J. Phys. Chem. 1983, 87, 18-20
18
Anisotropic Solvent Translational Diffusion in Solutions of Poly(y-benzyl-L-glutamate) Michael E. Moseley’ DepaHment of Isotope Research, The Weizmann Institote of Science, 76 100 Rehovot, Israel (Received September 21, 1982; I n Final Form: November 3, 1082)
Solvent translational diffusion in oriented solutions of PBLG in chloroform is relatively fast and exhibits anisotropy in which the diffusion in the direction along the parallel helical chains is 20-30% faster than that perpendicular to the helices. This phenomenon is explained in terms of the supramolecular structure of the helices in solution. The effect of a denaturant on the solvent diffusion behavior is also examined. As the classic model of biological lyophases, the polypeptide poly (y-benzyl-L-glutamate) (PBLG) has been the topic of a wide variety of experimental and theoretical s t u d i e ~ . ~ -In~ organic solvents such as chloroform and dichloroethane at sufficiently high concentrations>’ PBLG molecules exist as stiff rodlike a-helices roughly 2.5 nm in diameter and 91 nm long (M, = 130 000)8which aggregate in successively twisted layers of parallel helices forming a cholesteric supramolecular structure. In a strong magnetic field, the cholesteric structure spontaneously “unwinds” to produce an oriented nematic mesophaseg in which the long axes of the molecular helices align to within f2Oo of the magnetic field direction.lOpll Several studies9J2-14have shown that the nematic aggregation matrix restricts the isotropic rotational tumbling of the solvent molecules, resulting in observable dipolar and quadrupolar splittings in their NMR spectra. The magnitude of this splitting is a measure of how “ordered” the solvent is in the liquid crystalline environment. However, until now nothing has been known about the effect that the PBLG nematic suprastructure has on the translational behavior of the solvent. This Letter reports that, although the solvent translation is relatively fast, diffusion in the direction along the aligned helices (parallel to the nematic director), D,,,is 20-30% faster than that perpendicular to the director, D , in PBLG-chloroform solutions. A similar behavior is also observed for methane dissolved in solution.
mix over a period of 1 month and allowed to orient in the magnetic field for 24 h prior to measurement. Chloroform and methane translational diffusion coefficients parallel and perpendicular to the magnetic field (Oil and D,, respectively) were measured from the CHCIBand CHI proton resonances by the Fourier transform pulsed magnetic field gradient spin echo NMR technique described elsewhere.15J6 The measurements, performed on a Bruker WH-90 (21.1 kG) spectrometer, utilized a dual quadrupole gradient coil design mounted in a fixed orientation on the proton fixed-frequency probe. The two coils were used to create magnetic field gradients of known strength (-9 G/cm) parallel and perpendicular to the direction of the magnetic field. In corresponding separate experiments Dll and D , were then measured through collection of the latter half of the spin echo FID as a function of the gradient duration and subsequent Fourier transformation to resolve the chloroform and methane echo contributions. The reported diffusion values were typically within error limits of * 5 % (95% confidence intervals).
(4) Fasman, G. D., Ed. “Poly-y-amino Acids”, Marcel Dekker: New York, 1967. (5) DuPr6, D. B.; Samulski,E. T.in “Liquid Crystals, the Fourth State of Matter”;Saeva, F. D., Ed.;Marcel Dekker: New York, 1979; pp 203-47. (6) Robinson, C. Trans. Faraday SOC.1956,52, 571. (7) Robinson, C.; Ward, J. C.; Beevers, R. B. Discuss. Faraday SOC. 1958, 25, 29. (8) Doty, P.; Bradbury, J. H.; Holtzer, A. M. J. Am. Chem. SOC.1956,
Results and Discussion The degree of solvent ordering was monitored from the observed splitting (Av) of the deuterium quadrupolar doublet of CDC13 as a function of temperature and the resulh are shown in Figure 1. These values agree well with those previously reported1’ on a similar system. The absolute order parameters S,, for CDC13obtained from the observed splittings were calculated from the relation S,, = 2Av/(3e2qQ/h) with the value of e2qQ/h taken as 164.5 kHz.’* Absolute order parameters of chloroform (2% w/w) in thermotropic nematics are about 0.13,18 a factor of 40 larger than the values in Figure 1. Translational diffusion coefficients, D,, and D , , for chloroform and methane in PBLG solution are given in Figure 2 as functions of temperature. The Arrhenius plots were linear and the corresponding activation energies were determined to be E,,= 10.1 f 0.7 kJ/mol and E , = 11.8 f 0.8 kJ/mol for CHC13 with El,= 8.8 f 0.4 kJ/mol and E , = 9.7 f 0.4 kJ/mol for CH4. The self-diffusion of neat chloroform at 25 “Chas been measuredlg to be D = 2.7 x lo4 m2s-l, while corresponding chloroform diffusion rates in thermotropic nematic mesophases are on the order of D = 0.1 X lo4 m2 s-l.16 From the results in Figure 2 then, it is apparent that chloroform diffusion is largely unimpeded in PBLG solution despite the very large viscosities ( [ q ] 1.3 dL/gP observed in these systems. The same
(9) Sobajima, S. J . Phys. SOC.Jpn. 1967,23, 1070. (10) Fung, B. M.; Gerace, M. J.; Gerace, L. S . J. Phys. Chem. 1970, 74, 83. (11) Arwoll, R. D.; Vold, R. L. J. Am. Chem. SOC.1971, 93, 5335. (12) Panar, M.; Phillips, W. D. J . Am. Chem. SOC.1968, 90, 3880. (13) Gill, D.; Klein, M. P.; Kotowycz, G. J . Am. Chem. SOC.1968,90, 6870. (14) Samulski, E. T.; Tobolsky, A. V. Mol. Cryst. 1969, 7, 433.
