Macromolecules 1984,17, 2447-2450
2447
molecules must arise from interactions pther than those of short range. Whether these non-short-range differences are, in fact, the helix-helix interactions which are the only such interactions introduced in the two-chain theory,' whether the two-chain theory in its more complete formlo treats them correctly, and whether sense can be made of the observed differences in terms of the 39 out of 284 amino acid sites (11of which are at the hydrophobic, helix-helix contact surface) at which CY- and @-tropomyosin chains differ remain open questions.
Acknowledgment. Thii study was supported by Grant No. GM-20064 from the Division of General Medical Sciences, US.Public Health Service.
0
IO
20
30 40 50 60 70 00 t / T
Figure 2. Percent helix vs. Celsius temperature for reduced
Filled symbols: @-tropomyosinin (NaCl)m(NaPi)so(D'M')z(7.4). 4.72 mg ml-', x = 1.0 mM. Open symbols: 0.0100 mg mL-', x = 0.5 mM. Variously shaped symbols designate different runs. Solid curves are spline curves.
Q0 2
0
IO
20
References and Notes (1) Skolnick, J.; Holtzer, A. Macromolecules 1982,15, 303-314. (2) Skolnick, J.; Holtzer, A. Macromolecules 1982,15,812-821. (3) Holtzer, M. E.;Holtzer, A.; Skolnick, J. Macromolecules 1983, 16,173-180.4 (4) Holtzer, M. E.; Holtzer, A.;
16,462-465.
Skolnick,J. Macromolecules 1983,
(5) Skolnick, J.; Holtzer, A. Macromolecules 1983,16,1548-1550. (6) Mak, A.;Lewis, W.; Smillie,L. FEBS Lett. 1979,105,232-234. (7) Scheraga, H. Pure Appl. Chem. 1978,50,315-324. (8) Skolnick, J. Macromolecules 1983,16,1069-1083. (9) Skolnick, J. Macromolecules 1983,16, 1763-1770. (10) Skolnick, J. Macromolecules 1984,17,645-658. (11) Cummins, P.;Perry, S. Biochem. J. 1973,133, 765-777. (12) We describe complex aqueous solvent media by giving the chemical formula of each solute in parentheses with its millimolarity as subscript, followed by the pH in parentheses. Other abbreviations are as follows: DTT, dfthiothreitol; CD, circular dichroism; GdmC1, guanidinium chloride. (13) Holtzer, A.; Clark, R.; Lowey, S. Biochemistry 1965, 4, 2401-2411. (14) Edelhoch, H. Biochemistry 1967,6,1948-1954. (151 . . Crimmins. D.: Isom.. L.:. Holtzer. A. Coma Biochem. Phvsiol. 1981,69B;35-46. (161 Betteridae. D.: Lehrer, S. J. Mol. Biol. 1983. 167. 481-496. (17) Edward; B.;Sykes, B. Biochemistry 1980,19,' 2577-2583,eapecially Table I. (18) Mattice, W., personal communication (1980).
30 40 50 60 7 0 00 t/oc
Figure 3. Percent helix vs. Celsius temperature for reduced Filled symbols: tropomyosins in (NaCl)m(NaP~)m(D'M')o.a(7.4). a-tropomyosin at 0.104 mg mL-1. Open symbols: 8-tropomyosin at 0.100 mg ml-l. Variously shaped symbols designate different runs. Solid curves are spline curves.
