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J. Phys. Chem. B 2007, 111, 3062-3068
Reaction Dynamics of Excited Ketoprofen with Triethylamine Hiroyuki Suzuki,† Tadashi Suzuki,*,† Teijiro Ichimura,† Koichi Ikesue,‡ and Michinori Sakai‡ Department of Chemistry and Materials Science, Tokyo Institute of Technology, 2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan, and Fundamental Research Laboratories, Hisamitsu Pharmaceutical Co., Inc., 1-25-11 Kannondai, Tukuba, Ibaraki 305-0856, Japan ReceiVed: December 8, 2006; In Final Form: January 11, 2007
The reaction dynamics of ketoprofen (KP) with and without triethylamine (TEA) in methanol both in the ground and the excited states was studied by laser flash photolysis and the pump-probe emission spectroscopy. After the excitation, triplet KP abstracted a hydrogen atom from methanol to form KP ketyl radical (KPH). In the presence of TEA, the acid-base equilibrium state was found to be KP + TEA a KP- + TEAH+ in the ground state. The equilibrium constant was determined to be 32 ( 7. Excited KP- rapidly underwent decarboxylation to form a carbanion resonant with the 3-ethylbenzophenone ketyl biradical anion (3-EBP-), followed by a proton-transfer reaction with TEAH+ to produce the 3-ethylbenzophenone ketyl biradical (3EBPH). Furthermore, 3-EBPH was found to make a complex with TEA, whose equilibrium constant was obtained to be 18 ( 2 M-1. The complex formation ability of 3-EBPH was discussed compared with benzophenone ketyl radical (BPH).
1. Introduction
2. Experimental
The photoactivatable aryl ketone derivatives have been extensively studied as a biochemical probe in protein, nucleic acid, and lipid biochemistry.1 These photoactive ketones are used extensively in drugs and sunscreening agents.2-5 Benzophenone (BP) is one of the most famous aryl ketones. The photochemistry of BP describes an efficient intersystem crossing (the quantum yield is almost unity), so many reactions can easily take place from the lowest excited triplet state 3nπ*: charge transfer, energy transfer, hydrogen atom abstraction, etc. Benzophenone ketyl radical (BPH) is well-known as a photoproduct of BP. The 1:1 complex formation of BPH with aliphatic amines such as triethylamine (TEA) has been previously reported.6,7 Ketoprofen (KP), tiaprofenic acid, suprofen, and tolmetin are known as nonsteroidal-anti-inflammatory drugs8 containing a diaryl ketone chromopher; however, the side effect on human skin, that is to say, photosensitization in ViVo was reported.9-14 The basic photoreaction mechanisms of these drugs were studied extensively.15 Because KP has a carboxyl group, in the presence of base, the deprotonated KP (KP-) was reported to absorb UV light to yield a carbanion, through the rapid decarboxylation reaction16-23 (Scheme 1). The decarboxylation mechanism was clarified in detail by Borsarelli et al. in terms of thermodynamics.17 At pH < 7.4, the carbanion was protonated to yield the 3-ethylbenzophenone ketyl biradical (3-EBPH), and finally, 3-ethylbenzophenone (3-EBP) was formed. At pH > 7.4, the carbanion produced 3-EBP not through the intermediate of 3-EBPH. However, the reaction mechanism of photosensitization in ViVo has not been clarified yet. In this article, we will elucidate the reaction mechanism of KP with TEA by means of nanosecond laser flash photolysis as a prototype of the KP reaction with amines. † ‡
Department of Chemistry and Materials Science. Fundamental Research Laboratories.
