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J. Phys. Chem. B 2001, 105, 3300-3305
The Effect of Pressure on the Conformation of Two Sets of m-Phenylene Ethynylene Oligomers in PMMA and PtBMA† A. Zhu, M. J. Mio, J. S. Moore, and H. G. Drickamer* School of Chemical Sciences and The Frederick Seitz Materials Research Laboratory, UniVersity of Illinois, 600 S. Mathews AVenue, Urbana, Illinois 61801-3792 ReceiVed: NoVember 14, 2000; In Final Form: February 8, 2001
The effect of pressure has been studied on the emission characteristics of two sets of m-phenylene ethynylene oligomers in solid poly(methyl methacrylate) (PMMA) and polytertbutylmethacrylate (PtBMA). For oligomer 1, the tetramer, decamer, and hexadecamer (4-mer, 10-mer, and 16-mer) were used. For oligomer 2, the chain lengths 10, 16, 20, and 24 were examined. In liquid solution larger oligomers can exist either in a folded (helical) or unfolded state (random coil) or a mixture thereof depending upon the compatibility with the solvent. The 10-mer and 16-mer of oligomer 1 and 10-mer, 16-mer, 20-mer and 24-mer of oligomer 2 exist primarily in a folded conformation in the polymeric media at 1 atm. With increasing pressure, they unfold so that, between 30 and 60 kbar, one can establish the equilibrium constant as a function of pressure. The pressure derivative of this quantity gives the difference in partial molar volume ∆V h ′ ) VF - VU. In all cases ∆V h ′ decreased with increasing pressure. ∆V h ′ is larger for oligomer 1 than for 2, it increases with oligomer size and is larger in PtBMA than in PMMA. The 4-mer of oligomer 1 is unfolded at all pressures in both media, as is the 10-mer in PMMA.
Introduction Pressure denaturation experiments have long been used to analyze the stability of the solution phase and solid-state conformations of biological macromolecules. Analyses of this type are particularly beneficial when investigating the change in the native shape of protein1,2 and DNA3 molecules. With this focus in mind, we have employed high-pressure techniques to investigate the folding equilibrium of a unique class of nonbiological molecules known as foldamers.4 Foldamers are chain molecules that exhibit compact, ordered conformations in solution and in the solid state. Oligo(m-phenylene ethynylene)s are one family of such molecules. The collapsed state of these molecules (a helical conformation) presumably achieves stability in solution through nonselective and nondirectional interactions termed solvophobic forces.5 This refers to the driving force due to the solvent, which is roughly proportional to the change of surface area upon solvation, where the free energy change of equilibrium is a linear function of solvent surface tension. Therefore, an exploration of the change in partial molar volume of m-phenylene ethynylene oligomers in the solid state may shed light on the putative solvophobic folding event. To accomplish this, previously described spectroscopic assignments were utilized. In solution, these oligomers have been well-characterized spectroscopically by both UV-vis absorption6 and fluorescence emission7 spectrophotometry. Fluorescence emission spectroscopy specifically, is especially revealing with respect to deducing the solution phase conformations. Typically, oligomers too short to fold or those placed in solvents where folding is not energetically favorable (i.e., CHCl3) display a sharp, strong fluorescence emission band at 28000-29000 cm-1. † This work was supported in part by U.S. Department of Energy, Division of Materials Science Grant DEFG02-96ER45439, through the University of Illinois at Urbana-Champaign, Frederick Seitz Materials Research Laboratory. * To whom correspondence should be addresssed.
Figure 1. Structure of the two oligomers.
Those long enough to fold, when placed in polar solvents (i.e., CH3CN), display a broad, red-shifted emission band centered around 23000-24000 cm-1 at 1 atm. This second case is indicative of extended π-π stacking, and has been cited as evidence for the collapsed helical conformation possible with meta-linked phenylene ethynylene molecules. This study involves two sets of oligomers of m-phenylene ethynylene as shown in Figure 1. The measurements are made
10.1021/jp004189c CCC: $20.00 © 2001 American Chemical Society Published on Web 03/15/2001
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Figure 2. One atmosphere spectra of the 4-mer, 10-mer, and 16-mer of oligomer (1) in PMMA and PtBMA.
