12374
J. Phys. Chem. B 2001, 105, 12374-12377
Effect of Pressure on the Conformation of Two Oligo (m-Phenylene Ethynylene) Foldamers Containing a Piperazine or Terpene Derivative as Guest 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 AVe., Urbana, Illinois 61801-3792 ReceiVed: July 16, 2001; In Final Form: October 9, 2001
The effect of pressure to 65 kbar has been measured on the conformation of two oligo (m-phenylene ethynylene) foldamers with 16 and 24 monomer units containing a piperazine or terpene derivative as a guest. The piperazine is cylindrical, whereas the terpene is roughly spherical. The results are compared with previously described studies on oligomers with no guest. In all cases, the oligomer tends to transform from the coiled (helical) conformation to an open conformation with increasing pressure. Equilibrium constants (K) were extracted from the luminescence spectra. The difference in partial molar volume ∆V h ′ between the conformations was obtained from the pressure derivative of K. For the oligomer with no guest, ∆V h ′ decreases rapidly with increasing pressure which shows that the helical conformation is more compressible. With the guest present, the decrease in ∆V h ′ is much smaller, and in some cases, there is an increase over some range of pressure. The presence of the guest drastically reduces the compressibility of the helical form, presumably because the guest fills the void space created by the helical cavity. A detailed comparison is made between the two oligomers with and without the guests present.
Introduction Foldamers are chain molecules which can assume an ordered (e.g., helical) structure or a random structure in solution or in the solid state.1 A family of such foldamers, the oligo (mphenylene ethynylene) have been extensively studied in this laboratory.2 Recently, we have investigated the effect of pressure to 65-70 kbar on the structure of two series of these foldamers dissolved in solid poly(methyl methacrylate) (PMMA) and in poly(tertiarybutyl methacrylate) (PtBMA).3,4 The two media have similar electronic interaction but different packing. Typically in these media, the oligomers with the helical conformation emit fluorescence in the range of 20 000-24 000 cm-1, whereas the uncoiled oligomers emit in the range 26 00028 000 cm-1. As discussed in refs 3 and 4 and below, from an analysis of the spectra and straightforward thermodynamics, we can extract an equilibrium constant, and from pressure dependence of K, we can obtain the difference in volume between the two conformations. Typical spectra and a qualitative analysis appear in ref 3. In ref 4, a detailed quantitative analysis is presented for the two related sets of oligomers. The oligomers unfold with increasing pressure which indicates that the partial molar volume of the helical form is larger. ∆V h′ decreases by a factor of ∼3 between 35 and 60 kbar which shows that the helical conformation is more compressible. It has been shown in this laboratory that the helical form of one of these oligomers can incorporate guests.5 In this paper, we present pressure studies on oligomers 16 and 24 with two different guests. The structures of these foldable oligomers are shown in Figure 1. (They are the oligomers labeled (2) in refs 3 and 4.) The piperazine derivative (P) [cis-(2s,5s)-2,5-dimethylN,N′-diphenylpiperazine] is rodlike, whereas the terpene guest (T) [1s-endo-(-)-borneol] is more nearly spherical. Their structures are also shown in Figure 1. * To whom correspondence should be addressed.
Figure 1. Structure of m-phenylene ethynylene oligomer and of piperazine and terpene derivative guest.
Experimental Section The synthesis of the oligomers and guest and identification of the structure with the guests is described in detail elsewhere.1a,b,2b The samples were prepared by dissolving the polymer and oligomer in a minimal amount of CH2Cl2, rapidly drying it, and maintaining it in a vacuum oven for several days. The high-pressure optical techniques have been presented elsewhere.6 Here we only note that the measurements were made in a diamond anvil cell (DAC) using the 325 nm line of a HeCd laser for excitation. In all cases, the results presented represent the average of five to eight separate loads. The scatter among the results from different loads was never over (10%
10.1021/jp012725a CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001
Conformation of Two Oligo Foldamers
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12375
Figure 2. Typical spectra of oligomer 16 plus terpene in PMMA and PtBMA (A) in PMMA and (B) in PtBMA.
Figure 3. Typical spectra of oligomer 24 plus piperazine in PMMA and PtBMA (A) in PMMA and (B) in PtBMA.
and was usually within (5%. Samples held at ∼60 kbar for 18 h showed no change with time. Results For the oligomer with no guest (NG), fits were made between 30 and 65 kbar and ∆V h ′ calculated from 35 to 65 kbar. At lower pressure, the amount of unfolding was too small for accurate fitting. In a number of cases, the fits could be successfully made at slightly lower pressure for the oligomers plus guest (P or T). The fits assume that the spectra can be represented by two Gaussian bands. Typical spectra are shown in Figures 2 and 3. In Figure 4, we show superimposed spectra for oligomer 24 plus piperazine at a number of pressures, which demonstrates the existence of the pressure equivalent of an isobestic point. Upon the release of pressure, the spectra returns to the shape and peak location corresponding to ∼5-10 kbar. Because the initial spectra with and without guest differ only modestly, it is not possible to determine whether the guest is included in the refolded oligomer. This would seem unlikely.