An experimental setup has been described elsewhere.24 Briefly, a XeCl excimer laser (COMPex 102; 308 nm, 200 mJ, 20 ns time duration) was used as an excitation light source, and a Xe lamp (Ushio UXL-300D; 300W) passing via a sample flow cuvette (NSG T-59FL-10: 10 mm light path length) was used as a monitoring light source. The monitoring light was detected with a monochromator (Nikon P-250)/photomultiplier tube (Hamamatsu R928) system. The spectral resolution was 2 nm. The signal was measured by a digital oscilloscope (Sony Tektronix TDS380P, 2 GS/s, 400 MHz) and transferred to a personal computer. The signal was averaged 40 shots to improve the S/N ratio. Time-resolved emission spectra for transient species were measured by a pump-probe technique with the XeCl excimer laser and a second harmonic of a Nd3+:YAG laser (Continuum Powerlite 8010, 532 nm, 800 mJ, 6 ns time duration). The two lasers were synchronously fired with a pulse generator (Stanford Research System, DG535). The fluorescence detection system was the same system as the flash photolysis measurements described above. KP (purity 99.9%) was purified by recrystalization. Triethylamine (TEA), diethylamine, and tri-n-propylamine (GR grade) were purchased from Tokyo Kasei Kogyo Co., Ltd., and methanol, benzene, and acetonitrile (GR grade) were purchased from KANTO KAGAKU and were used from a fresh bottle. In all the time-resolved spectrum measurements, the concentration of KP was 1.25 mM. The sample solutions were deaerated by bubbling with argon gas (stated purity 99.95%) to eliminate oxygen and were flowed in the sample cuvette to eliminate the influence of the photoproduct. Absorption spectra were measured with a double beam spectrophotometer (Jasco Ubest V-550). All measurements were carried out at room temperature. 3. Results and Discussion 3.1. Equilibrium between KP and KP- in the Ground State. Figure 1 shows absorption spectra of KP in methanol
10.1021/jp0684372 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/23/2007
Reaction Dynamics of Ketoprofen with TEA
J. Phys. Chem. B, Vol. 111, No. 11, 2007 3063
SCHEME 1: Reaction Scheme of KP in Phosphate Buffer
(30.3 µM) with and without TEA (60.8 µM). The absorption spectrum in the absence of TEA was found to consist of three bands: a weak broad band at around 331 nm ( ) 150 M-1 cm-1) and two intensive bands at 254.2 nm ( ) 16900 M-1 cm-1) and 206.8 nm ( ) 23300 M-1cm-1). These absorption bands were safely assigned to the S1(nπ*), S2(ππ*), and S3(ππ*) states of KP, respectively, in comparison with BP. The absorption spectrum was totally changed with TEA concentration, and two isosbestic points were observed at 240 and 262 nm. The existence of two isosbestic points implied that an equilibrium state should exist in the KP and TEA system. Miyasaka et al. reported the formation of a weak CT complex between BP and N,N-diethylaniline, when the equilibrium constant was estimated to be 0.1∼0.5 by a Bensei-Hildebrand plot.25,26 On the other hand, in phosphate buffer at pH 7.4, KP was reported to exist as the deprotonated form of KP (KP-) in the ground state because of a weak acid (pKa ) 4.7).18,19 An acid-base
Figure 1. Absorption spectra of KP in methanol (30.3 µM) with (dotted line) and without TEA (60.8 µM) (solid line). Two isosbestic points are clearly observed at 240 and 262 nm.
equilibrium of KP in the presence of TEA (a weak base, pKa ) 10.7527) in methanol would be established. Figure 2 shows plots of absorbance at 308 nm against the concentration of TEA. From having the isosbestic points mentioned above, the following equilibrium states can be expected as the equilibrium of the complex formation (case 1) and the acid-base equilibrium (case 2): case 1 KP + TEA a KP‚‚‚TEA(C) and case 2 KP + TEA a KP- + TEAH+. First, case 1, where KP and TEA should make a complex (C) was considered. The equilibrium constant is given by
K)
[C] [KP][TEA]
(1)
Absorbance, OD(λ), is represented as
Figure 2. Plots of absorbance at 308 nm (b) against the concentration of TEA in methanol. The concentration of KP is 1.25 mM. The broken line shows the fitting curve obtained by eq 3, and the solid line shows the fitting curve obtained by eq 6.