in dilute solution in solid polymeric media. Poly(methyl methacrylate) (PMMA) and polytertbutyl methacrylate (PtBMA) have similar electronic characteristics (polarizability and polarity), but the internal geometry should be different. Experiments were performed to 60-65 kbar, which provided an increased polymer density of 38-40%. From the pressure derivative of the equilibrium constant between the conformations, it is straightforward to extract the difference in partial molar volumes. ∆V h ′ as a function of the length of the oligomer, the medium, and pressure as discussed. Experimental Section The syntheses of oligomer 1 (4-mer, 10-mer, 16-mer)8,9 and oligomer 2 (10-mer, 16-mer, 20-mer, 24-mer)7,10 have been described previously. Poly(methyl methacrylate) (PMMA, medium molecular weight) and polytertbutyl methacrylate (PtBMA, Tg ) 107 °C) were purchased from Aldrich and Polysciences, Inc., respectively. The polymers were used without further purification, because neither of them gave any emission when irradiated at the excitation wavelength. The oligomer and polymer were dissolved in spectral grade chloroform, and then the transparent solution was poured into a glass dish to form a blend film. After the solvent evaporated at room temperature, the film was then put in a vacuum oven for several days. The films were stored under vacuum. The concentrations of oligomers were 0.01 wt % for oligomer 4, 0.024 wt % for oligomer 10, 0.038 wt % for oligomer 16, 0.048 wt % for oligomer 20, and 0.057 wt % for oligomer 24.
Figure 3. One atmosphere spectra of the 10-mer, 16-mer, 20-mer, and 24-mer of oligomer 2 in PMMA and PtBMA.
The high-pressure spectroscopic measurements were made in a diamond anvil cell (DAC). The absorption and emission techniques have been described in detailed elsewhere,11-13 and the description need not be repeated here. Results The excitation at 325 nm is well out on the tail of the absorption peak. The absorption showed a significant red shift (1200-1500 cm-1 in 60 kbar) but below 32000-33000 cm-1, the absorption coefficient never significantly exceeded 0.1. The change in absorbance at 325 nm never exceeded 25%. Figures 2-3 present one-atmosphere emission spectra for the three methyl oligomers 1 and the four hydrogen oligomers 2, respectively, in PMMA and PtBMA. By comparison with the emission spectrum in solution, it is clear that the tetramer of oligomer 1 and the decamer of oligomer 2 show little if any folding while the higher oligomers are largely folded. We discuss the pressure results first for the decamer and hexadecamer of the methyl oligomer 1 and the three longer oligomers of the hydrogen series, 2. In Figures 4-6 we present spectra in the range 30-60 kbar fitted with two peaks in PMMA for the 16-mer of oligomer 1 and for the 24-mer of oligomer 2 in the two media. The other oligomers exhibited qualitatively similar spectra. In Figure 7 we present superimposed spectra at four pressures for the 16mer of oligomer 1 in PtBMA. Allowing for the pressure induced
3302 J. Phys. Chem. B, Vol. 105, No. 16, 2001
Figure 4. Spectra of oligomer 16 (1) in PMMA at four pressures.
Zhu et al.
Figure 6. Spectra of oligomer 24 (2) in PtBMA at three pressures.
Figure 7. Spectra of oligomer 16 (1) in PtBMA at four pressures, illustrating the existence of an isosbestic point.
Figure 5. Spectra of oligomer 24 (2) in PMMA at three pressures.
peak shifts, one sees clear evidence for the equivalent of an isobestic point. The other oligomers in PtBMA give comparable results. The peak separations in PMMA are too small to present clear-cut pictures. As can be seen, the spectra could be fit with two Gaussian peaks with constant half width. At pressures lower than 30 kbar, the fits were generally unreliable. The lower energy peak exhibits a large red shift and is clearly the major peak in the 1
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Figure 8. log K vs pressure for oligomer 10 (1) and 16 (1) in PMMA.
Figure 9. log K vs pressure for oligomer 10 (1) and 16 (1) in PtBMA.