presented to support it." The present more detailed study not only confirms this conclusion but provides a data base broad enough to support a detailed test of the theory of the a-helix-to-random-coil transition in these molecules. In advance of an attempt to fit these data to the extant theory,'O it is premature to speculate on the results. However, one conclusion can be stated because it is independent of the two-chain theory. The observed difference in stability between a- and /3-tropomyosin cannot result from differences in the short-range (a and s ( T ) ) interactions. We have been aware for some time that the helix content predicted from the helix-coil theory for single chains of a-tropomyosin and @-tropomyosinat 30 "C are almost identical.lS We have extended the same calculation to cover the entire experimentally accessible temperature range (0-80 "C)and find that predicted differences never exceed a fraction of a percentage point in helix content. It is thus immediately apparent (and dependent only on the theory for single chains) that observed differences between the thermal denaturation curves of the cy2 and 8 2
Halato-Telechelic Polymers. 10. Effect of the Ionic End Groups on the Glass Transition Temperature R. JERdME,* J. HORRION, R. FAYT, and PH. TEYSSIE Laboratory of Macromolecular Chemistry and Organic Catalysis, University of LiCge, Sart- Tilman 4000, LiCge, Belgium. Received March 23,1984 The incorporation of ions induces drastic modifications in the physicomechanicalproperties of organic p~lymen.l-~ The obvious technical importance of the subject has promoted an in-depth investigation of both the supermolecular structure and the thermal transitions of ion-containing polymers and, more especially, of ionomers. It is worth recalling that ionomers usually result from the incorporation of relatively few ionic groups into nonpolar polymers by the random copolymerization of common organic monomers with ionizable comonomers. Eisenberg3 has reviewed the effect of ions on the glass transition temperature (T) of ionomers based mainly on styrene: butadiene: ethyf acrylate: and ethylene? The glass transition temperature increases with increasing salt (metal acrylate or methacrylate) content and, although no meaningful correlation exists as yet, the effect would seem more pronounced as the Tgof the host material decreases? Some experimental results* support that cross-linking by
0024-9297/84/2217-2447$01.50/00 1984 American Chemical Society
Macromolecules, Vol. 17, No. 11, 1984
2448 Notes
c
A
A
0
Figure 1. Schematic illustration of polymers cross-linked through covalent bonds (A) and ionic aggregates (B). For convenience, the ionic aggregates are pictured as spherical multiplets without presuming their actual nature and geometry. The packing density of the chain segments is increased at the cross-links of A-type polymers, whereas it is largely reduced at the surface of the ionic
aggregates (B-type polymers).
volume (Figure 1B). Conclusively, it is believed that the insensitivity of Tg of polybutadiene to the presence of metal carboxylate end groups results from the annihilation of two opposite effects, i.e., restriction of chain-end mobility and reduced crowding at the branch points. Recent Observations A quite different behavior is observed when a,w-dicarboxylatopolystyrene (M,, = 22 000) is neutralized with alkali metal and alkaline earth cations (Table I). The glass transition temperature of the neutralized polymer is markedly less than that of the carboxylic acid form (365 K). Also the size of the specific cation employed markedly affects the Tgwith the largest cations having the lowest T for both the alkali metal and alkaline earth series. d h e n neutralization is performed simultaneously with two different alkaline earth cations, i.e., Ba2+( Wh = 0.38) and Mg2+(WMg= 0.62), the experimental Tg (313 K) obeys Fox's relationship (312 K)
anionic species is a mechanism for raising T ,whereas other ones' agree rather well with values predicted by a copolymerization equation. It is likely that both cross-linking 1 = -wBa and a copolymerization effect play a role in fixing the Tg + - wM, of ionomers. The effectiveness of various ions in raising TPS,,M, TBB% Tgvr the glass transition temperature depends on the q / a ratio where Tg&,T and Tgh,Mp are the Tg of the polymer (electrostatic field), where q is the counterion charge and neutralized w!