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OD(λ) ) λKP[KP]d + λTEA[TEA]d + λC[C]d ≈ λKP[KP]d + Cλ[C]d
(2)
where the terms of λKP, λTEA, and λC represent the extinction coefficients of KP, TEA, and C, respectively, and [KP], [TEA], and [C] are the concentrations of KP, TEA, and C; d is the optical path length (1 cm). Here, the extinction coefficient of TEA, λTEA, at 308 nm can be regarded as almost zero. From eqs 1 and 2, the final form of OD (λ) against the concentration of TEA is represented by
OD(λ) ) λC[KP]0d +
(λKP - λC)[KP]0d 1 + K[TEA]
(3)
The best fitting curve using eq 3 is shown as a broken line in Figure 2. Here, [KP]0 ) 1.25 mM, 308nm ) 326 M-1 cm-1, KP and K are the fitting parameters. However, the curve and 308nm C could not reproduce the experimental results. Therefore, case 1, where KP and TEA is equilibrated with the KP‚‚‚TEA complex, is not appropriate. Second, the acid-base equilibrium of case 2 was considered. The equilibrium constant K is represented by
K)
[KP-][TEAH + ] [KP][TEA]
Figure 3. Estimated concentration of each component with the amount of added TEA: the concentration of KP (solid line), KP- and TEAH+ (broken line), and neutral TEA (dotted line).
(4)
OD(λ) is given as
OD(λ) ) λKP[KP]d + λTEA[TEA]d + λ λ + KP -[KP ]d + TEAH+[TEAH ]d λ ≈ λKP[KP]d + KP -[KP ]d
(5)
λ λ where KP - and TEAH+ represent the extinction coefficients of + KP and TEAH , and [KP-] and [TEAH+] are the concentrations of KP- and TEAH+, respectively. The extinction coefλ ficients of λTEA and TEAH + at 308 nm are regarded as almost zero. From eqs 4 and 5, the final form of OD(λ) against the concentration of TEA is represented by
Figure 4. Transient absorption spectra of KP in methanol (1.25 mM) immediately after the laser pulse (O) and at 1.8 µs (b). The time profiles monitored at 335 and 600 nm are shown in the inset.
M-1 cm-1), and 205.8 nm ( ) 23 500 M-1 cm-1), respectively. These extinction coefficients of KP- in methanol agreed well with those of KP- reported in phosphate buffer at pH 7.4.20 Figure 3 shows the concentration of KP, KP-, TEA, and TEAH+ under the condition that the concentration of KP was maintained at 1.25 mM with the amount of added TEA, estimated with the obtained equilibrium constant K,
λ λ OD(λ) ) λKP[KP]0d + (KP - - KP)d ×
[KP] ) [KP]0 - [KP-]
([KP]0 + [TEA]0)K -x([KP]0 - [TEA]0) K + 4K[KP]0[TEA]0
(7)
2 2
([KP]0 + [TEA]0)K -
2(K - 1)
(6) where [TEA]0 is the concentration of added TEA and [KP]0 is the initial concentration of KP (1.25 mM). From eq 6, the initial concentration of KP (1.25 mM), and the 308nm value (326 M-1 KP 308nm cm-1), the best-fit curve was obtained with K and KP used as parameters shown as a solid line in Figure 2. The fitting curve excellently reproduced the experimental results, where 308nm K ) 32 ( 7 and KP ) 714 M-1 cm-1. The extinction coefficients of KP- agreed well with the experimental value determined by measuring the absorbance of KP- in the presence of NaOH (721 M-1 s-1), revealing the reliability of this experimental results. It is concluded that the equilibrium, KP + TEA a KP- + TEAH+, should be established. Therefore, the broken line of the absorption spectrum in Figure 1 should result from only KP-. The absorption maxima of S1(nπ*), S2(ππ*), and S3(ππ*) states of KP- were as follows: 320-380 nm region ( ) 100-200 M-1 cm-1), 255.6 nm ( ) 15 900
[KP-] )
x([KP]0 - [TEA]0)2K2 + 4K[KP]0[TEA]0 2(K - 1)
(8)
[TEA] ) [TEA]0 - [TEAH+]
(9)
[TEAH+] ) [KP-]
(10)
The abundance of each component in the ground state will be reflected on the reaction of KP and KP- in the excited state. 3.2. Reaction Dynamics of Excited KP in Methanol. The transient absorption spectrum of KP in methanol (1.25 mM) was measured with the 308 nm excitation (Figure 4). Immediately after the irradiation, two absorption bands were observed at 330 and 530 nm. The time course of the transient absorption was also shown in the inset of Figure 4. Both band maxima observed at 1.8 µs after the laser pulse were red-shifted to 335 and 550 nm. Cosa et al. tentatively assigned the spectra to the lowest excited triplet (3KP*) absorption and the KP ketyl
Reaction Dynamics of Ketoprofen with TEA
J. Phys. Chem. B, Vol. 111, No. 11, 2007 3065
Figure 6. Transient absorption spectra of KP in methanol (1.25 mM) with TEA (5 mM) immediately after the laser pulse (b), at 1.8 µs (O), and at 90 µs (2).