(15) Goldfarb, D.; Moseley, M. E.; Labes, M. M.; Luz. 2. Mol. Cryst. Liq. Cryst. 1982, 89, 119. (16) Moseley, M. E.; Loewenstein, A. Mol. Cryst. Liq. Cryst., in press. (17) Fung, B. M.; Martin, T . H. J . Chem. Phys. 1974, 61, 1698. (18) Vold, R. R.; Vold, R. L.; Szeverenyi, N. M. J . Phys. Chem. 1981, 85, 1934. (19) Stilbs, P.; Moseley, M. E. Chem. Scripta 1980, 15, 76.
Experimental Section The sample was prepared by dissolving PBLG (MilesYeda Ltd., M , = 130000), 16.9% w/w, in CHC13:CDC13 (50:50 v/v), after which it was degassed and sealed under a methane atmosphere (8 atm). The sample was left to (1) (a) Sir Charles Clore Postdoctoral Fellow, 1981-2. (b) Present address: Department of Pharmaceutical Chemistry, University of California, San Francisco, CA 94143. (2) Kelke, H.; Hatz R. ‘Handbook of Liquid Crystals”; Verlag Chemie: Weinheim, West Germany, 1980. (3) Govil, G.; Hosur, R. V., Eds. ”Conformation of Biological Molecules. NMR-Basic Principles and Progress”; Springer-Verlag: Berlin, 1982.
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0022-3654/83/2087-00 18$01.50/0 0 1983 American Chemical Society
The Journal of Physical Chemktty, Vol. 87,No. 1, 1983
Letters I
800
I
1
I
‘30
TABLE I : Chloroform Order Parameters and Chloroform and TFA Translational Diffusion Coefficients in PBLG/Chloroform/TFA Solution (17% w/wPBLG, 50:50 v/v CHCl,:CDCl,) as Functions of TFA Concentration at 30 “ C CHCI,
%
TFA, v/v 0 9 20
S N
I
19
CDCI, 103S,,
lO’Dli, m 2s -
lO’D1, m z s-!
2.4 0.57
2.1
1.7
1.6 1.0
1.4
0.30
1.0
TFA
109D11,lO’D1, mz s - l mz s - l 0.3 0.3
0.3 0.3
500
120
I IO0
1
c0
-IC0
t
300
200
A00
1°C)
Figure 1. The observed splittings (Av)of the deuterium quadrupolar doublet of chloroform (CDCI,) and the corresponding calculated absolute order parameters S, in an oriented PBLG solution as functions of temperature.
(“C) 394
299
33
32
210
I26
46
-I00
I
I
-29
I
35
36
37
38
34
1 0 ~( K - I1) ~
Flgure 2. Translational diffusion coefficients of chloroform and methane in the directions along (D and perpendicular to (D i)the aligned PBLG chain helices as functions of temperature. The lines through the experimental points are best fits used in determining the activation energies for diffusion.
is true for methane diffusion, with diffusion rates in PBLG solution an order of magnitude larger than corresponding values in thermotropic nematics. The fast diffusion and slight solvent ordering observed is most likely due both to the absence of specific solvent interactions (H-bonding,etc.) with the polymer matrix and to the relatively large ratio (9:l) of noninteracting solvent molecules to PBLG monomer units. There appears to be fast exchange between the few solvent molecules in the highly ordered anisotropic regions close to the polymer chain and the many more molecules in the largely isotropic regions in between the parallel helices. If we assume the PBLG volume fraction to be 0.17, the free area between two neighboring chains is roughly equal to the chain radius (1.2 nm).20 In light of the above, it is somewhat surprising to find translational diffusion anisotropy of chloroform and methane to be as large as DII/D, = 1.2-1.3, which is comparable to that found16 at similar temperatures for these molecules in thermotropic nematic mesophases. It is suggested that the observed anisotropy in PBLG is the result of the spatial long-range translational restraints arising from the polymer nematic suprastructure of long, relatively stationary stiff rods. These restraints are unique from those in thermotropic nematics in the large degree ~~~~~
~~
(20) Tobolsky, A. V.; Samulski, E. T. In “Liquid Crystals and Plastic Crystals”, Gray, G . W.; Winsor, P. A.; Eds.; Wiley: New York, 1974; Vol. 1, pp 176-98.
