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J. Phys. Chem. 1996, 100, 14192-14197
Conformational Properties of Nickel(II) Octaethylporphyrin in Solution. 2. A Low-Temperature Optical Absorption Spectroscopy Study Antonio Cupane,*,† Maurizio Leone,† Lorenzo Cordone,† Harald Gilch,‡ Wolfgang Dreybrodt,‡ Esko Unger,‡ and Reinhard Schweitzer-Stenner*,‡ FB1-Institut fu¨ r Experimentelle Physik, UniVersita¨ t Bremen, 28359 Bremen, Germany, and Istituto di Fisica and INFM-GNSM, UniVersity of Palermo, I-90123 Palermo, Italy ReceiVed: NoVember 7, 1995; In Final Form: March 26, 1996X
We have measured the absorption spectrum of Ni(II) octaethylporphyrin in CH2Cl2 and in a 50% v/v isopentane/ ethyl ether mixture as a function of temperature between 150 and 300 K and 40 and 300 K, respectively. The Soret band can be decomposed into two subbands whose frequencies differ by 220 cm-1. By analogy with resonance Raman results (Jentzen et al. J. Phys. Chem. 1996, 100, 14184-14191 (preceding paper)), we attribute the low-frequency subband to a conformer with a nonplanar macrocycle structure, whereas the high-frequency subband is interpreted as resulting from a planar conformer. The subbands’ intensity ratios exhibit a solvent-dependent van’t Hoff behavior between 300 and 160 K. Crystallization of CH2Cl2 prevents measurements at lower temperatures. For Ni(II) octaethylporphyrin in the glass-forming isopentane/ethyl ether mixture, the intensity ratio bends over in a region between 150 and 100 K and remains constant below. These data can be fitted by a modified van’t Hoff expression which also accounts for the freezing of the above conformers into a nonequilibrium distribution below a distinct temperature Tf. The fit yields a freezing temperature of Tf ) 121 K and a transition region of 52 K. In accordance with the Raman data we found that the nonplanar conformer has the lowest free energy and is therefore dominantly occupied at low temperatures. Furthermore we found that the Soret band’s profile is Voigtian with a temperature-dependent Gaussian contribution. The latter results from a bath of low-frequency modes to which the electronic transition into the B state is vibronically coupled. This most likely comprises out-of-plane modes of the porphyrin, in particular those involving the central metal atom, and molecular motions within the liquid environment. At temperatures above the glass transition of the solvent, the amplitudes of these motions increase above the values predicted by a purely harmonic model. This is indicative of strong nonharmonic contributions to their potential energy.
Introduction The elucidation of the conformational and dynamic properties of the active site is an essential prerequisite for the understanding of functional properties of heme proteins. In the natural systems, however, the situation is complicated by the fact that the conformation of the highly flexible Fe protoporphyrin IX is determined by the simultaneous presence of several effects such as the iron-proximal histidine linkage, van der Waals contacts between heme group peripheral substituents and amino acid side chains within the heme pocket, interactions between the proximal histidine and the methine carbon atoms of the porphyrin core, heme-ligand interactions, etc.1 Nickel(II) octaethylporphyrin (NiOEP) plays a central role as a model substance for the properties of porphyrin embedded in heme proteins, in particular because its periphery is not highly crowded by the peripheral substituents and the ionic radii of Ni(II) and Fe(II) are closely similar. Studies on the conformational properties of NiOEP in solution are therefore expected to provide information on the intramolecular interactions relevant to the structure as well as on their dependence on the physicochemical properties of the solvent medium. NiOEP crystallizes in three different forms.2 Two of them designated as triclinic A and B contain planar porphyrins but with different bond lengths and ethyl orientations. The * Authors to whom all correspondence should be addressed. † Universita ¨ t Bremen. ‡ University of Palermo. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(95)03304-1 CCC: $12.00