Figure 5. (a) Transient absorption spectrum (O) and emission spectrum (b) of KPH, where the time interval between two lasers was 1 µs. (b) Plots of emission intensity monitored at 590 nm against the time interval.
radical (KPH) absorption from analogy with the transient absorption spectrum of benzophenone.21 We carried out a quenching experiment with KI to confirm the transients. It was found that the decay rate monitored at 330 and 530 nm became faster, and the generation amount of the long-lived transient became smaller with KI. The quenching rate constant was obtained to be kq ) (2.2 ( 0.1) × 109 M-1 s-1 by SternVolmer plots. The rate constant thus obtained agreed well with the diffusion controlled rate in methanol (1.1 × 1010 M-1s -1 at 293 K).28 Therefore, the transient absorption spectrum observed immediately after the laser irradiation turned out to be T-T absorption of KP. The transient, whose absorption maxima were 335 and 550 nm, should be yielded from 3KP*, because the decay rate constant of 3KP* and the rise time constant of the transient monitored at 335 nm were identical (k ) (5.6 ( 0.2) × 106 s-1). The transient absorptions at 335 and 550 nm were not observed when benzene and acetnitrile were used as a solvent. This transient should be KPH, produced from a hydrogen atom abstraction of 3KP* from a methanol molecule. Figure 5a shows the emission spectrum of KPH, where the time gap between the excimer laser and the YAG laser was 1 µs. The transient absorption spectrum of KPH was also shown in Figure 5a. The absorption and emission spectra exhibited a good mirror image. The fluorescence lifetime was within our experimental resolution (99%) existed as a deprotonated form of KP (KP-) in the ground state (see Figure 3). The transient absorption spectra indicated three kinds of transient species: transient A (600 nm), transient B (525 nm), and transient C (545 nm). The quenching experiment with KI was carried out. However, the KI could not show any influence on the lifetime and the absorption intensity for transients A, B, and C, revealing that they should be neither 3KP* nor the transient species produced from 3KP*. The transient A, whose lifetime was determined to be 60 ns, would be a carbanion, because the absorption spectrum of the carbanion observed in phosphate buffer at pH 7.418,19,21 appeared in the same spectral range. The lifetime of the carbanion in the presence of TEA, however, was shorter than the reported one (120 ns in phosphate buffer). This reason will be discussed later. Figure 7 shows the transient absorption spectra of KP in methanol (1.25 mM) at 1.8 µs after the laser pulse when the concentration of TEA varied from 0 to 1.5 mM. There was an observed isosbestic point at 535 nm. The absorption intensity of KPH (550 nm) decreased with TEA, and that of transient B increased (525 nm). The existence of the isosbestic point may imply an equilibrium state, such as KPH + TEA a KPH‚‚‚TEA complex. Kajii et al. reported the formation of the BPH and TEA complex, and the disappearance rate of BPH and the formation rate of the BPH‚‚‚TEA complex were identical.6 However, the abundance ratio of KP against KPwas small (ca. 0.09) under the experimental condition of TEA (1.5 mM) (see Figure 3); furthermore, the formation rate of transient B (1.7 × 107 s-1, see below) did not correspond to the disappearance rate of KPH (∼103 s-1).
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Figure 9. Plots of the decay rate constant of carbanion against the net concentration of neutral TEA (O) and TEAH+ (b).
Figure 8. (a) Transient absorption spectrum of KP in methanol (1.25 mM) with TEA (1.5 mM) observed at 1 µs after the laser pulse (O) and the emission spectrum (b), when the time interval between the two lasers was set at 1 µs. (b) Time profile of transient absorption (dot) monitored at 600 nm and the plots of the emission intensity (O) monitored at 590 nm against the time interval between the two lasers.