TABLE 1: Peak Maxima (cm-1) for Oligomer 1 oligomer 4-mer 10-mer 16-mer
pressure/kbar
in PMMA
in PtBMA
0 30 60 0 30 60 0 30 60
27 600 26 850 26 200 23 950 21 400 25 210 19 810 25 050 24 440 21 550 25 520 20 070 25 180
27 510 26 750 26 120 23 800 20 700 26 350 20 020 25 770 24 200 21 910 27 690 19 750 26 740
TABLE 2: Peak Maxima (cm-1) for Oligomer 2 oligomer
pressure/kbar
in PMMA
in PtBMA
10-mer
0 30 60 0 30 60 0 30 60 0 30 60
27 320 26 200 25 800 24 410 21 920 25 420 20 930 25 200 24 380 21 880 25 380 20 300 25 130 24 250 21 510 25 610 19 950 25 200
26 500 25 900 25 800 24 100 21 120 26 810 19 910 26 460 23 920 21 510 26 900 19 950 26 220 24 210 21 560 26 620 19 840 26 370
16-mer 20-mer 24-mer
atm spectrum, corresponding to the folded conformation. The higher energy peak grows strongly with increasing pressure and corresponds to the unfolded conformation as exhibited, e.g., by the tetramer of oligomer 1. The peaks are, in general, more clearly separated in PtBMA than in PMMA. The spectral changes are largely reversible as noted below. In Tables 1 and 2 we indicate the peak maxima as a function of pressure for all oligomers. As presented in detail in the discussion below it is possible to extract at each pressure from 30 kbar up an equilibrium constant, K, in terms of the ratio of the areas. The values of K vs pressure appear in Figures 8-11. For each oligomer in each medium, a minimum of three separate loads was employed. To establish whether equilibrium really was obtained, several samples were left under pressure overnight, and in one case, a sample was left at 55 kbar for 5 days. The fits were identical within our accuracy to those obtained immediately after
Figure 10. log K vs pressure for oligomer 16 (2), 20 (2), and 24 (2) in PMMA.
compressing. Upon release of pressure the sample relaxed, but usually to spectra resembling those at 5-10 kbar rather than 1 atm. Spectra were obtained on samples released from various pressures from 30 to 60 kbar but always to an applied pressure of 1 atm. Spectra at three pressures for tetramer of oligomer 1 and the decamer of oligomer 2 are shown in Figures 12 and 13 in PMMA and in PtBMA. At each pressure the intensities in the two media are normalized at the maximum to emphasize differences in shape and energy. For the tetramer 4 (oligomer 1) there is, as expected, no folding. The peaks show a significant red shift with increasing pressure. There is a somewhat structured tail at low energy that grows modestly with pressure. At present, we have no assignment for this tail. The decamer 10(2) exhibits quite different behavior in the two media. In PMMA the spectra correspond rather closely to
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Figure 11. log K vs pressure for oligomer 16 (2), 20 (2), and 24 (2) in PtBMA.
Figure 13. Spectra of oligomer 10 (2) in PMMA and in PtBMA at three pressures.
quantitative to permit extraction of accurate values for ∆V h ′. Data taken for these two oligomers at twice the concentration did not differ significantly from the results in Figures 12 and 13. Discussion The equilibrium constant between the unfolded and folded conformations can be written as
K)
XU AU QF ) XF AF QU
(1)
Where U and F stand for the unfolded and folded conformations, XU and XF are the mole fractions, AU and AF are the areas under the emission peaks, and QU and QF are the quantum yields. From elementary thermodynamics we deduce
hF - V h U ∆V ∆V h V h′ ∂ ln K )) ) ∂p RT RT RT Then, Figure 12. Spectra of oligomer 4 (1) in PMMA and in PtBMA at three pressures.
those observed for 4 (oligomer 1) discussed above. In PtBMA there is clearly significant folding which decreases with increasing pressure as for the higher oligomers. The spectra could be fit with two peaks, but the trends were not sufficiently
∆V h ′ ) RT
[
( ) ]
∂ ln(AU/AF) QF /∂P - ∂ ln ∂P QU
(2)
(3)
We assume the ratio of quantum yields is independent of pressure. Given the magnitude of the change in AU/AF in, say 15 kbar, a 10-15% change in the ratio of quantum yields in this pressure range would have no significant effect on our
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TABLE 3: ∆V h ′ Data for Oligomer 1 polymer
oligomer
PMMA
10-mer 10-mer 16-mer 16-mer 10-mer 10-mer 16-mer 16-mer
PtBMA
pressure range /kbar
∆V h ′/cc/mol
∆V h ′PtBMA/∆V h ′PMMA
30-45 45-60 30-45 45-60 30-45 45-60 25-40 45-60
0.83 0.66 1.58 1.08 1.45 1.18 2.03 1.08
1.69 1.78 1.28 1.00
TABLE 4: ∆V h ′ Data for Oligomer 2 polymer
oligomer
PMMA