tetragonal crystal C, however, contains nonplanar porphyrins whose pyrrole rings are tilted. The planar and nonplanar conformations can be distinguished by resonance Raman spectroscopy, because nonplanar distortions cause a downshift of some prominent marker bands’ frequencies in the macrocycle’s Raman spectrum,3,4 namely ν2, ν3, ν10, ν11, and ν19. For NiOEP in solution, the Raman band arising from the ν10 mode is significantly broadened and has its frequency between the respective band positions in the spectra of triclinic A/B and C. Moreover the frequency of the ν10 mode was found to increase with decreasing tempeature.5 This led Spiro and co-workers5 to the conclusion that NiOEP is slightly nonplanar at room temperature but approaches a planar structure at low cryogenic temperatures. As it was shown in the preceding paper,6 the band profiles of the core size marker bands ν2, ν10, and ν19 of NiOEP in CS2 and CH2Cl2 can be decomposed into two different subbands. The low- and high-frequency subbands of ν10 and ν19 were attributed to conformers with nonplanar and planar macrocycles, respectively. The analysis of the temperature dependence of corresponding subbands’ intensity ratios yields the enthalpic and entropic part of the free energy difference between the conformers. Alltogether these data suggest that, in organic solvents, NiOEP exists as a mixture of planar and nonplanar conformers with the nonplanar structure being energetically stabilized with respect to the planar one. Resonance excitation profiles (REP) of the above Raman bands show that the subbands of ν10 and ν19 exhibit different resonance energies, i.e. the REPs of the low-frequency subbands © 1996 American Chemical Society
Conformation of Ni(II) Octaethylporphyrin in Solution. 2
J. Phys. Chem., Vol. 100, No. 33, 1996 14193
are red-shifted with respect the those of the corresponding highfrequency components. This prompted the present study in which we report the temperature dependence of the Soret band of NiOEP in two different solvents, namely dichlormethane (CH2Cl2) and 50% v/v isopentane/ethyl ether (IPEE). The data cover the temperature region 300-160 K for CH2Cl2 and 30040 K for IPEE. We show that the Soret band of NiOEP can also be decomposed into two subbands which we assign to a planar and a nonplanar conformer, respectively. The enthaplic and entropic differences between these conformers and their dependence on the solvent’s physical properties are derived from the temperature dependence of the subbands integrated intensities. The temperature dependence of the Gaussian width, in turn, provides information on the dynamic properties of the conformers. The agreement between optical spectroscopy and Raman data is excellent; in fact, we believe that the combined use of different experimental, computational, and theoretical approaches is necessary in order to gain insight on the conformational, dynamic, and functional properties of these systems. Materials and Methods Material. NiOEP was purchased from Sigma Chemie. Purification and sample homogeneity control were perfomed as described in the previous paper.6 Dichloromethane, isopentane, and ethyl ether were purchased from Fluka and were of UV spectroscopy grade. Sample concentrations were chosen in order to obtain maximum absorbance values between 1 and 2 in the whole temperature range investigated. Optical Spectroscopy. Spectra in the 500-300-nm region were recorded in digital form at 0.5-nm intervals with a PCIBM controlled Cary Varian 2300 spectrophotometer; the scan speed was 0.5 nm/s and the integration time 0.5 s; the slitwidth was less than 0.2 nm in the whole wavelength range. Measurements were performed on NiOEP dissolved in two different solvents: dichloromethane (DCM) and 50% v/v isopentane in ethyl ether (IPEE). The temperature range investigated was 300-160 K in DCM and 300-40 K in IPEE and was limited by sample cracking. Samples were cooled at a rate of 1.5 K/min and were kept at each temperature for equilibration; thermal cycling yielded fully reproducible results. Baselines (cuvette + solvent) were measured as a function of temperature in separate experiments and subtracted from the spectra. The experimental setup and methods for optical absorption measurements in the temperature range 300-20 K have been described in previous publications.7 Data Analysis. The procedure adopted to analyze the Soret band profile at various temperatures has been reported previously.7,8 Therefore we confine ourselves on illustrating only the principal aspects. In view of the results6 reported in the previous paper we assume that the optical bands are composed of two subbands SbI and SbII due to the existence of porphyrin conformers with planar and nonplanar macrocycles. For each individual subband the band profile is described as the convolution of two terms:
Sbi(ν) ) Miν[Li(ν)XGi(ν)]
(1)
where i labels the subbands and Mi is a constant proportional to the square of the electric dipole matrix element. The first term, L(ν), is the usual Lorentzian line shape for an absorption band. It can be written as
{[ ]
Li(ν) )
Ni,k S mi,ke-Si,k i,k
∑ ∏ k)0
{mi,k}
mi,k!