Figure 4. Spectra of oligomer 24 plus piperazine in PtBMA at a number of pressures showing the pressure equivalent of an isobestic point.
∂ ln K ∆V h VF - VU ∆V h′ )) ) ∂p RT RT RT
Discussion In analyzing the data, we write the equilibrium constant K in the form
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
(2)
then
∆V h ′ ) RT
[
{( )} ]
∂ ln ({AU}/{AF}) QF + ∂ ln /{∂P} ∂P QU
(3)
It is assumed that QF/QU varies slowly if at all with pressure. In any case, the variation should always be in the same direction, so the effect should not be significant. Therefore, we ignore the second term within the brackets in eq 3.
12376 J. Phys. Chem. B, Vol. 105, No. 49, 2001
Zhu et al.
TABLE 1: Ratio of K Values (PtBMA/PMMA)a Olig24
Olig16
pressure
NG
P
T
NG
P
T
30 35 40 45 50 55 60 65
0.52 0.52 0.51 0.49 0.48 0.50 0.52 0.55
0.46 0.54 0.57 0.56 0.57 0.58 0.63 0.70
0.43 0.47 0.51 0.55 0.57 0.62 0.68 0.72
0.26 0.27 0.30 0.31 0.32 0.33 0.35 0.36
0.50 0.55 0.62 0.67 0.72 0.75 0.76 0.76
0.45 0.46 0.47 0.49 0.49 0.51 0.54 0.58
a
NG ) no guest, P ) Piperazine guest, T ) Terpene guest.
Figure 6. ∆V h ′ vs pressure for oligomer 24 in PMMA (A) and in PtBMA (B). b, piperazine guest; O, terpene guest; 1, no guest.
Figure 5. ∆V h ′ vs pressure for oligomer 16 in PMMA (A) and in PtBMA (B). b, piperazine guest; O, terpene guest; 1, no guest.
In Table 1, we present the ratio of equilibrium constants PtBMA/PMMA at a series of pressures for the oligomer with no guest (NG), with piperazine guest (P), and with terpene guest (T). Clearly, for all systems and at all pressures, the unfolding is significantly less in PtBMA than in PMMA. Apparently the large tert-butyl groups provide a looser packing with lesser driving force to assume the smaller volume state. Plots of ∆V h ′ vs pressure are shown in Figure 5 for oligomer 16 in PMMA and PtBMA and in Figure 6 for oligomer 24 in the two media. All ∆V h ′’s shown are in the units of cc/mol of oligomer. It is evident that ∆V h ′ changes less rapidly with pressure when there is a guest present. The presence of the guest apparently reduces the compressibility of the helical oligomer. This is not surprising in view of the open structure of the oligomer without the guest. There is no reason to expect any significant change in the compressibility in the neighborhood of an unfolded oligomer even if the guest should be interacting with a segment of the oligomer. From Figure 5, it can be observed that in oligomer 16 there is a distinct difference between ∆V h ′ for the two guests. Below
∼45 kbar, the oligomer with the piperazine guest exhibits a much larger ∆V h ′ than the same oligomer with the terpene guest. In PMMA, the change with pressure is essentially parallel, whereas in PtBMA above 45 kbar, there is a significantly more rapid decrease for the oligomer with the piperazine guest. For the oligomer 24, the ∆V h ′ involving either guest is very nearly the same magnitude. It is of interest to note that at low pressure there is actually an increase in ∆V h ′ with pressure for the oligomer with the piperazine guest which indicates that in the region the open form is less compressible than the helical form with guest. In the same region there is essentially no effect of pressure on ∆V h ′ for the oligomer 24 with the terpene guest. Figure 7 presents the ratio of the ∆V h ′’s in the two media. For oligomer 24, the differences in behavior for no guest, terpene, and piperazine are second order, but for the 16 oligomer, whereas the material with no guest and terpene show similar behavior not drastically unlike that for the longer oligomer; oligomer 16 with the piperazine guest shows a drastic decrease in the ratio of ∆V h ′’s at the higher pressure, quite different than the increase exhibited by the other systems. This difference is much larger than any experimental error. It is of interest to compare the ratio of ∆V h ′ for oligomer 24 to that of oligomer 16. In the case of no guest, the ratio is 1.5 ( 0.15 over the entire pressure range in either PMMA or PtBMA, which indicates that ∆V h ′ per monomer unit is independent of length of oligomer. This point is discussed in more detail for the series of oligomers in ref 4. For the case of the terpene guest the ratio is 2.2 ( 0.15 from 30 to 45 kbar and above ∼50 kbar, it decreases to ∼1.8 ( 0.15. The behavior is essentially the same in both PMMA and PtBMA. For the piperazine guest, the ratio in PMMA is 1.2 ( 0.15 for 30-55 kbar and decreases slightly at 60 kbar. In PtBMA, the ratio is 1.1 ( 0.15 to ∼55 kbar and then increases to 1.3 ( 0.15 at 60
