2422
J. Phys. Chem. 1996, 100, 2422-2429
Solvent Effects on the Resonance Raman and Electronic Absorption Spectra of Bacteriochlorophyll a Cation Radical Yasuhito Misono,† Leenawaty Limantara,‡ Yasushi Koyama,‡ and Koichi Itoh*,† Department of Chemistry, School of Science and Engineering, Waseda UniVersity, Shinjuku-ku, Tokyo 169, Japan, and Faculty of Science, Kwansei Gakuin UniVersity, Uegahara, Nishinomiya 662, Japan ReceiVed: August 4, 1995; In Final Form: October 20, 1995X
Resonance Raman and electronic absorption spectra of bacteriochlorophyll a cation radical (BChl a•+) were recorded in 14 different kinds of solvents. The frequency of the ring-breathing Raman band of BChl a•+ was in the region of 1596-1599 cm-1 in solvents forming the pentacoordinated state in neutral bacteriochlorophyll a (BChl a), while it was in the region of 1584-1588 cm-1 in solvents forming the hexacoordinated state. BChl a•+ exhibited a key absorption band in the regions 546-554 and 557-563 nm in the above penta- and hexacoordinating solvents. Therefore, it has been concluded that the penta- and hexacoordinated states are retained even after conversion of BChl a into BChl a•+ (one-electron oxidation). Application of this rule to the case of 2-propanol solution showed transformation from the penta- to the hexacoordinated state upon one-electron oxidation in this particular solution. The coordination states of BChl a•+ could be correlated with the donor number (DN) and the Taft parameters, β and π*, of the solvent: The hexacoordinated state was formed in solvents with DN g 18 or β > 0.5 showing higher electron-donating power, while the pentacoordinated state was formed in solvents with π* > 0.65 showing higher dielectric stabilization. Preferential solvation of a hexacoordinating solvent over a pentacoordinating solvent in their mixture (methanol/ acetone or tetrahydrofuran/methylene chloride) was seen in both BChl a and BChl a•+, but the constant of preferential solvation was increased approximately 3 times by one-electron oxidation.
Introduction The primary process of photosynthesis in Rhodobacter sphaeroides takes place in the reaction center (RC), which is a membrane-bound protein complex containing bacteriochlorophyll a (BChl a, Figure 1), bacteriopheophytin a (BPheo a), and quinones.1,2 Photoexcitation of the RC causes charge separation between one of the special pairs BChls and BPheo and results in the formation of a BChl a cation radical (BChl a•+) and a BPheo a anion radical. The charge-separated state is supposed to be stabilized through specific pigment-pigment and pigment-peptide intermolecular interactions in the membrane protein, e.g., ligation through the magnesium atom, hydrogen bonding through the carbonyl groups, and stacking (aggregation) of the pigments. In order to elucidate the molecular mechanism of the charge separation, resonance Raman (RR) and infrared (IR) spectroscopies have been applied to BChl a•+ both free in solution and bound to the RC.3-9 Cotton et al.3,4 measured the RR spectra of BChl a•+ produced by electrochemical oxidation in methylene chloride. Lutz et al.5 prepared BChl a•+ by oxidation with I2 and observed its RR spectrum in glassy methanol at 30 K. The RR band ascribable to the ring-breathing vibration near 1600 cm-1 showed a definite change in frequency upon one-electron oxidation, which was ascribed to changes in bond order in the bacteriochlorine macrocycle. Mattioli et al.6 measured the FT-Raman spectra of the oxidized RC of R. sphaeroides R26 and suggested that the positive charge was localized on one of the special pairs of bacteriochlorophylls based on the fact that the frequency shift of the C131 keto carbonyl stretching band, which was induced * Author to whom correspondence should be addressed. † Waseda University. ‡ Kwansei Gakuin University. X Abstract published in AdVance ACS Abstracts, January 1, 1996.
0022-3654/96/20100-2422$12.00/0
Figure 1. Molecular structure of BChl a.