Γi
‚
Nk
}
[ν - νi,0(T) - ∑mi,kRi,kνi,k]2 + Γi2 k)0
(2) where Γi is a damping factor related to the finite lifetime of the excited state; the product extends to all high-frequency porphyrin vibrations (hνk,i > kBT) which become Franck-Condon active by the transition into the B states. The sum runs over their occupation numbers. The coupling of the electronic transition with high-frequency modes is described by linear and quadratic coupling constants Si and Ri, respectively. The term L(ν) therefore takes into account the natural (homogeneous) width of the electronic transition and coupling with “high-frequency modes”. We recall that we consider “high-frequency modes” those for which transitions from the excited vibrational states of the electronic ground state are negligible. The coupling of the electronic transition with a bath of “lowfrequency modes” introduces the second term, Gi(ν), in eq 1 and gives rise to a gaussian broadening of the Lorentzian band:
Gi(ν) ) σi(T)-1 exp[-ν2/2σi2(T)]
(3)
where σi is the temperature-dependent half-width. “Lowfrequency modes” are those whose vibrational frequency is on the same order of, or smaller than, kBT. Transitions from excited vibrational levels do therefore occur. When the effects of coupling with both high- and low-frequency modes are taken into account, the shape of each subband is described by a superposition of Voigtians (convolution of a Lorentzian with a Gaussian). Results and Discussion Spectral Analysis. Figure 1 shows the Soret band of NiOEP in IPEE taken at various temperatures between 300 and 40 K. Marked changes of the band shape occur upon lowering the temperature. In particular, the total absorbance increases, the band shifts to lower energies, and a structure emerges at the high-energy side which results from vibronic transitions into the first vibrationally excited states. Figure 2 shows the Soret band of NiOEP in IPEE taken at 40 K (left panel), 170 K (center panel), and 300 K (right panel). The solid lines therein result from fitting eqs 1-3 to the data. All fits are satisfactory, as judged by the residuals depicted in the upper panels of the respective figures. Analogous results are obtained at all the other temperatures investigated and also for NiOEP in DCM. Values of the relevant parameters are reported in Table 1. Some further comments about the fitting are given in the following: (1) The peak frequency shift between the two subbands has been taken as ∆0 ) ν0(SbII) - ν0(SbI) ) 220 cm-1. This is very close to the values obtained from the differences between the Q-band resonance positions of resonance Raman bands.6 This value has been assumed to be temperature independent. (2) The Lorentzian half-width Γ ) 335 cm-1 has been obtained from the fits to the low-temperature spectra, where it is unambiguously determined by the red edge of the band. We assumed that this value is identical for both subbands and temperature independent. In a separate fit, in which this parameter was allowed to vary, variations of no more than 10% were observed.
14194 J. Phys. Chem., Vol. 100, No. 33, 1996
Cupane et al. TABLE 1: Values of Spectral Parameters Obtained from Fits to the Soret Band of NiOEP Dissolved in IPEE and DCMa solvent
T (K)
S350
S1520
Γ (cm-1)
σ (cm-1)
∆ν0 (cm-1)
ν0 (cm-1)
IPEE IPEE DCM DCM
40 300 160 300
0.16 0.16 0.16 0.16
0.046 0.046 0.046 0.046
335 335 335 335
67 229 135 244
220 220 220 220
25401 25424 25397 25351
a The values of S353, S1520, Γ, and σ were assumed to be identical for both subbands. ν0 values refer to SbI.
Figure 1. Soret band of NiOEP in IPEE at various temperatures. From top to bottom: 40, 100, 150, 170, 200, 260, and 300 K.
