40 1
J. PhyS. Chem. 1082, 86, 401-406
Dielectric and Spectroscopic Studles of PentachlorophenoGAmine Complexes Z. Malatskl, M. Rospenk, L. Sobczyk,’ Institute of Chemistry, Unhrersny of Wroclew, 50-383 Wroclew, Polend
and E. Qrech Instttute of Fundamental Chemistry, Technical UnlversRy of Szczecln, 71-065 Szczecln, Polend (Received: June 18, 198 1)
Dipole moments in CC4,infrared absorption spectra under various conditions, and UV spectra of a number of pentachlorophenol (PCP) complexes with nitrogen bases were measured over a broad ApK, range. From the dipole moment measurements an inversion region of ApK, was found for which a 50% proton transfer can be expected. The complexes from this region exhibit certain anomalies in their IR spectra, in particular a broad continuous absorption, a strong temperature effect on the absorption in the far-infrared, and particular sensitivity to changes in solvent polarity. The W spectra revealed proton-transfer equilibria for a number of complexes, both in solutions and in the solid state.
Introduction Complexes of phenols with amines are model hydrogen-bonded systems owing to the possibility for an almost continuous control of the donor-acceptor properties of interacting components. There is already a lot of information on the effect of ApK, taken as a measure of the interaction power on various properties of complexes. The data available until now indicate that in all cases there is a specific ApK, range at which the potential-energy curve for proton motion exhibits either two minima of similar energies or one broad minimum.1~2 The purpose of the present work is to investigate variations in dipole moments and IR and UV spectra under various conditions (solvent and temperature effects) according to ApK, for pentachlorophenol (PCP) complexes with various amines. These complexes were selected because of easy preparation of pure complexes even with weak amines. In addition, there is additional information on charge distribution in these complexes from NQR measurements? Moreover, studies performed by Denisov and Schreiber indicate that pentachlorophenol may form with amines both hydrogen-bonded (HB) complexes and proton-transfer (PT) add~cts.~J’ Experimental Section Pentachlorophenolates of ternary amines were prepared by crystallization from acetonitrile solution. The composition of the complexes was determined on the basis of the chlorine content from the elemental analysis. Dipole moments of pentachlorophenol, amines, and complexes were determined in CC14 solutions at 25 O C (l,&bis(NJV-dimethy1amino)naphthalene in benzene solution) by measuring permittivity by the heterodyne beat method and density by the pycnometric method. IR spectra were recorded on Perkin-Elmer Model 180 and 621 spectrophotometersfor solutions in KBr cells, for suspensions in Nujol and hexachlorobutadiene using (1)L.Sobczyk, K.Bunzl, and H. Ehgelhardt in T h e Hydrogen Bond, m,p. Recent Devdopmen~in Theory and E.perimentem, G. Zundel, and C. Sandorfy, Eds. North-Holland Publishing CO., Amsterdam, 1976,p 937. (2) G. Zundel and A. Nagyrevi, J. Phys. Chem., 82,685 (1978);R. Lmdemann and G. Zundel, J. Chem. SOC., Faraday Trans. 2, 73, 788 (1977). (3) E. Grech, J. Kalenik, and L.Sobczyk,J . Chem. SOC.,Faraday 75, 1587 (1979). ( 4 ) G. S. Denisov and V. M. Schreiber, Vestn. LGU,No. 10,M) (1975); No. 4,61 (1976). (5)E.Grech, Z. Dega-Szafran,and M. Szefran, Pol. J. Chem., 62,1589 (1978). 0022-3654/82/20860401$01.25/0
polyethylene, CsI, KBr, and KRS-5plates, and for pellets in KBr. Temperature dependences of the IR spectra were recorded by using a low-temperature attachment for liquid nitrogen and a temperature-control unit of our own design. Electronic spectra were recorded on a Beckman UV 5240 spectrophotometer. Samples for measuring the electronic spectra of crystalline complexes were prepared by thorough rubbing of the substance in silicone grease.