%' Ba, Mg, and a mixture of Ba and Mg, a the distance between the centers of the charges in the respectively, and WB,and WMgare the weight percent of contact ion pair.6 Finally, the degree of clustering of the polystyrene neutralized with Ba and Mg, respectively. The ions, particularly at moderately high ionic concentrations, neutralization was achieved as described elsewhere;13it can also decrease segmental mobility sufficiently to have proceeded in dry toluene through addition of the due an appreciable effect on TB' The T i s of styrene and ethyl amount of barium methoxide and magnesium methoxide acrylate ionomers show meaningful changes in slope when plotted as a function of the ionic comonomer ~ o n t e n t . ~ ~to ~ the carboxy-telechelicpolybutadiene. Complete reaction was ensured by distilling off the formed methanol. The Below a given content, the initial rise in T is linear and attributed to the ion aggregation into smah tight multineutralization of a,w-dicarboxylatopoly(a-methylstyrene) (A?" = 10OOO) is responsible for a relatively less important plets. Above that concentration, the rate of increase acdepression of Tgthan that due to polystyrene (Table 11). celerates and is supposed to reflect the onset of clustering. Again, Ba2+is more efficient than Mg2+. Furthermore, the The rather complex behavior of ionomers makes these Tgof the neutralized polymer is always lower when it is compounds evidently unsuitable for an unambigous apmeasured simply after neutralization, solvent removal, and proach of the effect of ions attached onto polymeric drying up to constant weight (25 "C, lo4 torr), rather than backbones. This is why an extensive study of model ionafter compression molding of the bulk HTP (Table 11). It containing polymers, i.e., halabtelechelic polymers (HTP), has been recently undertaken:'*16 while the ionic groups is however noteworthy that after an annealing at 200 "C for 30 min the nonmolded samples exhibit practically the are more or less randomly distributed as pendant groups same Tgas that observed after compression molding. in ionomers, they are selectively attached at both ends of linear chains in HTP. Discussion and Conclusions Polydiene-based HTPs have been largely investigated As ion pair interactions are governed by an Arrheniusup to now, and the neutralization of a,w-dicarboxylatotype activation the ion aggregation is expected = 4600) with divalent metal has no polybutadiene to decrease when temperature increases. In the glass detectable effect on TB'15On the other hand, the dynamic transition region of polystyrene (T, = 365 K) and polymechanical properties of a,w-alkaline earth dicarboxylat~polybutadiene~~J~ and -polyisoprene16show (a-methylstyrene) ( T g= 427 K), 7, could be assumed unambiguously that the ionic end groups aggregate and shorter than t , and scrambling of the ionic aggregates give rise to a relaxation mechanism obeying an Arrheniwould already occur in the polymeric matrix near Tg. In us-type temperature dependence. In the glass transition that temperature range, the polystyrene and poly(aregion of polybutadiene (193 K), the ionic aggregates may methylstyrene) chain ends would not be permanently attached at the surface of stable particles but would exchange be represented as stable cross-links. At low temperature, more or less easily. The decrease in Tgcould be a result the relaxation time (7,) of the ion pair interactions is likely to be longer than the time required for the glass transition of the increased mobility of the chain ends combined with detection ( t ) . No modification of the ionic aggregates is more free volume around branching points. In order to therefore observable in time and they behave as permanent account more accurately for the experimental results, the tie points of the chains. It is well-known that the chemical thermal dependence of the ion aggregation process must (sulfur, peroxide, etc.) cross-linking of polymers restrains be discussed. At low temperatures, the ionic aggregates the chain mobility (or reduces the free volume; Figure lA), are stable (7, >> t ) and their size is the largest (Figure 2A). As temperature increases, 7, decreases to become finally resulting in the effective raising of TB'When attached at the surface of large and stable ionic aggregates, the chain shorter than t, and the ion pairs in the aggregates exchange faster. The temporarily free ion pairs and/or small-size ends suffer also a loss of mobility, the effect of which might multiple& can either move within the aggregate or diffuse however be counterbalanced by a decrease in their packing density. Indeed, because of the steric hindrance between into surrounding ones. As charge separation requires a prohibitive energy in nonpolar media, the diffusing species metal carboxylate groups, the chain-end overlap is exin alkaline earth dicarboxylato polymers are likely triplet pected to be markedly reduced and to create extra free
(a,,
Macromolecules, Vol. 17, No. 11, 1984
Notes 2449 Table I1 Effect of Neutralization on T,of a,w-Dicarbo~ylatopoly(a-methylstyrene)~ (W,= 10 000) T.. K after molding at 200 “C ~~
before molding 427 363b 359b
cation
, G,
cnl
>>t