Figure 3. Resonance Raman spectra of NiOEP in CS2 at 300 K (upper spectrum) and at 170 K. The excitation wavelength is 406.7 nm.
modes of NiOEP in CH2Cl2. Though their data cover the Q-band rather than the B-band region the analysis yielded good estimation of the parameter cBBA1g of the ν4 mode, which describes the intramanifold FC-type coupling within the B states.11 This parameter is related to the coupling strength S used in the present study by12
Sk ) (cBBA1g/Ωk)2
Figure 2. Fittings of the Soret band of NiOEP in IPEE at 40 K (left), 170 K (center), and 300 K (right) in terms of eqs 1-3. Triangles are the experimental points; for the sake of figure readability, not all of the experimental points have been reported. Dashed lines are the spectral profile calculated according to eqs 1-3 and the extrapolation; the continuous line is the overall calculated band profile. The residuals are also shown in the upper panel, on an expanded scale.
(3) Only two “high-frequency modes” are needed in the fits, one at 350 cm-1 and the other at 1520 cm-1. Their Sk values are listed in Table 1. Since our resolution of the vibronic structure is limited by the intrinsic homogenous width of the subbands, i.e. 335 cm-1, these modes should be considered as effective ones averaging over the nearby frequency region. Hence one may identify the 350 cm-1 mode with the ν8 doublet at 360 cm-1, whereas the 1520 cm-1 “mode” could well reflect contributions from ν4 (1383 cm-1), ν3 (1520 cm-1), and ν2 (1602 cm-1), which show a comparable intensity in the resonance Raman spectrum with excitation in the Soret band region (Figure 3).6,9 This notion is corroborated by the following argument. In a recent study Bobinger et al.10 have measured and analyzed the resonance excitation profiles of several high-frequency
(4)
where Ωk is the frequency of the kth mode in cm-1. Bobinger et al. obtained cBBA1g ) 120 cm-1 for ν4. Raman spectra of NiOEP measured with 406 nm excitation reveal very similar intensities for ν2 and ν4, whereas the ν3 intensity is by a factor 1.3 larger (Figure 3). Thus their coupling strength would add up to an effective value of [2(120 cm-1)2 + (1.3 × 120 cm-1)2]1/2 ) 220 cm-1. The corresponding effective Sk value can be estimated to 0.022 by use of eq 4. This is by a factor of 2 smaller than the Sk value of the effective 1520 cm-1 mode used in the fit to the B band (Table 1), but this discrepancy is still in the limit of uncertainty of the cBBA1g parameter derived from the resonance excitation profiles. Moreover, by comparing the intensities of the ν8 doublet and the ν4 band in the Raman spectrum taken with 406 nm excitation, the total coupling parameter cBBA1g of the former can be estimated to 110 cm-1. This corresponds to Sk ) 0.093, which is again by a factor of 2 smaller than the S value of the 350 cm-1 “mode” employed in the fit to the Soret band (Table 1). This shows that in particular the ratio S350/S1520 obtained from the absorption data is consistent with the resonance Raman spectrum measured with Soret excitation. (4) Temperature-independent S values are in agreement with resonance Raman data.12 Moreover we assumed the S values of the two subbands to be indentical. This may well be an oversimplification, but the data do not allow us to unambiguously discriminate between corrsponding S values of the two
Conformation of Ni(II) Octaethylporphyrin in Solution. 2
Figure 4. Ratio of conformer populations as a function of reciprocal temperature: filled symbols, sample in IPEE; open symbols, sample in DCM. The solid lines represent fits in terms of eq 6 (see Results and Discussion).