by oxidation of the RC, was nearly the same as that induced by oxidation of monomeric BChl in solution. Ma¨ntele et al.7 observed the IR spectra of BChl a•+, which was prepared by electrochemical oxidation in methanol and tetrahydrofuran (THF), and correlated the terminal carbonyl stretching frequencies in the 1650-1740 cm-1 region to the modes of hydrogen bonding. Comparing these results to the light-induced change in the IR spectrum of the RC of R. sphaeroides R26, the authors concluded that the hydrogen bonding of the C132 ester group is absent in the charge-separated state. Morita et al.8,9 recorded © 1996 American Chemical Society
Bacteriochlorophyll a Cation Radical the temperature dependence of the light-induced IR difference spectra of both the chromatophores and the RCs from various photosynthetic bacteria, assigned the C131 keto-carbonyl stretching bands in the 1703-1715 cm-1 region based on comparison to free BChl a•+ in hydrogen-bonding and non-hydrogenbonding solvents, and interpreted the difference spectra in terms of hydrogen bonding and charge delocalization. The results of the above vibrational spectroscopy of the RCs in the charge-separated state have not been fully correlated with specific interactions of BChl a•+ with its surroundings. This is mainly due to the lack of data systematically correlating the RR and IR spectra of the cation radical with its states of ligation, hydrogen bonding, and stacking. As a first step in this line of investigation, we collected the RR spectra of BChl a•+ in acetone, methanol, methylene chloride, and mixtures of acetone and methanol10 and obtained the following conclusions. (1) The ring-breathing vibration (mainly the Ca-Cm stretching) appeared at 1601, 1587, and 1600 cm-1 in acetone, methanol, and methylene chloride, respectively. Thus, by changing the solvent from pentacoordinating acetone to hexacoordinating methanol, the ring-breathing frequency in BChl a•+ (νr+) was lowered by 14 cm-1, which roughly corresponded to the lowering of the ring-breathing frequency in BChl a (νr) by 17 cm-1. The fact suggested that the states of coordination are retained even after one-electron oxidation. (2) On addition of only 10 vol % of methanol to acetone, the νr+ frequency of 1601 cm-1 in pure acetone shifted to 1592 cm-1, the frequency change being almost 64% of the shift caused by the total replacement of acetone by methanol. The result indicated that the cation radical is preferentially solvated with methanol rather than with acetone. In the present investigation, solvent effects on BChl a•+ have been examined more systematically. Both the RR and electronic absorption spectra of BChl a•+ were recorded in 14 different kind of solvents. The RR spectra were recorded in both THF/ methylene chloride and acetone/methanol mixtures. The results confirmed the conclusions of the above preliminary experiments10 and established the relationships, the states of coordination vs the RR and the electronic absorption spectra. The ringbreathing frequencies are compared among the (ground) S0 state of BChl a, the T1 state of BChl a, and the (ground) D0 state of BChl a•+ (hereafter, abbreviated as S0 BChl a, T1 BChl a, and D0 BChl a•+ or more simply, the S0, T1 and D0 states). Experimental Section Thin-Layer Electrochemical Cells for RR and Absorption Measurements. Two kinds of thin-layer electrochemical cells were built for the measurements of the RR and the electronic absorption spectra of BChl a•+. Figure 2A shows a cell for both the RR and the electronic absorption measurements. A platinum plate (8 mm × 2 mm), a platinum wire (0.3 mm L), and a silver rod (4 mm L) were embedded in a cylindrical Teflon block (80 L × 20 mm) and used as the working electrode, the counter electrode, and the pseudoreference electrode, respectively. A glass window (50 mm L × 1 mm) was fixed onto the Teflon block by the use of a pair of stainless rings. The thickness between the window and the working electrode was fixed to 100 µm by using a Teflon spacer. The cell was filled with each sample solution by using a peristaltic pump after cleaning it with the same solvent. When necessary, the sample solution was kept flowing through the cell by using the pump (ca. 3 mL/min) during the RR measurement; this method was effective to avoid the effects of side reactions caused by irradiation. Figure 2B shows another cell used only for the absorption measurements; it is similar to those used by Ma¨ntele et al.7 and
J. Phys. Chem., Vol. 100, No. 6, 1996 2423
Figure 2. Thin-layer electrochemical cells used for (A) both electronic absorption and RR measurements and for (B) electronic absorption measurements only.
by Morita et al.8,9 A gold mesh with a thickness of 1 µm (15 mm × 35 mm, 1000 mesh), a platinum foil with a thickness of 20 µm, and a silver foil with a thickness of 20 µm were used as the working electrode, the counter electrode, and the pseudoreference electrode, respectively. All the electrodes were assembled between a pair of CaF2 windows (50 mm L × 5 mm). The thickness of the cell was fixed at 200 µm by the use of a Teflon spacer. Each sample solution was injected with a syringe from the inlet, i.e., a hole on the window of each cell. Preparation of sample solutions. BChl a was extracted from the cells of R. sphaeroides R26, and purified by column chromatography and then by high-pressure liquid chromatography, as described previously.11,12 BChl a was dissolved in a mixture consisting of a solvent and a supporting electrolyte (0.3 mol/L), and the sample solution was degassed with dry argon gas. Tetra-n-butylammonium hexafluorophosphate (TBAPF6) was used as the supporting electrolyte for solvents, methylene chloride, 3-pentanone, n-butyronitrile, 2-butanone, and THF, while tetra-n-butylammonium tetrafluoroborate (TBABF4) was used for the rest of the solvents. The oxidation potential for the preparation of BChl a•+ in each solvent was determined by cyclic voltammetry performed in the thin-layer cells described above. Table 1 lists the oxidation potentials of BChl a•+ (vs Ag/Ag+). Completion of oxidation was checked by the diminution of oxidation current to a virtual zero value. Absorption spectroscopy also was applied to confirm the completion of oxidation by observing the disappearance of the Qy band of BChl a near 770 nm. A reflection mode was used for the cell in Figure 2A, and a transmission mode was used for the cell in Figure 2B. The concentration of BChl a was 1 mmol/L for the electronic absorption measurements and either 1 or 2 mmol/L for the RR measurements. As for solvents, ethyl acetate, 2-propanol, THF, and ethanol of spectroscopy grade were purchased from Kanto Chemical Co., Inc.; n-butanol of spectroscopy grade was purchased from Kishida Chemicals; other solvents of special grade were
2424 J. Phys. Chem., Vol. 100, No. 6, 1996
Misono et al.