Results and Discussion Dipole Moments. In the calculations of dipole moments for weak-amine complexation, equilibrium constants were taken into account. Under measuring conditions the complex dissociation to free components reached up to 30% for the weakest amine, 3-cyanopyridine. Equilibrium constants were determined from the intensity of the “free”-phenolband. The polarity of the hydrogen bond, 4,i.e., the chargetransfer moment vector along the bridge /O-H
2%
*N
was calculated by assuming that the OH bond moment in phenol is 1.51 D while the C-O-H angle is 117°.6 For unsymmetrical amines the calculations allowed for freedom of amine rotation around the hydrogen bond consistent with the direction of a free electron pair in the nitrogen atom. For the complexes with 1,8-bis(NJV-dimethylamino)naphthalene, Aji was calculated by assuming that the proton is attached symmetrically to both nitrogen atoms maintaining the same C-0-. -H+ angle. All results of calculations are summarized in Table I, where the data concerning the moments of free amines are obtained from our own independent measurements. Equilibrium constants for the complexes with trialkylamines have not been determined. Free-phenol concentration under experimental conditions was neglegibly low. The dependence of Aji vs. ApK, is presented in Figure 1,which shows a typical curve, characteristic of all acidbase systems investigated until now. In studies performed so far, one specific base and acids of different pK values have been u s u d y applied. In the present work we have managed to collect a series of complexes over a broad ApK, range with One proton donor‘ Since the complexes with pentachlorophenol could not cover the full range of ApK,, including the situations where the proton transfer takes
-
(6) R. Nouven and P. Huyskens, J. Mol. Struct. 16,459 (1973). (7)P. Huyskens and Th. Zeegere-Huyskena, J. Chirn. Phys. Phys.Chim. B i d , 61,81 (1964).
0 1982 American Chemical Society
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Malarski et al.
The Journal of Physical Chemistry, Vol. 86, No. 3, 1982
TABLE I: Dipole Moments and Polarities of Hydrogen Bonds in the Pentachlorophenola Complexes with Nitrogen Bases in CCl, at t = 25 “C PPKZl Kp(comno. nitrogen base (base), D (base) (complex) plex), D A,? A PKa 4.55 -3.91 3.45 1.35 10.8 1.51 3-cyanopyridine 1 1.42 4.71 - 2.41 1.98 12.5 3-bromopyridine 2.85 2 14.5 1.50 4.30 -1.75 2.42 3.51 4-acetylpyridine 3 16.4 1.40 3.45 -0.73 4-formyl pyridine 1.80 4.53 4 -0.33 52.5 1.32 5.28 quinoline 2.23 4.93 5 5.31 -0.04 71.0 1.30 pyridine 2.30 5.22 6 1.43 0.14 55.5 5.78 2.63 5.40 isoquinoline 7 1.34 0.77 68.5 5.19 4-methylpyridine 2.12 6.03 8 0.99 1.45 5.65 85.3 3-meth ylpyridine 2.55 6.25 9 1.87 2.30 6.18 7.13 92.0 2,4,6-trimethylpyridine 2.20 10 4.67 5.60 8.47 0.85 9.93 11 tributylamine 6.16 5.49 8.49 0.95 10.75 12 triethylamine 7.10 6.90 7.5 1.92 12.34 1,8-bis(N,N-dimethylaino ). 13 naphthalene (in benzene)
‘Pentachlorophenol:
p =
1.75 D, pKa = 5.26.
1 -4
-2
0
Figure 1. Plot of Afi vs. ApK,. from ref 18.
2
6
4
8 ApVo
-
I I
e
l
The points marked by 0 are taken
place, the diagram includes some points taken from the review artic1e.l The dependence of A6 vs. ApK, may be described by means of a proton-transfer equilibrium model; i.e., polarizability is a linear function of the contribution of the state with proton transfer. The equilibrium constant of proton transfer may, in turn, be described, according to Huyskens and Zeegers-Huyskens, by means of the equation log KpT = EApK, + C’ where constants C’ and 5 depend on the medium. The value of C’is defined by ApK, (4.46) at which the population of both states, i.e., HB and PT, is the same. Calculations based upon the results obtained for the pentachlorophenol complexes yield C’ = -1.47 and = 0.33. Average NQR %C1 frequencies obtained for solid PCP complexes may be correlated with ApK, (1.20) in a similar wag which led to the values of C’and 6 equal to -0.91 and 0.76; i.e., in solids the inflection point (where up to 50% proton transfer is formally reached) occurs for much weaker bases. This is in agreement with expectation since the crystal lattice favors the polar PT state. It is worth noting a constant value of GHB for complexes at ApK, < 1. It is much higher than for other systems investigated till now. This results most certainly from the fact that in free PCP the OH group is bounded by the intramolecular hydrogen bond OH. 43. The formation of the complex follows a break in this bond and change in the conformation of phenol. For steric reasons an intermolecular O-H...N bond could not be formed in the
-
l h
I I
----
-
- --:-
,
I
I
nA ~l
1/
-\
I
I
Flgure 2. Evolution of protonic absorptlon bands on increasing the ApK, value for pentachlorophenolatesof (a) pyrazine, (b) 4-acetylpyridine, (c) isoqukrdine, (d) 4-methylpyr!dlne, (e) N-methylmorpholine, (f) imidazole, (g) 4-(N ,Ndimethylamlno)pyridine, and (h) triethylamine, in KBr.