of chain rigidity and in the relatively long chain lengths (0.1 pm) compared to the pathlengths (9 pm) over which solvent diffusion is measured in these experiments. This is the first experimental report of solvent translational diffusion in ordered polypeptide solutions. Bishop and Dimarzio earlier proposed21 several simple models using continuum and polymer lattice counting techniques to predict spherical small molecule (solvent) diffusion behavior in a nematic array of long parallel smooth cylinders, such as that reasoned to be found in oriented lyotropic liquid crystals. Considering a PBLG volume fraction of 0.2, their models predict absolute D,, and D, values for chloroform which are within 30% of the experimental results in Figure 2. The models also predict anisotropy values of 1.2-1.3. This is in very good agreement with the observed results for chloroform and indicates that it is the degree of chain rigidity and long-range molecular order in the nematic network that gives rise to the observed anisotropy in an oriented system with fast solvent diffusion. In an effort to expand on these results, preliminary measurements show a similar diffusion behavior for 1,2dichloroethanein oriented PBLG-dichloroethane solutions (16.9% w/w PBLG). Dichloroethane diffusion rates are roughly 50% slower than those reported here for chloroform and show slightly larger activation energies and diffusion anisotropies. In addition, fast anisotropic solvent diffusion in PBLG-benzene solutions has also been observed. An interesting and biologically important aspect of current polypeptide research concerns the intramolecular phase transition from the a-helical chain conformation to a flexible random coil upon addition of a denaturant such as trifluoroacetic acid (TFA).3*4The action of the acid in the breakdown of the polypeptide structure has been shown to have large effects in lowering the solution shear and rotational v i s ~ o s i t y 8while ~ ~ ~decreasing ~~~ solvent ordering24and reorientation times,25for example. As a preliminary investigation, solutions with TFA added to PBLG/chloroform (17% w / w PBLG, 50:50 v/v CHC13:CDC13)were prepared with TFA concentrations of 0,9, and 20% v/v. The order parameter S,, of CDC1, and the diffusion coefficients (Dliand Dl)of CHCl, and TFA were then measured and the results tabulated in Table I. So that these values could be compared with the diffusion rates in Table I, a solution of 15:85 v/v TFA:chloroform (50:50 v/v CDC13:CHC13)was prepared without PBLG. The TFA and chloroform diffusion rates at 30 “C were and 2.9 X m2 s-l, respecmeasured to be 2.0 X tively. (21) Bishop, M.; Dimarzio, E. A. Mol. Cryst. Liq. Cryst. 1974,28,311. (22) Teramoto, A.; Nakagawa, K.; Fujita, H. J. Chem. Phys. 1967,46, 4197. (23) Patel, D. L.;DuPrB, D. B. Mol. Cryst. Liq.Cryst. 1979,53, 323. (24) Laszlo, P.; Paris, A.; Marchal, E. J . Phys. Chem. 1973, 77, 2925. (25) Chien, M.; Samulski, E. T.; Wade, C. G . Macromolecules 1973, 6, 638.
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The Journal of Physical Chemistry, Vol. 87, No. 1, 1983
From the slow TFA diffusion observed in the PBLG solutions, it can be inferred that the acid proton is essentially “bound” to sites on the chain for relatively long periods of time, in conclusion similar to findings from relaxation studies on the TFA proton in PBLG solution.25 The large decrease in the chloroform diffusion rates and the disappearance of the diffusion anisotropy upon addition of TFA occurs along with a factor of two decrease in the solution intrinsic viscosity.22 As a possible explanation it is reasoned that, as the rigid nematic suprastructure is progressively broken down by the action of the denaturant, the increase in chain flexibility leads to the
Letters
observed decrease in the diffusion rates. Also, the breakdown of the anisotropic long-range order into an environment of randomly situated chains averages out any local differences in D,, and D,, resulting in the disappearance of solvent diffusion anisotropy and ordering.
Acknowledgment. The author expresses his thanks to Professor A. Loewenstein for preparation of the methane sample and to Professor Z. Luz for his support in this work. Registry No. PBLG,25014-27-1; PBLG (SRU), 25038-53-3;
TFA,76-05-1;chloroform, 67-66-3;methane, 74-82-8;1,2-dichloroethane, 107-06-2.