TABLE 1: Oxidation Potential (V vs Ag/Ag+) Applied to the Pt-Plate Electrode and the Au-mesh Electrode in the Preparation of BChl a•+ in Each Solvent oxidation potential (V vs Ag/Ag+) solvent
Pt
Au
(a) methylene chloride (b) 2-octanone (c) acetone (d) propionitrile (e) 3-pentanone (f) n-butyronitrile (g) 2-butanone (h) ethyl acetate (i) 2-propanol (j) n-butanol (k) tetrahydrofuran (l) ethanol (m) methanol (n) n-propanol (o) benzonitrile (p) N,N-dimethylformamide
0.65 0.90 0.70 0.65 0.50 0.65 0.55 1.00 0.80 1.00 0.45 0.65 0.35 0.80
0.65 0.80 0.60 0.60 0.40 0.40 0.40 1.10 0.80 0.90 0.45 0.55 0.35 0.60 0.70 0.65
purchased from Kanto Chemical Co. Methylene chloride was dehydrated with CaCl2, acetone and 2-butanone with CaSO4, and methanol with molecular sieves 4A;13 these solvents were distilled prior to use. The rest of the solvents were used without further purification. The supporting electrolyte, TBABF4, was prepared as follows.14,15 Hydrogen tetrafluoroborate (HBF4) was added to a saturated aqueous solution of an equimolar tetra-n-butylammonium bromide. White crystals, which appeared at room temperature, were collected by filtration. After thorough washing with water, the crystals were dried under vacuum, recrystallized 3 times from ethanol, and then dried again under vacuum. The other supporting electrolyte, TBAPF6, was prepared by the same procedure using hydrogen hexafluorophosphate instead. RR Measurements. RR spectra were measured by using a spectrometer (SPEX 1877 Triplemate) equipped with an imageintensified CCD detector (Princeton Instruments, Model ICCD 576 G/B) as described previously.10 The 418.5 nm excitation line was generated as the second harmonic of a Ti-sapphire laser (Solar Co. Ltd., Model CF 131 M), which was pumped by the second harmonic 532 nm pulses from a Nd:YAG laser (Spectron Laser Systems, Model SL404 1010; pulse duration, ca. 15 ns; repetition rate, 10 Hz). The 418.5 nm line was used to selectively measure the RR spectra of BChl a•+, which shows a characteristic absorption band in the region 420-430 nm. Its power was reduced to less than 1 mJ/pulse to avoid the generation of the excited states and the decomposition of BChl a. A 90° scattering was collected in the RR measurements; the incident angle of the excitation beam was fixed at 60° with reference to the surface normal of the working electrode of the cell. Indene and the solvent (listed in Table 1) were used for the frequency calibration of each RR spectrum. The Raman bands due to the supporting electrolytes were not detected in the 1200-1800 cm-1 region under the present resonance conditions. Subtraction of the solvent Raman bands was performed carefully by changing the subtraction factor by small steps. The RR spectra in 2-octanone, 3-pentanone, 2-butanone, and ethyl acetate, however, still contain bands due to the solvents, each of which is marked with an asterisk in the spectra. Electronic Absorption Measurements. Electronic absorption measurements in the transmission mode were performed by the use of a Shimadzu UV-3101 PC spectrophotometer, while those in reflection mode were performed by the use of a rapidscan spectrophotometer (Unisoku Scientific Instruments, Model RSP601) that was equipped with a multichannel detector (MOS high-sensitivity image sensor) and a 50 W halogen lamp.
A curve-fitting technique, based on the least-squares method, was applied in order to accurately determine the band positions of BChl a•+ in the 546-575 nm region (band E, Vide infra); a Gaussian function was assumed for each component. Results and Discussion Classification of the Coordination States of BChl a•+ Based on the νr+ Frequencies. Figure 3 shows the RR spectra (λex ) 418.5 nm) of BChl a•+ in the 14 solvents. The RR spectra could not be obtained in (o) benzonitrile and (p) N,Ndimethylformamide because of the strong fluorescence background probably due to partial decomposition of the sample during electrolysis. The RR spectra in methylene chloride and in methanol (spectra a and m of Figure 3) are similar to those reported by Cotton et al.4 and Lutz et al.,5 respectively. The strong RR bands in the 1584-1599 cm-1 region can be assigned, based on their frequencies and intensities, to the ringbreathing mode consisting of all the Ca-Cm stretchings.10,16 The ring-breathing frequencies in BChl a (νr) and those in BChl a•+ (νr+) are listed in Table 2. The correlation between the νr frequency and the state of coordination was first shown by Callahan and Cotton for neutral BChl a;17 it is 1610 cm-1 in the pentacoordinated state and 1595 cm-1 in the hexacoordinated state. Umemoto et