Conformation of Two Oligo Foldamers
J. Phys. Chem. B, Vol. 105, No. 49, 2001 12377 constant K gives, for oligomers with no guest, a ∆V h ′ which decreases rapidly with increasing pressure, indicating that the helical form is more compressible. In the presence of a piperazine or terpene guest ∆V h ′ shows a much smaller pressure dependence. A number of regularities in the behavior of ∆V h′ are observed as well as some anomalies. Because ∆V h ′ is in the range of 0.5-2.0 cm3/mol and the molar volumes of the oligomers are in the range of 5000-7500 cm3/mol, very small perturbations in molecular properties or interaction could account for these anomalies. 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 the Urbana-Champaign Frederick Seitz Materials Research Laboratory. References and Notes (1) (a) Soth, M. J.; Nowick, J. S. Curr. Opin. Chem. Biol. 1997, 1, 120. (b) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (c) Barron, A. E.; Zuckermann, R. N. Curr. Opin. Chem. Biol. 1999, 3, 681. (d) Stegers, K. D.; Soth, M. J.; Nowick, J. S. Curr. Opin. Chem. Biol. 1999, 3, 714. (e) Kirshenbaum, K.; Zuckermann, R. N.; Dill, K. A. Curr. Opin. Struct. Biol. 1999, 9, 530. (f) Degrado, W. F.; Schneider, J. P.; Hamuro, Y. J. Pept. Res. 1999, 54, 206. (g) Archer, E. A.; Gong, H.; Krische, M. J. Tetrahedron 2001, 57, 1139.
Figure 7. Ratio of ∆V h ′PtBMA/∆V h ′PMMA. A, oligomer 16; B, oligomer 24; b, piperazine guest; O, terpene guest; 1, no guest.
kbar. It appears that the cylindrical piperazine guest packs more efficiently in the longer oligomer, whereas the opposite is true for the essentially spherical terpene. It must be noted that the molecular weights of oligomer are in the order 5000 and 7500, and the densities are of the order of one, so that ∆V h ′ of 0.5-2.0 cm3/mol constitutes a miniscule fraction of the molar volume, and very small perturbations in physical properties or interaction with the environment can cause relatively large changes in ∆V h ′. At this time, a more detailed accounting would be pure speculation. Summary The effect of pressure on foldamers is to cause the helical form to unfold. The pressure derivative of the equilibrium
(2) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science 1997, 277, 1793. (b) Prince, R. B.; Saven, J. G.; Wolynes, P. G.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 3114. (c) Prince, R. B.; Okada, T.; Moore, J. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 233. (d) Gin, M. S.; Yokozawa, T.; Prince, R. B.; Moore, J. S. J. Am. Chem. Soc. 1999, 121, 2643 (e) Gin, M. S.; Moore, J. S. Org. Lett. 2000, 2, 135. (f) Prince, R. B.; Brunsveld, L.; Meijer, E. W.; Moore, J. S. Angew. Chem., Int. Ed. 2000, 39, 228. (g) Brunsveld, L.; Prince, R. B.; Meijer, E. W.; Moore, J. S. Org. Lett. 2000, 2, 1525. (3) Zhu, A.; Mio, M. J.; Moore, J. S.; Drickamer, H. G. J. Phys. Chem. B 2001, 105, 3300. (4) Zhu, A.; Mio, M. J.; Moore, J. S.; Drickamer, H. G. Chem. Phys. Lett. 2001, 342, 337. (5) (a) Prince, R. B.; Barnes, S. A.; Moore, J. S. J. Am. Chem. Soc. 2000, 122, 2758. (b) Tanatani, A.; Mio, M. J.; Moore, J. S. J. Am. Chem. Soc. 2001, 123, 1792. (6) (a) Jurgenson, C. W.; Drickamer, H. G. Phys. ReV. B 1984, 30, 7202. (b) Yang, G.; Dreger, Z. A.; Drickamer, H. G. J. Phys. Chem. B 1997, 101, 4218. (c) Dreger, Z. A.; Yang, G.; White, J. O.; Drickamer, H. G. J. Phys. Chem. B 1997, 101, 9511. (d) Yang, G.; Li, Y.; White, J. O.; Drickamer, H. G. J. Phys. Chem. B 1999, 103, 5181.