conformers. A further comment concerns the treatment of the mode at 350 cm-1 as a “high-frequency mode”. In fact kBT is equal to 215 cm-1 at T ) 300 K, so that 〈n〉350 ) 0.2. A simple calculation shows, however, that for S ) 0.16 the weight of the anti-Stokes 1-0 transition even at 300 K is only 2%. It can therefore be safely neglected. (5) Values of the Gaussian width σ(T) are also taken to be equal for the two subbands. This implies that the strength of the two conformers’ coupling to low-frequency motions is identical. It is necessary to mention that satisfactory fitting to all band shapes can also be achieved by invoking only one band whose band shape is described by eqs 1-3. This reduces the number of fitting parameters by one. We found that this model yields χ2 values larger by about a factor of 2 at all temperatures. Even these fits could only be obtained by allowing the coupling parameter S350 to vary as a function of temperature. Thus we obtained 0.026 at 300 K, 0.12 at 180 K, and 0.22 at 40 K. This prediction was directly checked by measuring the NiOEP resonance Raman spectra at 300 and 180 K with Soret excitation. The spectra are shown in Figure 3. They do not show any indication of larger intensities of low-frequency modes at low temperature. This demonstrates that the one band model does not suffice for the B band in NiOEP, in contrast to what has been observed for various heme proteins and model compounds.7,8 It was therefore disregarded. Temperature Dependence. In the preceding paper6 we have shown that the prominent structure-sensitive Raman lines of Ni(OEP) in solution can be decomposed into two subbands the intensity ratio of which exhibit a strong dependence of temperature. The latter was rationalized by invoking a two-state model which assumes two different conformers of the porphyrin’s macrocycle, i.e. a nonplanar ruffled and a nearly planar one. This model accounts for the temperature dependence of the Raman lines ν19 and ν10. In analogy with the resonance Raman data reported in the previous paper, we consider the two subbands shown in Figure 2 as arising from different conformers of the NiOEP molecule. On the basis of the previously reported resonance excitation profiles6 and computational predictions,13 we attribute the lowfrequency subband SbI to a conformer with a nonplanar (ruffled) macrocycle, whereas the high-frequency subband (SbII) is assigned to a more planar conformer. A van’t Hoff plot of the subbands’ intensity ratios is shown in Figure 4. It shows a van’t Hoff behavior between 300 and 150 K for both solvents. For IPEE it bends over at lower
J. Phys. Chem., Vol. 100, No. 33, 1996 14195 temperatures and stays constant below 100 K. A similar behavior has earlier been observed for subbands of spectral marker bands of myoglobin, namely the νFe-His Raman band of horse heart deoxymyoglobin14 and the IR band resulting from the stretching mode of heme-bound CO in sperm whale myoglobin.15 These data were interpreted as indicative of taxometric conformational substates of the protein.15,16 While these substates are in thermodynamic equilibrium at room temperature, they freeze out below a temperature Tf which can be attributed either to the glass transition of the solvent (for the IR data)15,16 or the protein environment of the prosthetic heme group.14 The data depicted in Figure 4 suggest that a similar freezing mechanism is operative for NiOEP in IPEE, which should be assigned to a structrual arrrest of molecular motions in the solvent (glass transition). Thus our data demonstrate a striking similarity between porphyrins embedded in a liquid and a protein matrix. This supports the notion that at least parts of the proteins undergo liquid-like motions.17,18 In order to fit the data in Figure 4, we therefore follow the approach proposed by Gilch et al.,14 in that we introduce a heuristic temperature Teff. At high temperatures, where equilibrium is maintained, Teff equals the actual temperature T, whereas it is equal to the freezing temperature Tf in the lowtemperature region. In the intermediate region, the effective temperature deviates from the actual temperature due to incomplete equilibration. A reasonable function with such properties is given by
Teff )
Tf T + T - Tf Tf - T 1 + exp 1 + exp ∆T ∆T
(
)
(
)
(5)
where ∆T is the width of the transition region. The temperature dependence of the above subbands’ absorption ratios can be fitted by the following modified van’t Hoff equation:
A(SbI) A(SbII)
)
(SbI)
e(∆SI,II/R)e(-Hi,ii/RTeff)
(SbII)
(6)
where A(SbI) and A(SbII) are the integrated intensities of subbands I and II, respectively. (SbI) and (SbII) denote the intrinsic integrated absorption of the two conformers, which may not be identical.19 As shown by the solid line in Figure 4 the quality of the fit to the data observed for NiOEP in IPEE 3 gives support to the analysis proposed and yields a ∆HI,II value of -3.5 kJ/mol. The obtained prefactor (i.e., [(SbI)/(SbII)] e(∆SI,II/R)) is -2.9 J/mol K. The Tf and ∆T values are 121 and 52 K. The corresponding data observed for NiOEP in DCM can be fitted to a normal van’t Hoff equation. That yields -5.8 and -3.6 J/mol K for ∆HI,II and the prefactor, respectively. The latter ∆HI,II value is within a factor 1.5 identical to the analogous value obtained from the corresponding (two-state) analysis of temperature dependence of resonance Raman band shapes.6 In view of the various approximations made in the analysis of both data sets, we consider this agreement as satisfactory. In particular, both sets of data indicate that the ruffled conformer has a lower enthalpy and entropy with respect to the planar one. Moreover, the solvent dependence of thermodynamic parameters underlines the notion inferred from the resonance
14196 J. Phys. Chem., Vol. 100, No. 33, 1996
Cupane et al.