al.18 showed that the νr Raman band appears in the 1606-1612 cm-1 region in the pentacoordinated state and that it appears in the 1593-1598 cm-1 region in the hexacoordinated state. On the basis of this criterion,18 we can classify the coordination states of BChl a as those listed in Table 2. (In ethyl acetate, this classification by the νr frequency does not agree with the classification by the wavelength of the Qx absorption (Vide infra), and therefore, this solution will be excluded from the following discussion. The νr Raman band appeared as a doublet, at 1597 and 1608 cm-1 in benzonitrile and at 1595 and 1608 cm-1 in N,N-dimethylformamide, showing that BChl a takes both the penta- and hexacoordination states in these solvents.18 Therefore, these solvents also will be excluded from the following discussion.) BChl a forms an aggregated state in methylene chloride,19-21 but its νr frequency (1611 cm-1) is in the same region as those of the pentacoordinated monomers. Therefore, the νr frequency cannot determine whether BChl a is in the aggregated state or in the monomeric state.11,17 Figure 4 shows the correlation between the νr and νr+ frequencies. The νr+ frequencies split into two regions, and a clear correlation is seen between the νr+ value and the state of coordination (except for the case of 2-propanol), if one assumes that the states of coordination are retained even after conversion from BChl a to BChl a•+. It can be concluded that BChl a•+ takes the pentacoordinated state in solvents a through g and that it takes the hexacoordinated state in solvents j through n. On the basis of the classification of the νr and νr+ frequencies, it can be concluded that in 2-propanol (i), the bacteriochlorophyll molecule changes the state of coordination from the penta- to the hexacoordinated state upon conversion from BChl a to BChl a•+. As explained in the next section, this interpretation will be confirmed by electronic absorption spectroscopy (Vide infra). There are some additional Raman bands that can be used to differentiate the coordination states of BChl a•+. (1) The Raman band in the 1320-1340 cm-1 region can be assigned to a coupled mode consisting of the Ca-N stretchings and the Cm-H in-plane bendings.10,16 In the pentacoordinated state (in solvents a-g), it appears in the 1330-1337 cm-1 region, while in the hexacoordinated state (in solvents j-n), it appears in the 13241327 cm-1 region. (2) The RR spectra of BChl a•+ in solvents, methylene chloride (a), propionitrile (d), ethyl acetate (h), and
Bacteriochlorophyll a Cation Radical
J. Phys. Chem., Vol. 100, No. 6, 1996 2425
Figure 3. RR spectra of D0 BChl a•+ in 14 different kind of solvents: (a) methylene chloride; (b) 2-octanone; (c) acetone; (d) propionitrile; (e) 3-pentanone; (f) n-butyronitrile; (g) 2-butanone; (h) ethyl acetate; (i) 2-propanol; (j) n-butanol; (k) tetrahydrofuran; (l) ethanol; (m) methanol (n) n-propanol.
TABLE 2: Solvent Effects on the Ring-Breathing Frequencies (νr and νr+) and on the Qx Absorption (λmax) and the Band E (λmax+) of BChl a and BChl A+, Respectively solvent
νr (cm-1)
λmax (nm)
νr+ (cm-1)
λmax+ (nm)
DNa
βb
π* b
(a) methylene chloride (b) 2-octanone (c) acetone (d) propionitrile (e) 3-pentanone (f) n-butyronitrile (g) 2-butanone (h) ethyl acetate (i) 2-propanol (j) n-butanol (k) tetrahydrofuran (l) ethanol (m) methanol (n) n-propanol (o) benzonitrile (p) N,N-dimethylformamide
1611 Vc 1608 V 1606 V 1611 V 1610 V 1609 V 1609 V 1597 VI 1613 V 1596 VI 1598 VI 1599 VI 1596 VI 1598 VI (1608 + 1597)g V and VIg (1608 + 1595)g V and VIg
581 V, Ad 582 V 580 V 579 V 578 V 579 V 579 V 584 V 584 V 606 VI 596 VI 606 VI 609 VI 607 VI 584 V 583 V
1599 Ve 1599 V 1598 V 1597 V 1596 V 1596 V 1595 V 1593 V 1588 VI 1588 VI 1586 VI 1585 VI 1584 VI 1584 VI
542 Vf 542 V 544 V 545 V 543 V 547 V 553 V 544 V 554 VI 558 VI 557 VI 559 VI 557 VI 558 VI 546 V 562 VI
0.0
0.00
0.82
17.0 16.1
0.48 0.37 0.45
0.71 0.71 0.72 0.71 0.67 0.55 0.48 0.47 0.58 0.54 0.60 0.52 0.90 0.88
16.6 17.1 20.0 19.0 20.0 18.0 11.9 24.0
0.48 0.45 0.95 0.88 0.55 0.77 0.62 0.41 0.69
a Taken from ref 22. b Taken from ref 23. c The coordination states classified in terms of the λ max value (see text). V and VI indicate the pentaand the hexacoordinated states, respectively, and A indicates the aggregated state. d The coordination states classified in terms of the νr frequency. e The coordination states classified in terms of the λmax+ value. f The coordination states classified in terms of the νr+ frequency. g Taken from ref 18. See text. h The donor number (DN) and the Taft’s parameters, β and π*, are also listed.