phenyl ring plane. For complexes from the ApK, < 1 region, a constant value of A2 might indicate that it is the polarization effect which contributes predominantly to AC. Infrared Spectra. Let us first analyze the spectra of solid complexes since in this case it is possible to investigate changes occurring from weaker HB complexes through a transition state up to the PT ion pairs. The evolution of the IR spectra with increasing ApK, is illustrated in Figure 2, where the envelopes of OH stretching frequency bands separated roughly from other absorption bands, as well as broad absorption bands in the low-frequency range, are drawn. The appearance of this absorption is characteristic of the complexes from the inversion region. This result is consistent with the findings
The Journal of Physial Chemktry, Vol. 86, No. 3, 7982 403
Pentachlorophenol-Amine Complexes
TABLE 11: Characterization of the Protonic Absorption for the Crystalline Amine Pentachlorophenolates" us(O-H.**N)* v,(O-*..H-N+) % PT continuum of no. base ApK, (NQR)3 envelope components protonic absorption 1 2 3 4 5 6
pyrazine 3-cyanopyridine 3-bromopyridine 4-formylpyridine pyridine isoquinoline
-4.61 - 3.91 - 2.41 -0.73 -0.04 0.14
-0 0 0 0
7
4-methylpyridine
0.77
50
8
2,4,6-trimethylpyridine
1.87
9 10
imidazole N-meth ylmorpholine
1.73 2.12
11 12 13
2-methyl-4-aminopyridine trie thylenediamine 4-(N, N-dimeth y1amino)pyridine tri-n-butylamine triethylamine quinuclidine 1,8-bis(N,N-dimethylamino)naphthalene tetrabutylammonium hydrogen bis( pentachlorophenolate)
14 15 16 17
18
a
2780 2750 2740 2700 2640 2600
2670 m, 2940 s 2650 m, 2930 s 2640 s, 2900 s 2600 s, 2880 s 2550 s, 2900 m 2480 s, 2900 m
2200
2000 w, 2500 w
80 90
2350 1960
1950 m, 2550 s 1960m .
2.12 3.56 4.35
100 100
2650 2300 2640
1940 w, 2700 vs 1900 w, 2300 s 1900 w, 2640 vs
4.67 5.49 5.69 7.10
100 100
2340 2350 2200
2340 s, 2600 w 2280 8,2400 vs, 2590 w 1900 m , 2350 m
13
800-1500 w, 1350 max 800-1500 m, 1250 max, Evans holes, temp effect w 100-1600 vs, 650 max, Evans holes, temp effect vs 300-1500 s, 850 max, Evans holes, temp effect m 400-1500 s, 950 max, Evans holes, temp effect w 950-1500 m, 1320 max
100
300-850 m, 500 max, Evans holes, temp effect m 200-1400 s, 900 max, Evans holes, temp effect w
2400 vw
T = 300 K, in KBr, in Nujol-CsI-polyethylene.
of Zundel,2who has shown that the maximum intensity of the absorption continuum occurs near the inflection point, Le., for ApK, at which one should expect 50% proton transfer. If the NQR resonance frequency3 is assumed to be a measure of the degree of proton transfer, then the maximum intensity of the continuum occurs, in fact, exactly at ApK, characterizing the inflection point. Even in cases where continuous absorption occurs, in most systems a band appears on the high-frequency side which might be assigned to stretching vibrations. It has almost as a rule a fine structure and is asymmetric so it is difficult to determine the peak position. However, one could roughly estimate the center of gravity and then correlate it with the ApK, value. The diagram obtained is shown in Figure 3. Two regions can be distinguished (I) the region correspondingto the HB complexes, without proton transfer, and (11) the region corresponding to the ion pairs. At ApK, of about 1.4, the inflection point o c m which is marked in a certain sense by a collapse. For the 4-methylpyridine complex the stretching frequency band disappears completely. NQR studies show that in this case the PT percentage is exactly 50. It is difficult to explain the behavior of two complexes formed by 4-(N,N-dimethylamin0)pyridine and 2-methyl-4-aminopyridine (indicated by crosses in Figure 3) for which the stretching frequency bands appear at high frequencies and there is no continuous absorption at low frequencies. This is probably due to the lattice effecta which favor the ionic states. In any case NQR studies indicate that in the case of these two complexes we are dealing with a strong polar bond. A sharp decrease in the stretching frequency in the inversion region seems to be typical for all hydrogenbonded systems. Such behavior was found, for instance, in the hydrogen bonds in Mannich bases8for which a large number of data were collected. All investigated systems are described in Table 11, where those cases in which the continuum occurs are indicated. (8) A. Sucharda-Sobayk and Sci. Chim., 26, 549 (1978).