16-mer 16-mer 20-mer 20-mer 24-mer 24-mer 16-mer 16-mer 20-mer 20-mer 24-mer 24-mer
PtBMA
pressure range /kbar
∆V h ′/cc/mol
∆V h ′PtBMA/∆V h ′PMMA
30-45 45-60 30-45 45-60 30-45 45-60 30-45 45-60 30-45 45-60 30-45 45-60
0.924 0.52 1.60 0.76 1.53 0.80 1.46 0.87 1.64 1.10 1.96 1.00
1.58 1.67 1.03 1.32 1.28 1.25
conclusions, especially as any change in QF/QU with pressure would be very similar for all systems. The values of ∆V h ′ obtained are presented in Tables3 and 4. Although the ∆V h ′s are a very small fraction of the molar volumes, there are some consistent trends. The partial polar volumes are always larger for the folded conformation. The values of ∆V h ′ shown in Tables 3 and 4 have an accuracy of (10%. At pressures below 30 kbar the scatter in K (the ratio of areas) was significantly greater so that no useful values of ∆V h ′ could be obtained at low pressure. This difference decreases significantly in all cases with increasing pressure, indicating a greater compressibility in the neighborhood of the folded configuration. A comparison between the two oligomers [1 and 2] can be made for the hexadecamer. Oligomer 1 with the methyl group exhibits considerably larger ∆V h ′ than 2 that with the hydrogen, so that with methyl group there is a significant difference in the packing in the folded state. In general, ∆V h ′ is measurably larger in PtBMA than in PMMA. The electronic interactions should be the same, but the bulky tertiary butyl groups affect the relative packing efficiency. This effect is larger for the shorter than for the longer oligomers. If one normalizes ∆V h ′ for the chain length, the value per monomer unit is always within ( 20% and is usually much closer. The possibility exists that non-hydrostatic (shear) effects could exist within the polymer matrix, which could contribute to the
unfolding. To test this hypothesis oligomers 4 (1), 10 (1), and 16 (1) were dissolved in PVCl and spectra were taken as a function stretch in our controlled stretch apparatus.14 Even when elongated to three times the starting length, there was no measurable effect on the peak location or shape. As indicated under results, it is not possible to extract significant quantitative information about the 4-mer of oligomer 1 or the 10-mer of oligomer 2. Summary The emission spectra for two sets of oligomers in solid polymeric media (PMMA and PtBMA) are presented. Except for the shortest oligomers they exist in folded form at 1 atm, but with increasing pressure they unfolded. From the equilibrium constant between two forms, we extract the difference in partial molar volume ∆V h ′ ) VF - VU. ∆V h ′ varies approximately in proportion to the length of the oligomers. It is larger for the oligomers with the methyl group (1) than for the oligomers with hydrogen (2) and larger in PtBMA than in PMMA. It decreases with increasing pressure. Acknowledgment. The authors thank the U.S. Department of Energy, Division of Materials Science, for support under Grant DEFG 02-96ER45439 through the University of Illinois at Urbana-Champaign Frederick Seitz Materials Research Laboratory. References and Notes (1) Weber, G.; Drickamer, H. G. Q. ReV. Biophys. 1983, 16, 89-112. (2) Heremans, K.; Smeller, L. Biochim. Biophys. Acta 1998, 1386, 353-370. (3) Macgregor, R. B., Jr. Biopolymers 1998, 48, 253-263. (4) Mio, M. J.; Hill, D. J.; Prince, R. B.; Lahiri, S.; Hughes, T. S.; Tanatani, A.; Zimmerman, N. W.; Moore, J. S. A Field Guide to Foldamers. Chem. ReV. 2001. To be submitted for publication, (5) Sinanoglu, O. Solvent Effects in Molecular Associations. In Molecular Associations in Biology; Pullman, B., Ed.; Academic Press: New York, 1967; pp 427-460. (6) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277, 1793-1796. (7) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 3114-3121. (8) Gin, M. S.; Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 2643. (9) Mio, M. J.; Prince, R. B.; Moore, J. S.; Kuebel, C.; Martin, D. C. J. Am. Chem. Soc. 2000, 122, 6134-6135. (10) Tanatani, A.; Mio, M. J.; Moore, J. S. Chain-Length Dependent Affinity of Helical Foldamers for a Rod-Like Guest. J. Am. Chem. Soc. 2001, 117. In press. (11) Jurgenson, J. W.; Drickamer, H. G. Phys. ReV. B 1984, 30, 7202. (12) Yang, G.; Dreger, Z. A.; Drickamer, H. G. J. Phys. Chem. B 1997, 101, 4218. (13) Dreger, Z. A.; Yang, G.; White, J. O.; Drickamer, H. G. J. Phys. Chem. B. 1997, 101, 9511. (14) Yang, G.; Li, Y.; Zhu, A.; White, J. O.; Drickamer, H. G. Macromolecules 2000 33, 3173.