tetrahydrofuran (k), exhibit a weak RR band in the region of 1712-1719 cm-1. (The Raman bands with an asterisk in this region are due to the solvents.) Morita et al.9 assigned the IR bands near 1750 and 1710 cm-1 to the C132 ester and C131 keto-carbonyl stretchings, respectively. Referring to their assignments, we assign the RR bands to the C131 keto-carbonyl (C131dO) stretching vibration. Its frequency seems to depend on both the coordination and the aggregation states as follows:
1712 cm-1 in propionitrile forming a pentacoordinated monomer; 1715 cm-1 in methylene chloride forming a pentacoordinated aggregate; 1719 cm-1 in tetrahydrofuran forming a hexacoordinated monomer. Thus, upon transformation from the penta- to the hexacoordinated monomer, the C131dO stretching shows a high-frequency shift by 7 cm-1, which contrasts with the low-frequency shift of νr+ by 17 cm-1. The result suggests an electron redistribution in the bacteriochlorine macrocycle
2426 J. Phys. Chem., Vol. 100, No. 6, 1996
Figure 4. Correlation between the νr frequencies of S0 BChl a and the νr+ frequencies of D0 BChl a•+. See Figure 3 for the labels of the solvents a-g and i-n. Symbols O and 0 indicate the solvents forming the penta- and hexacoordinated states, respectively, in S0 BChl a. Symbol 4 indicates the 2-propanol solution (i), where transformation from the penta- to the hexacoordinated state takes place upon conversion from S0 BChl a to D0 BChl a•+.
upon transformation from the penta- to the hexacoordinated state; the π-electrons in the conjugated system may become more delocalized, while those of the terminal carbonyl group may become more localized. Classification of the Coordination States of BChl a•+ Based on Absorption Spectra. Figure 5 shows the absorption spectra of BChl a•+ in the region of 300-1000 nm in 16 different solvents. The absorption bands can be classified into seven groups, i.e., (A) 361-369 nm, (B) 397-401 nm, (C) 419-429 nm, (D) 505-530 nm, (E) 546-575 nm, (F) 869-885 nm, and (G) 902-937 nm. Table 2 lists the wavelength of the Qx absorption of BChl a (λmax) and the wavelength of band E of BChl a•+ (λmax+) in each solvent. In addition to the above absorption bands, another band was observed in the 335-342 nm region for some solvents, i.e., methylene chloride (a) and alcohols (i, j, l, and n). Callahan and Cotton17 observed the absorption spectra of neutral BChl a in 15 different solvents, and they established that the position of the Qx band (abbreviated to λmax) can be used as a marker of the coordination states. Umemoto et al.18 extended this study by measuring the absorption spectra of BChl a in 24 different solvents, and they concluded that λmax is in the region of 575-588 nm for the pentacoordinated state and that it is in the region of 593-610 nm for the hexacoordinated state. According to this criterion, the coordination states of BChl a in each solvent can be classified into either the penta- or hexacoordinated state, as listed in Table 2. Classifications of the coordination states using this criterion for the solvents ethyl acetate (h), benzonitrile (o), and N,N-dimethylformamide (p) are not consistent with those using the νr frequency, and, as already mentioned, these solvents are excluded from the discussion in this paper. Figure 6 shows the correlation between λmax and λmax+. The λmax+ values are split into two regions, and a clear correlation is seen between the λmax+ value and the state of coordination if one assumes that the states of coordination are retained even after conversion from BChl a to BChl a•+. It can be concluded that BChl a•+ takes the pentacoordinated state in solvents a-g and that it takes the hexacoordinated state in solvents j-n. Thus, the wavelength of band E (λmax+) serves as a marker to differentiate the coordination states of BChl a•+; the λmax+ value
Misono et al. is in the region of 546-554 nm in the pentacoordinated state, and it is in the region of 557-562 nm in the hexacoordinated state. Application of this criterion to the case of a 2-propanol solution shows that the bacteriochlorophyll molecule changes the coordination state from the penta- to the hexacoordinated state upon one-electron oxidation. This conclusion is in complete agreement with that obtained by resonance Raman spectroscopy (Vide supra). The reason for this change in the coordination state in 2-propanol will be discussed in the next section. Several other features of the absorption spectra in Figure 5 may also be related to the states of aggregation and hydrogen bonding. Bands A, B, C, D, F, and G are considerably blueshifted in aggregate-forming methylene chloride compared to those in the rest of pentacoordinating and monomer-forming solvents. This may be ascribed to the aggregated state of the BChl a•+ in methylene chloride. It is reasonable to consider that hydrogen bonding of solvent alcohols to the terminal C131 keto group may affect the adsorption spectral feature. Additional bands observed in the 339-342 nm region for alcohols (i, j, l, and n) are ascribable to the hydrogen-bonding interaction. Similarly, the coordination of the terminal C131 keto group to the magnesium atom may cause absorption spectral change. An additional band at 335 nm in methylene chloride can be ascribed to the coordination of the terminal group of one BChl a•+ molecule to the magnesium atom of the neighboring BChl a•+ molecule in the aggregated state. Correlation