Figure 5. (left) 2 values as a function of temperature: filled symbols, sample in IPEE; open symbols, sample in DCM. (right) 0 values as a function of temperature: filled symbols, sample in IPEE; open symbols, sample in DCM; triangles, subband I; circles, subband II.
Raman data that solvent-porphyrin interactions affect the structure of the latter. As mentioned above, our data indicate that coupling processes between porphyrins and liquid solvents are similar to those observed for porphyrin-protein systems. In this case, porphyrin and solvent should be considered as a dynamic entity in which vibrational and relaxational motions of the solvent are coupled to structural fluctuations of the porphyrin atoms.17,18 Information on the dynamics of the conformers can be inferred from the temperature dependence of parameters σi2 and ν0. Within the framework of the harmonic approximation, the temperature dependence of the Gaussian width can be expressed as8
σh2(T) ) NSlRl2〈ν〉2coth(h〈ν〉/2kBT) + 2σin
(7)
where kB is the Boltzmann constant, N, 〈ν〉, Sl, and Rl are respectively the total number, the effective frequency, and the effective linear and quadratic coupling constant of the lowfrequency modes coupled to the transition. “h” is used as a label for σ to indicate that eq 7 is only valid in the harmonic regime. σin reflects an eventual temperature independent Gaussian inhomogeneous broadening. Moreover, due to the presence of quadratic coupling (Rl), the frequency ν0 becomes dependent on temperature also:
ν0(T) ) ν00 - 1/4N〈ν〉(1 - Rl) coth(h〈ν〉/2kBT) + C (8) where ν00 is the purely electronic (0-0) transition frequency and C takes into account other temperature-independent contributions related to the coupling with the low-frequency bath. Figure 5 displays the temperature dependence of σ2 and ν00. The left panel shows the thermal behavior of σ2 in which the solid line results from a fit of eq 7 to the data. A significant deviation dσ ) σ(Τ) - σh(T) becomes apparent for temperatures above 120 K. That observation correlates nicely with the Tf value derived from the temperature dependence of the subbands’ intensity ratio. Thus it indicates that both effects, i.e. the freezing in of the porphyrin conformers and the deviation of σ(T) from a harmonic behavior have one common reason, i.e. a glassy transition of the solvent. It is interesting to compare the magnitude of the above deviation from anharmonicity with corresponding obtained from the B-band analysis of unligated and ligated myoglobin and of protoporphyrin IX.20 The latter was dissolved in a methanol/ glycerol/NaOH mixture. A glycerol/water mixture was used as solvent for the proteins. The spectral analysis yielded dσ (300 K) values of approximately 37 cm-1 for carbon monoxymyoglobin, 61 cm-1 for deoxymyoglobin, 60 cm-1 for hemeCO, and approximately 118 cm-1 for NiOEP in IPEE. Hence
it seems that dσ becomes somewhat enhanced for porhyrins in a pure liquid. The physical interpretation of dσ deserves some further comment. For heme proteins and heme complexes, it was argued that it results from the coupling of porphyrin and protein motions which become anharmonic above the freezing temperature of the protein solvent environment. In a recent paper Leone et al.20 showed that dσ involves out-of-plane motions of the central iron atom, which are modulated by protein/solvent motions. The metal ion's interaction with the porphyrin macrocycle is most likely caused by pseudo-Jahn-Teller coupling between the electronic ground state of the porphyrin and low lying electronic states of the central metal atom.21 The involvement of out-of-plane motions comprising the central iron atom in the Soret band thermal broadening is not surprising because in particular the hexacoordinated heme group is subjected to A2u-type distortions (doming), which may cause normal modes of this symmetry to become Raman active via intrastate Franck-Condon coupling.22 The situation is somewhat different for NiOEP, however, since at least the macrocycles of the planar conformers should have D4h symmetry so that out-of-plane modes cannot contribute to the Soret transition. One possible coupling mechanism is B1g-type pseudo-JahnTeller coupling between the porphyrin ground state and an excited charged transfer state of Ni(II) which has recently been shown to contribute to the resonance excitation profiles of B1gtype porphyrin modes.23 The present data on NiOEP corroborate the notion that molecular motions within the liquid/protein environment of a porphyrin strongly effects the structural and