The transient absorption spectrum of transient B excellently agreed with that of 3-EBPH reported in the phosphate buffer.18 Therefore, it is strongly suggested that transient B will be 3-EBPH, which was only observed in the acid phosphate buffer solution. To clarify the reaction mechanism of the 3-EBPH formation, a pump-probe technique was applied on measuring the emission spectrum of 3-EBPH, because the transient absorption spectra for the transients largely overlapped. Figure 8a shows the transient absorption spectrum of KP in methanol (1.25 mM) with TEA (1.5 mM) observed at 1 µs after the excimer laser and the emission spectrum of 3-EBPH, when the time gap between the two lasers was 1 µs. The transient absorption and the emission spectra show a good mirror image, so that this emission should result from 3-EBPH. Both of the transient absorption and emission spectra of 3-EBPH were found to be blue-shifted in comparison with those of KPH (Figure 5a). The fluorescence lifetime was within our experimental resolution ( 8.7).18 This reaction was also observed when diethylamine and tri-n-propylamine were used as amines. It is just clarified that TEAH+ should be a key species to form 3-EBPH. 3.4. Complex Formation between 3-EBPH and TEA. Figure 10 shows the transient absorption spectra of KP in methanol (1.25 mM) with TEA (5-200 mM) at 1.8 µs after the laser pulse. There was an observed isosbestic point at 537.5 nm. The absorbance of 3-EBPH decreased (525 nm), and the absorbance increased at longer wavelength (545 nm) with TEA. Therefore, the following equilibrium to produce a 3-EBPH‚‚‚TEA complex (C′) was considered.
Reaction Dynamics of Ketoprofen with TEA
J. Phys. Chem. B, Vol. 111, No. 11, 2007 3067 SCHEME 2: Reaction Scheme of KP with and without TEA in Methanol
Figure 11. The plots of transient absorption intensity observed at 555 nm (b), 560 nm (O), and 565 nm (2) with various concentration of added TEA. The solid line shows best fitting lines obtained with eq 13.
3-EBPH + TEA a C′ KC′ )
[C′] [3 - EBPH][TEA]
(11) (12)
where KC′ is the equilibrium constant. The plots of absorbance at 555, 560, and 565 nm against the concentration of TEA are shown in Figure 11. OD(λ) is derived as λ [3 - EBPH]0d + OD(λ) ) C′
λ λ (3-EBPH - C′ )[3 - EBPH]0d
1 + KC′[TEA]
(13)
λ where [3-EBPH]0 is the initial concentration of 3-EBPH. C′ λ and 3-EBPH are the extinction coefficients of the complex and 3-EBPH, respectively. The plots, shown in Figure 11, were λ λ analyzed with the values of [3-EBPH]0, 3-EBPH , and C′ as fitting parameters. The equilibrium constant was successfully obtained to be KC′ ) 18 ( 2 M-1. The complex formation of BPH and TEA was reported by Kajii et al.6 The spectral feature for the 3-EBPH‚‚‚TEA complex was similar with that of the BPH‚‚‚TEA complex. Therefore, the complex may be understood as a hydrogen-bond complex. The equilibrium constant of the BPH‚‚‚TEA complex was reported to be KC′ ) 60-103 M-1 in several solvents.6,7 The obtained KC′ value for the 3-EBPH‚‚‚TEA complex was smaller than that for the BPH‚‚‚TEA complex. The value of KC′ was known to be rather related with the dielectric constant (r) of solvent: the larger the r was, the smaller KC′ was. Abe et al. showed the linear relationship between the Gibbs free energy change (∆G ) -RT ln KC′) and (r - 1)/(r + 2).7 We have to compare the KC′ value for these complexes in the same solvent polarity. So, we estimated the KC′ value of the BPH‚‚‚TEA complex in methanol (r ) 33.7)29 to be 53 M-1. This value was lager than that of the 3-EBPH‚‚‚TEA complex; therefore, the complex formation ability of 3-EBPH was found to be smaller than that of BPH. The ability would be considered to result from the charge distribution of the carbonyl group. A side chain of the ethyl radical group of 3-EBPH, as a result, would affect the hydrogen-bonding energy. The reaction mechanism elucidated is summarized in Scheme 2. We clarified the reaction dynamics of KP with and without TEA in methanol both in the ground and the excited states.