between the Coordination States of BChl a•+ and the Taft Parameters of the Solvents. Table 2 lists also the donor number (DN)22 and the Taft parameters, β and π*,23-25 of each solvent. Callahan and Cotton17 showed a linear relation between the DN value and the wavelength of the Qx absorption in neutral BChl a. Although this linear relation between DN and λmax+ is not found in the present case of BChl a•+, it can be concluded that the pentacoordinated state is formed in solvents with DN e 17 and that the hexacoordinated state is formed in solvents with DN g 18. Obviously, a solvent with a large DN tends to form the hexacoordinated state in BChl a•+. Taft et al.23-25 proposed “solvatochromic parameters”, β and π*, that measure the electron-donating power of the solvent and the dielectric stabilization of a dipole by the solvent system, respectively. Figure 7 shows the correlations between (A) β and νr+, (B) β and 1/λmax+, (C) π* and νr+, and (D) π* and 1/λmax+. Parts A and B of Figure 7 exhibit clear classifications of both νr+ and 1/λmax+ in terms of β. Together with the values of β and the classification of the coordination states listed in Table 2, it can be concluded that the hexacoordinated state (the pentacoordinated state) is formed in solvents with a value of β larger (smaller) than 0.5. Dependence of the coordination states on both the DN and β parameters indicates that the electron-donating power of the solvent is most important in forming the hexacoordinated state in preference to the pentacoordinated state. Parts C and D of Figure 7 exhibit more clear classifications of both νr+ and 1/λmax+ in terms of π*. Together with the values of π* and the coordination states listed in Table 2, it can be concluded that the pentacoordinated state (the hexacoordinated state) is formed in solvents with a value of π* larger (smaller) than 0.65. This dependence of the coordination states on the π* value can be explained as follows. Since an axial dipole is expected to be formed between the central magnesium atom of BChl a•+ and the oxygen or nitrogen atom of the solvent in the pentacoordinated state,10 dielectric stabilization of this dipole by the solvent system with a large π* must enhance the
Bacteriochlorophyll a Cation Radical
J. Phys. Chem., Vol. 100, No. 6, 1996 2427
Figure 5. Electronic absorption spectra of D0 BChl a•+ in 16 different kinds of solvents: (a) methylene chloride; (b) 2-octanone; (c) acetone; (d) propionitrile; (e) 3-pentanone; (f) n-butyronitrile; (g) 2-butanone; (h) ethyl acetate; (i) 2-propanol; (j) n-butanol; (k) tetrahydrofuran; (l) ethanol; (m) methanol; (n) n-propanol; (o) benzonitrile; (p) N,N-dimethylformamide. Bands marked by an asterisk are due to byproducts during the electrochemical oxidation, and those marked by S0 are ascribable to the Qy band of the neutral species, which exists as a minor component in the sample solutions.
Figure 6. Correlation between the λmax values of S0 BChl a and the λmax+ values of D0 BChl a•+. See Figure 4 for the symbols and the labels of solvents.
formation of this pentacoordinated state. On the other hand, a pair of axial dipoles with the opposite direction should be canceled out in the hexacoordinated state. Therefore, dielectric stabilization is not necessary in the hexacoordinated state, and a strongly dielectric environment must push the equilibrium from the hexacoordinated state to the pentacoordinated state. Thus, a solvent with a large π* must enhance the formation of the pentacoordinated state. The transformation from the penta- to the hexacoordinated state upon one-electron oxidation in 2-propanol does not con-
tradict the above correlations between the coordination states and the solvent parameters. The high value of β (0.95) and the low value of π* (0.48) of 2-propanol should favor the formation of the hexacoordinated state. On the other hand, a steric hindrance in the particular structure of the secondary alcohol is unfavorable for the formation of the hexacoordination state. Presumably, the latter factor is more important in determining the coordination state of BChl a, resulting in the formation of the pentacoordinated state. In the case of BChl a•+, a decrease in electron density on the magnesium atom may enhance the electron-donating power of the solvent in coordination. This effect must overcome the steric hindrance, causing the formation of the hexacoordinated state in the cation radical. The four correlations, νr+ and 1/λmax+ vs β and π* (parts A-D of Figure 7), show that both νr+ and 1/λmax+ depend on both parameters, β and π*, and that the manner of contribution of each parameter varies depending on the coordination states. On the assumption that νr+ or 1/λmax+ is a linear function of π* and β for each coordination state, the following empirical relations are obtained by means of least-squares fitting to the observed values. For the νr+ value,
νr+ ) 18.95π* - 2.177β + 1584
(1)
for the pentacoordinated state and
νr+ ) 51.15π* - 8.635β + 1620
(2)
for the hexacoordinated state were obtained. Here, the π* value plays a major role in determining the νr+ value. For 1/λmax+,
2428 J. Phys. Chem., Vol. 100, No. 6, 1996
Misono et al.
Figure 7. Correlations of the νr+ frequencies and the 1/λmax+ values with the Tafts parameters, π* and β: (A) νr+ vs β; (B) 1/λmax+ vs β; (C) νr+ vs π*; (D) 1/λmax+ vs π*. See Figure 3 for the labels of solvents and Figure 4 for symbols.
