopy for g5 days in the resultant mixtures held at 23 °C. Other than when a reaction mixture was observed by optical microscopy, it was stored in the above chamber. Oxidation of 1’s GVs with NaOCl in the pH 8.0 Phosphate Buffer. To 0.50 mL of a GV sample at 23 °C, containing 0.20 mg (0.00040 mmol) of 1, prepared as above in the sample well of a modified microscope slide, 0.6 µL (0.0004 mmol) of 0.676 M NaOCl was added by a 10-µL syringe during 1 s, and the resultant reaction mixture was observed by phase-contrast optical microscopy. The GVs were destroyed within ca. 16 s; see Figure 1. The same reaction was performed in the pH 8.0 phosphate buffer prepared in D2O, and the reaction mixture was transferred to a 5-mm NMR tube after 5 min. Other than the signal for phosphate of the buffer, the 31P NMR spectrum of the reaction mixture contained only one singlet at δ 85.6, which corresponds to disulfide-linked dimer 4. Oxidation of 1’s GVs with Ceric Ammonium Nitrate in the pH 8.0 Phosphate Buffer. This reaction was performed as above using 0.20 mg (0.00040 mmol) of 1 and 1.6 µL (0.00080 mmol) of 0.503 M ceric ammonium nitrate. The GVs were destroyed within ca. 50 s, as observed by phase-contrast optical microscopy. Oxidation of 1’s GVs with Hydrogen Peroxide in the pH 8.0 Phosphate Buffer. This reaction was first performed as above using 0.20 mg (0.00040 mmol) of 1 and 0.5 µL (0.0004 mmol) of 0.843 M H2O2. The GVs were destroyed within ca. 260 min, as observed by phase-contrast optical microscopy; see Figure 2 and the Results and Discussion. The reaction was also performed on a larger scale. Surfactant 1 (6.95 mg, 0.00138 mmol) was smeared to one side of the bottom of an evaporation dish (5 cm × 1.5 cm). Then 17.4 mL of the pH 8.0 buffer was added, and the sample was allowed to stand undisturbed for 2 h at 23 °C in a chamber containing an aliquot of the buffer. Thereafter, 1.6 µL (0.00138 mmol) of 8.43 M H2O2 was added, and the reaction mixture was observed by phasecontrast optical microscopy. At 45 min after the addition of H2O2, the reaction mixture was plunge-frozen into liquid N2 and lyophilized. The residue was suspended in 1.0 mL of CDCl3, and the resultant mixture was filtered and analyzed by 31P NMR.

Oxidative Destruction of Vesicles The spectrum contained only two singlets at δ 111.5 and 86.2, corresponding to 1 and 4, respectively. Based on the relative heights of these singlets, the conversion of 1 to 4 was 71%. Oxidation of 1’s GVs with Sodium Hydrogenperoxymonocarbonate in the pH 8.0 Phosphate Buffer. This reaction was performed as above using 0.20 mg (0.00040 mmol) of 1 and 0.8 µL (0.0004 mmol total HOOCO2Na and H2O2) of the sodium hydrogenperoxymonocarbonate solution (see above). The GVs were destroyed within ca. 9 h, as observed by phase-contrast optical microscopy.

Langmuir, Vol. 19, No. 21, 2003 8725 Phase-Contrast Optical Microscopy. The instrumentation employed has been described previously.3a The photomicrographs of Figures 1 and 2 were taken at 200× magnification.

Acknowledgment. Acknowledgment is made to the U.S. Army Research Office and to the donors of the American Chemical Society Petroleum Research Fund for support of this research. LA0351109