4. Conclusion Reaction dynamics of KP with and without TEA was studied with laser flash photolysis. Excited KP was found to give rise
to 3KP*, whose absorption maxima appeared at 330 and 530 nm, by the quenching experiment with KI. 3KP* abstracted a hydrogen atom from a methanol molecule to yield KPH, whose absorption maxima were observed at 335 and 550 nm. By the pump-probe emission measurement, KPH was found to release red emission. This behavior is characteristic for benzophenone and its ketyl radical. In the presence of TEA, it was clarified that the acid-base equilibrium state, KP + TEA a KP- + TEAH+, was established in the ground state, whose equilibrium constant was determined to be 32 ( 7, and the extinction coefficients of KP and KP- at 308 nm were also obtained to be 326 M-1 and 714 M-1cm-1, respectively. Therefore, the net concentration of KP, neutral TEA, KP-, and TEAH+ were successfully estimated. Excited KP- was rapidly decarboxylated to produce a carbanion, whose absorption band was observed at around 600 nm. A carbanion was found for the first time to react with TEAH+ to yield 3-EBPH, whose absorption maximum was 525 nm. This reaction could be explained by the proton-transfer reaction at the diffusion controlled rate in the encounter complex of 3(3-EBP-‚‚‚TEAH+) in the solvent cage. It turned out that TEAH+ should be a key species to form 3-EBPH, which was not observed in the basic phosphate buffer solution. 3-EBPH was found to make a complex with TEA for the first time, and its equilibrium constant was determined to be 18 ( 2 M-1. It also turned out that the formation ability of 3-EBPH was smaller than that of BPH, suggesting the difference of the charge distribution of the carbonyl group due to the ethyl radical substitution. These experimental results imply that amines such as TEA should play an important role in initial photoreaction of KP;
3068 J. Phys. Chem. B, Vol. 111, No. 11, 2007 furthermore, the radical produced by the photodecarboxylation should react with some biomolecule containing the amino group in ViVo, followed by appearance of toxicity. References and Notes (1) Dorma´n, G.; Prestwich, G. D. Biochemistry 1994, 33 (19), 5661. (2) Schallreuter, K. U.; Wood, J. M.; Farwell, D. W.; Moore, J.; Edwards, H. G. M. J. InVest. Dermatol. 1996, 106, 583. (3) Silva, R.; Almeida, L. M. S.; Brandao, F. M. Contact Dermatitis 1995, 32, 176. (4) Alomar, A.; Cerda, M. T. Contact Dermatitis 1989, 20, 74. (5) Szczurko, C.; Dompmartin, A.; Michel, M.; Moreau, A.; Leroy, D. Photodermatol., Photoimmunol,. Photomed. 1994, 10, 144. (6) Kajii, Y.; Itabashi, H.; Shibuya, K.; Obi, K. J. Phys. Chem. 1992, 96, 7244. (7) Abe, T.; kawai, A.; Kajii, Y.; Shibuya, K.; Obi, K. J. Phys. Chem. A 1999, 103, 1457. (8) Steen, K. H.; Wegner, H.; Meller, S. T. Pain 2001, 93, 23. (9) Bosca´, F.; Carganico, G.; Castell, J. V.; Go´mez-Lecho´n, M. J.; Hernandez, D.; Mauleo´n, D.; Martı´nez, L. A.; Miranda, M. A. J. Photochem. Photobiol., B 1995, 31, 133. (10) Artuso, T.; Bernadou, J.; Meunier, B.; Piette, J.; Paillous, N. Photochem. Photobiol. 1991, 54 (2), 205. (11) Castelli, F.; Guidi, G. D.; Giuffrida, S.; Miano, P.; Sortino, S. Int. J. Pharm. 1999, 184, 21. (12) Condorelli, G.; Constanzo, L. L.; Guidi, G. D.; Giuffrida, S.; Rizzarelli, E.; Vecchio, G. J. Inorg. Biochem. 1994, 54, 257. (13) Traynor, N. J.; Johnson, B. E.; Gibbs, N. K. Toxicol. in Vitro 1996, 10, 619. (14) Parij, N.; Nagy, A.-M.; Fondu, P.; Ne`ve, J. Eur. J. Pharm. 1998, 352, 299.
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