1/λmax+ ) 6260π* + 1270β + 13 300
(3)
for the pentacoordinated state and
1/λmax+ ) -3400π* - 1560β + 20 900
(4)
for the hexacoordinated state were obtained. The π* and β values play comparative roles in this case. For both expressions of νr+ (eqs 1 and 2) and 1/λmax+ (eqs 3 and 4), the coefficient of π* exhibited opposite signs between the penta- and hexacoordinated states, a fact that suggests that a different mechanism of dielectric stabilization is in operation in each coordination state. Comparison between the Preferential Solvation of T1 BChl a and that of D0 BChl a•+. The constant of preferential solvation, K, which measures the solvation of the bacteriochlorophyll molecule with a hexacoordinating solvent in preference to that with a pentacoordinating solvent,11 was determined for the following four systems as combinations of two chemical species of S0 BChl a and D0 BChl a•+ and two kinds of solvent mixtures of methanol/acetone and tetrahydrofuran (THF)/ methylene chloride (see ref 11 for the definition of K). K(S0) ) 2.8 and K(D0) ) 7.8 were obtained in the methanol/acetone mixture, while K(S0) ) 22 and K(D0) ) 72 were obtained in the THF/methylene chloride mixture. Upon conversion from S0 BChl a to D0 BChl a•+, the constant of preferential solvation
increased by 2.8 times in the methanol/acetone mixture and by 3.3 times in the THF/methylene chloride mixture. In contrast, the constant of preferential solvation decreased slightly upon triplet excitation:11 K(S0) ) 22 and K(T1) ) 19 were obtained in the THF/methylene chloride mixture, while K(S0) ) 62 and K(T1) ) 57 were obtained in the pyridine/methylene chloride mixture. Thus, the constant of solvation decreased by 0.86 times in the former mixture and by 0.92 times in the latter mixture. The large increase in the constant of preferential solvation upon one-electron oxidation seen in the present case of BChl a•+ is ascribable to an enhanced intermolecular interaction between the positively charged cation radical and the strongly electrondonating methanol or THF. The K(S0) and K(D0) values in the THF/methylene chloride mixture are larger than those in the methanol/acetone mixture, an observation that can be explained in terms of the difference in the DN (or β) value of each solvent pair. Table 2 shows that the difference in DN (β) is 20.0 (0.55) in the THF/methylene chloride pair and 3.0 (0.14) in the methanol/acetone pair. Thus, the larger constants of preferential solvation in the THF/ methylene chloride mixture can be ascribed to the larger difference in the electron-donating power between the solvent pair. Comparison between the Ring-Breathing Frequencies of T1 BChl a and Those of D0 BChl a•+. Figure 8 compares the ring-breathing frequencies in the S0 state (νr), the T1 state (νr′′),
Bacteriochlorophyll a Cation Radical
J. Phys. Chem., Vol. 100, No. 6, 1996 2429
Figure 8. Correlations of the ring-breathing frequencies of T1 BChl a (νr′′) and D0 BChl a•+ (νr+) with those of S0 BChl a (νr). See Figure 3 for labels of solvents a-g and j-n and Figure 4 for the symbols. For the correlation between νr′′ and νr, labels in ref 11 are used in a set of parentheses: (a) acetonitrile; (b) diethyl ether; (c) acetone; (d) ethyl acetate; (e) 2-propanol; (f) 2-butanone; (g) 1-butanol; (h) propyl ether; (i) 3-pentanone; (j) tetrahydrofuran; (k) 2-octanone; (l) 1-hexanol; (m) methylene chloride; (n) 1-decanol; (o) carbon tetrachloride; (p) pyridine. Symbols are as follows: b and 9 represent T1 BChl a in the pentaand hexacoordinating solvents, and . and 3 represent D0 BChl a•+ in the pentacoordinated aggregates and in the LHC of R. sphaeroides R26, respectively.
TABLE 3: Regions of the Ring-Breathing Vibration Frequencies (in cm-1) in the Two Coordination States and in the Three Electronic States of Bacteriochlorophyll a BChl a (νr)a BChl a+ (νr+)b T1 BChl a (νr′′)e a
pentacoordinated state
hexacoordinated state
1612-1606 1599-1593 1591-1585
1598-1593 1588-1584 1581-1578
Taken from ref 18. b This work. c Taken from ref 11.