electronic properties of the latter, most likely via various non-covalent and electrostatic interactions.24 Since the motions are slow compared with Raman scattering and optical absorption,25 they create an inhomogeneity which is quasi-static on the time scale of our experiment. Above the glass transition temperature the amplitudes of the motions depend on temperature.17b,26 Therefore, by interacting with the porphyrin, they also cause a strong temperature dependence of the Gaussian half-width σ. The interpretation is consistent with the observation that dσ is larger for porphyrins dissolved in liquids than for the corresponding functional groups in heme proteins because the protein atoms move on a smaller scale compared with the molecules in a liquid.27 The assessment of the relative contributions of porphyrin vibrations involving the central metal atom and of motions within the protein/liquid environment to σ2(T) in various systems, however, requires detailed systematic studies on the effect of various glass-forming solvent, central metals, and peripheral substituents on the electronic and structural properties of porphyrins. Conclusions The absorption spectroscopy data reported in this work show that the Soret band of NiOEP dissolved in organic solvents can be decomposed in two subbands separated by 220 cm-1. In agreement with resonance Raman we attribute these two subbands to different NiOEP conformers having planar (highfrequency subband) and nonplanar (low-frequency subband) macrocycle structure. Absorption spectroscopy enables monitoring of the population ratio of the two conformers in a wide temperature range, including also temperatures where “freezing” of the solvent matrix has occurred. Therefore, not only the enthalpic and entropic difference between the conformers but also the critical temperature range in which the sample cannot reach equilibrium on the time scale of the experiment can be
Conformation of Ni(II) Octaethylporphyrin in Solution. 2 determined. Enthalpy and entropy differences are in reasonable agreement with analogous quantities estimated from resonance Raman spectroscopy: they show that the nonplanar, ruffled conformer is characterized by lower enthalpy and lower entropy with respect to the planar one. Moreover, all thermodynamic quantities are highly dependent upon solvent composition. This confirms that interactions with the solvent play a central role in determining the structure of the porphyrin macrocycle. From the temperature dependence of the subbands Gaussian width and peak frequency provide a description of the NiOEPsolvent entity as a highly mobile system, in which motion within the liquid are coupled to the porphyrin macrocycle. These motions are frozen in when the solvent undergoes a transition from the liquid to a glassy state. In this regard the porphyrinsolvent system parallels pophyrins embedded in protein matrices, thus underscoring the notion that proteins and liquids have some important physical properties in common. Acknowledgment. This project is financially supported by the European Community in the framework of the network program “The dynamcics of protein structure”, which is part of the “Human Capital Mobility Program”. References and Notes (1) (a) Gellin, B. R.; Karplus, M. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 801. (b) Gellin, B. R.; Lee, A. W.; Karplus, M. J. Mol. Biol. 1983, 171, 489. (2) (a) Cullen, D. L.; Meyer, E. F., Jr. J. Am. Chem. Soc. 1976, 96, 2095. (b) Brennan, T.; Scheidt, W. R.; Shelnutt, J. A. J. Am. Chem. Soc. 1988, 110, 3919. (3) (a) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.; Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077. (b) Hobbs, J. D.; Majumder, S. A.; Luo, L.; Sickelsmith, G. A.; Quirke, J. M. E.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1994, 116, 3261. (4) (a) Shelnutt, J. A.; Hobbs, J. D.; Majumder, S. A; Sparks, L. D.; Medforth, C. J., Senge, M. O.; Smith, K. M.; Miura, M.; Quirke, J. M. E. J. Raman Spectrosc. 1992, 23, 523. (b) Anderson, K. K.; Hobbs, J. D.; Luo, L.,Kimberley, D. S.; Quirke, J. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1993, 115, 123 (5) Czernuszewicz, R. S.; Macor, K. A.; Li, X.-Y.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem. Soc. 1989, 111, 3860. (6) Jentzen, W.; Unger, E.; Karvounis, G.; Shelnutt, J. A.; Dreybrodt, W.; Schweitzer-Stenner, R. J. Phys. Chem. 1996, 100, 14184-14191. (7) (a) Cupane, A.; Leone, M.; Vitrano, E.; Cordone, L.; Hitpold, U. R.; Winterhalter, K. H.; Yu, W.; DiIorio, E. . Biophys. J. 1993, 65, 2461.