L.Sobczyk, Bull. Acad. Pol. Sci., Ser.
cm"
3000
-
2400
.
2200 2000
-
1800
-
Flgure 3. Dependence of proton stretching frequencies on ApK, for crystalllne PCP complexes wlth amines.
400
800
1200
cm.1
FIgm 4. Temperatwe effect on the IR spectrum of the PCP complex wlth 4methylpyrMlne in NuJol.
This absorption is, as a rule, accompanied by numerous Evans holes. Meanwhile, a temperature effect occurs,
Malarski et al.
The Journal of phvsicd Chemistty, Vol. 86, NO. 3, 1982
404
V
L
\
\
1I io00
1500
2000
3000
I
y cm'
'
Flgue 5. Spectra of PCP complexes with (a) pyrldlne, (b) deuterated pyridlne, (c) triethylamine, and (d) deuterated triethylamlne, In CCI,.
consisting of an increase in absorption intensity at the low-frequency side of the continuum. This effect is particularly evident just in the 4-methylpyridine complex as shown in Figure 4. This picture is similar to that found for homoconjugated cations [N-H-**N]+(ref 9) which most probably is related to the shape of the double minimum potential curve. A decrease in temperature leads, according to the suggested interpretation, to barrier lowering and bridge symmetrization. The behavior of the IR spectra of complexes in solution is similar to that in the solid state, the inversion region being shifted to higher ApK, values. In CC14 the strongest broad absorption occurs for tributylamine and triethylamine. The plot of A6 against ApK, may indicate that the degree of proton transfer is about 60% for these complexes, and so it is amazingly consistent with the result obtained from UV spectra. Figure 5 illustrates the spectra of complexes with pyridine and tributylamine and of deuterated species. In the case of the pyridine complex, the symmetric OH stretching frequency band is very well shaped and the isotopic ratio of 1.17 may be accurately determined. A similar value of 1.20 was found for the tributylamine complex, though in this case the accuracy is much lower. On the other hand, the behavior of the band in the low-frequency region is different. In the pyridine complex a relatively narrow strong band appears in the 1100-1400-cm-' region which disappears on deuteration. At the same time a much more narrow band appears just below lo00 cm-l. The estimated isotopic ratio is about 1.3, and so it is only slightly lower than 42. In the case of the tributylamine complex, a broad absorption in the fingerprint region becomes only slightly changed on deuteration (ita intensity slightly diminishes). The position is similar to that found previously for the HCl complexes with oxygen bases from the transition region of ApK,1O and also for other acid-base complexes."J2 The dependence of the formation enthalpy for acid-base complexes on ApK, is a monotonic function13 with a characteristic shoulder for the inversion region. Thus, the (9)E. Grech, 2. Malareki, and L. Sobczyk, Spectrosc. Lett. 9,459 (1973);Pol. J. Chem., 62,131 (1978). (10)M.b p e n k , A. Koll,and L. Sobczyk, Ado. Mol. Relaxation Interact. Processes, 11, 129 (1977). (11)A. Sucharda-Sobczyk and L. Sobczyk, unpublished results. (12)F.Kohler and P. Huyskens, Ado. Mol. Relaxation Processes, 8, 125 (1976). (13)R.S.Drag0 and T. D. Epley, J. Am. Chem. SOC.,91,2883(1969).
Flgue 6. Effect of solvent on the I R spectrum of PCP complex with triethylamine: (a) CCI,, (b) chloroform, (c) CH,CI,, (d) acetonltrlle, (e) crystalllne complex In KBr.
dependence of Av(OH.--N) on ApK, indicates that, on exceeding the inversion point, the energy of the hydrogen bond itself begins to decrease and the disturbance of the N+-H bond under the effect of the anion becomes smaller; the system becomes a "purer" ion pair where Coulombic interactions are predominant. The solvent exerts a considerable influence on the situation in the complex. Figure 6 illustrates the behavior of the proton stretching frequency band for OH...N in several solvents for triethylamine pentachlorophenolate. A slow disappearance of the background absorption and a shift of the band which might be assigned to the N+-H*..O- frequency toward shorter wavelength resembles the picture obtained for that complex in KBr. The effect of solvent polarity on the IR spectrum of PCP complexes depends on which side of the inversion point they are located-left-hand side or righthand side; they are like a mirror images. Three different theoretical concepts were suggested to explain the effect of broad absorption. ZundeS4 as a starting point takes the superpolarizability of the hydrogen bridge described by the potential-energy curve with a low barrier for proton transfer between the two minima. The source of broad absorption, according to Zundel, is the electric-field fluctuations in the environment, leading to a broadening of tunnel transition levels. In ref 15 we have shown that the formation of a continuum may result from the modulation of the bridge length as a result of the coupling with low-frequency bridge vibrations if the potential curve with a low barrier double minimum both symmetric and asymmetric is assumed. Another concept was presented by Sokolov et al.,16 who explained the ab(14) G. Zundel in "The Hydrogen Bond", Vol. 11, P. Sehuster, G. Zundel, and C. Sandorfy, Eds., North-Holland Publishing Co.,Amsterdam, 1976,p 683. (15)H. Romanowski and L. Sobczyk, Chem. Phys., 19,361(1977);E. Grech, Z. Malarski, H. Romanowski, and L. Sobczyk, J.Mol. Struct., 47, 317 (1978).