and the D0 state (νr+). The regions of the νr, νr′′, and νr+ frequencies in these states are summarized in Table 3. The ringbreathing frequency clearly differentiates the T1 state and the D0 state although their spectral patterns are extremely similar to each other (compare Figure 3 of this paper and Figure 2 of ref 11). The ring-breathing frequencies in the pentacoordinated state are higher than those in the hexacoordinated state in all the cases of the S0, D0, and T1 states. For the dependence of the ring-breathing frequency on the electronic states, they are in the order νr > νr+ > νr′′ in both the penta- and hexacoordinated states. This order in the ring-breathing frequency can be explained as follows. According to the ab initio molecular orbital calculation of ethyl bacteriochlorophyllide a by Petke et al.,26 the two highest occupied molecular orbitals (HOMO’s), MO 1 and MO 2, are of nonbonding and bonding nature, respectively, with respect to the Ca-Cm bond, while the two lowest unoccupied orbitals (LUMO’s), MO 1* and MO 2*, are of definite antibonding nature. Therefore, the removal of one electron from the HOMO’s should cause a decrease in the Ca-Cm bond order and, thus, a decrease in the ring-breathing frequency (νr > νr+). In addition of one electron with parallel spin to the LUMO’s should cause a further decrease in the Ca-Cm bond order and in the ring-breathing frequency (νr+ > νr′′). The νr+ and νr′′ frequencies, shown in Figure 8 and summarized in Table 3, can be used to identify the electronic and the coordination states of bacteriochlorophyll a in the aggregated form and in the pigment-protein complexes; Nishizawa et al.11 recorded the transient Raman spectra of BChl a in aggregate-forming carbon tetrachloride and methylene chlo-
ride, and they detected the ring-breathing Raman bands at 1599 and 1597 cm-1. They first assigned the transient species to “aggregated T1 BChl a”, but now it is clear that the transient species should be assigned to the pentacoordinated D0 BChl a•+ instead (see symbols . in Figure 8). Nishizawa et al.11 recorded also the transient Raman spectrum of BChl a bound to the light-harvesting complex of R. sphaeroides R26. The observed ring-breathing frequency at 1598 cm-1 strongly suggests that the transient species should be assigned not to T1 BChl a 11 but to D0 BChl a•+ (see symbol 3 in Figure 8). Nishizawa and Koyama27 recorded the transient Raman spectrum of the RC of R. sphaeroides R26, and they detected a Raman band at 1601 cm-1. The most important piece of information obtained by the above classification of the electronic and coordination states of bacteriochlorophyll a is that the cation radical (D0 BChl a•+) in the aggregates and in the lightharvesting complex (LHC) is in the pentacoordinated state. Thus, resonance Raman spectroscopy is a powerful tool for probing the intermolecular interaction of bacteriochlorophyll a. Acknowledgment. We are grateful to Professor Hiroyoshi Nagae of Kobe City University of Foreign Studies for helpful discussions about the relationships νr+ and 1/λmax+ vs the solvent parameters. References and Notes (1) Deisenhofer, J.; Michel, H. In Chlorophylls; Scheer, H., Ed.; CRC Press: Boca Raton, FL, 1991. (2) Parson, W. W. Ann. ReV. Biophys. Bioeng. 1982, 11, 57. (3) Cotton, T. M.; van Duyne, R. P. Biochem. Biophys. Res. Commun. 1978, 82, 424. (4) Cotton, T. M.; Parks, K. D.; van Duyne, R. P. J. Am. Chem. Soc. 1980, 102, 6399. (5) Lutz, M.; Kleo, J. Biochim. Biophys. Acta 1979, 546, 365. (6) Mattioli, T. A.; Hoffmann, A.; Robert, B.; Schrader, B.; Lutz, M. Biochemistry 1991, 30, 4648. (7) Ma¨ntele, W. G.; Wollenweber, A. M.; Nabedryk, E.; Breton, J. Proc. Natl. Acad. Sci. U.S.A. 1988, 85, 8468. (8) Morita, E. H.; Hayashi, H.; Tasumi, M. Biochim. Biophys. Acta 1993, 1142, 146. (9) Morita, E. H.; Hayashi, H.; Tasumi, M. Chem. Lett. 1991, 1583. (10) Misono, Y.; Nishizawa, E.; Limantara, L.; Koyama, Y.; Itoh, K. Chem. Phys. Lett. 1995, 236, 413. (11) Nishizawa, E.; Limantara, L.; Nanjou, N.; Nagae, H.; Kakuno, T.; Koyama, Y. Photochem. Photobiol. 1994, 59, 229. (12) Nishizawa, E.; Koyama, Y. Chem. Phys. Lett. 1990, 172, 317. (13) In Purification of Laboratory Chemicals, 3rd ed.; Perrin, D. D., Armarego, W. L. F., Eds.; Pergamon Press: Oxford, 1988. (14) House, H. O.; Feng, E.; Peet, N. P. J. Org. Chem. 1971, 36, 2371. (15) Rousseau, K.; Farrington, G. C.; Dolphin, D. J. Org. Chem. 1972, 37, 3968. (16) Limantara, L.; Koyama, Y.; Katheder, I.; Scheer,H. Chem. Phys. Lett. 1994, 227, 617. (17) Callahan, P. M.; Cotton, T. M. J. Am. Chem. Soc. 1987, 109, 7001. (18) Umemoto, Y.; Furukawa, Y.; Koyama, Y. Unpublished work. (19) Cotton, T. M.; Heald, R. L. J. Phys. Chem. 1987, 91, 3891. (20) Cotton, T. M.; van Duyne, R. P. J. Am. Chem. Soc. 1981, 103, 6020. (21) Cotton, T. M.; van Duyne, R. P. J. Am. Chem. Soc. 1979, 101, 7605. (22) Gutmann, V. In Donor-Acceptor Approach to Molecular Interactions; Plenum Press: NY, 1978; p 20. (23) Kalmet, M. J.; Abbound, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (24) Kalmet, M. J.; Abbound, J. L. M.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 6027. (25) Kalmet, M. J.; Taft, R. W. J. Chem. Soc., Perkin Trans. 2 1979, 337. (26) Petke, J. D.; Maggiora, G. M.; Shipman, L. L.; Christoffersen, R. E. Photochem. Photobiol. 1980, 31, 243. (27) Nishizawa, E.; Koyama, Y. Unpublished work.
JP952295M