J. Phys. Chem., Vol. 100, No. 33, 1996 14197 (8) Cupane, A.; Leone, M.; Vitrano, E.; Cordone, L. Eur. Biophys. J. 1995, 23, 385. (9) Li, X-Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Stein, P.; Spiro, T. G. J. Phys. Chem. 1990, 94, 47. (10) Bobinger, U.; Schweitzer-Stenner, R.; Dreybrodt, W. J. Phys. Chem. 1991, 95, 7625 (11) (a) Schweitzer-Stenner, R. Q. ReV. Biophys. 1989, 22, 381. (b) Schweitzer-Stenner, R.; Bobinger, U.; Dreybrodt, W. J. Raman Spectrosc. 1991, 22, 65. (12) Stallard, B.; Callis, B. R.; Champion, P. M.; Albrecht, A. C. J. Chem. Phys. 1984, 80, 70. (13) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.; Ondrias, M. R. J. Am. Chem. Soc. 1993, 115, 581. (14) Gilch, H.; Dreybrodt, W.; Schweitzer-Stenner, R. Biophys. J. 1995, 69, 214 (15) (a) Ansari, A.; Berendzen, J.; Braunstein, D; Cowen, B. R.; Frauenfelder, H.; Kong, M. K.; Iben, I. E. T.; Johnson, J.; Ormos, P.; Sauke, T. B.; Scholl, R.; Schulte, A.; Steinbach, P. J.; Vittitow, R. D.; Young, R. D. Biophys. Chem. 1987, 26, 337. (b) Hong, M. K.; Braunstein, D.; Cowen, B. R.; Frauenfelder, H.; Iben, I. E. T., Mourant, J. R., Ormos, P.; Scholl, R.; Schulte, A.; Steinbach, P. J., Xie, A.-H.; Young, R. D. Biophys. J. 1990, 58, 429. (16) Iben, I. E. T.; Braunstein, D.; Doster, W.; Frauenfelder, H.; Hong, H. K.; Johnson, J. B.; Luck, S; Ormos, P.; Schulte, A.; Schulte, P.; Steinbach, P.; Xie, A.; Young, R. D. Phys. ReV. Lett. 1989, 62, 1916. (17) (a) Doster, W.; Cusack, S.; Petry, W. Nature 1989, 337, 754. (b) Cusack, S.; Doster, W. Biophys. J. 1990, 58, 243. (18) Doster, W.; Cusack, S.; Petry, W. Phys. ReV. Lett. 1990, 65, 1080. (19) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson, N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, W. A., III; Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085. (20) Leone, L.; Cupane, A.; Militello, V.; Cordone, L. Eur. Biophys. J. 1994, 23, 349. (21) Stavrov, S. S. Biophys. J. 1993, 65, 1942; (22) Li, X.-Y; Czernuszewicz, R. S.; Kincaid, J. R.; Spiro, T. G. J. Am. Chem. Soc. 1989, 111, 7015. (23) Unger, E.; Bobinger, U.; Dreybrodt, W.; Schweitzer-Stenner, R. J. Phys. Chem. 1993, 97, 9956. (24) (a) Findsen, E. W.; Shelnutt, J. A.; Ondrias, M. R. J. Phys. Chem. 1988, 92, 307. (b) Kolling, O. W. J. Phys. Chem. 1991, 95, 192. (25) Myers, A. B.; Mathies, R. A. In Biological Applications of Raman Spectroscopy; Spiro, T., Ed.; John Wiley & Sons: New York and Chichester, U.K., 1987; Vol. 2, pp 1. (26) Go¨tze, W.; Sjo¨rgen, L. Rep. Prog. Phys. 1992, 55, 241. (27) Nienhaus, G. U.; Frauenfelder, H.; Parak, F. Phys. ReV. B. 1991, 43, 3345.
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