The Journal of phvsical Chemlstty, Vol. 86, No. 3, 1982 405
Pentachlorophenol-Amine Complexes
TABLE 111: Position of the UV Absorption Bands in PCP and Its Salts and Complexes with Amines A , nm solvent
base
salt
HB form
n-heptane chloroform dichl or om e thane acetonitrile silicone grease
302.2, 292.5 302.5, 293.2 302.0, 293.0 302.1, 293.5 302, 292 NaPCPa NaPCPa TPCP TPCP THBPCPC
H20
Nujol (suspension) chloroform Nujol (suspension) Nujol(suspensi0n) n-heptane CCl, chloroform dichloromethane acetonitrile
triethylamine triethylamine triethylamine triethylamine triethylamine
312, ? 307.5 (295) 305, ? 311 (298) 313.5 (303) 316 (305)
silicone CCl, silicone CCl, silicone silicone
triethylamine 3-cyanopyridine 3-cyanopyridine 2,4,6-trimethylpyridine 2,4,6-trimethylpyridine N-methylmorpholine
302,292 302, 292 304, 295 303, 292 302,292
grease grease grease grease
PT form
Sodium pentachlorophenolate. (pentachlorophenolate).
Tetrabutylammonium pentachlorophenolate.
sorption continuum in the systems containing symmetrical H502+ and H302-ions on the grounds of interactions between proton vibrations and phonons. Each of the abovementioned effects is probably more or leas important depending on a specific given system. The results obtained for solutions in nonpolar solvents seem to support the concepts which ascribe particular importance to the shape of the proton motion potential. It seems unquestionable that continuous absorption has been discovered until now only in those systems which exhibit a more or less symmetric distribution of proton density in the bridge. Quite recently Zundel et al. have shown that for quantitative description of continuous absorption one should allow for both the bridge-length distribution and electric-field
fluctuation^.'^ UV Spectra. In order to study in more detail the situation of proton motion, we have measured the UV spectra of the PCP complex with selected amines under various conditions. PCP exhibits a doublet band in the long-wavelength W region at 292 and 302 nm which should be assigned to the r T* transition" characteristic of the r-electron system of the benzene ring, shifted toward long wavelength under the influence of the hydroxyl substituent and the chlorine atoms. In aqueous alkaline solution the long-wavelength band is shifted to 320 nm, which should be assigned to the hydrated PCP anion. A similar band slightly above 320 nm in CCll occurs in the presence of an excess of stronger amines. The situation is illustrated in Figure 7, where the spectra of pure pentachlorophenol are compared with those of its complexes with 4-methylpyridine,triethylamine, and sodium salt in aqueous solution. Figure 7 indicates apparently the presence of two forms of the complex HB and PT, and for the triethylamine complex the percentage of PT may be estimated as equal to about 60 f 10. This result is consistent with estimates performed on the
320, 295 331, 313 338,316 346,326 332, ? 321, 295 325, ? 325, 298 327,299 329,295,346 (ionization) 323,298 324, ? 323, ? 321, ?
Tetrabutylammonium hydrogen bis-
/j
06
Figure 7. UV spectra of (a) PCP and of complexes with (b) 4methylpyrkjlne and (c) triethylamine In CCI,. Curve d applies to sodlum PCP sal in water.
A
-
(16)N.D.Sokolov and V. A. Savelev, Chem.Phys., 22,361(1977);N. B. Librovich, V. P. Sakun, and N. D. Sokolov, ibid., 39, 351 (1979). (17)W.S. Pilingin, Zh.Obshch. Khirn., 45, 149 (1975). (18) H. Ratajczak and L. Sobczyk, J. Chern. Phys., 50, 556 (1969). (19)A. Hayd, G.Weidemann, and G. Zundel, J. Chem.Phys., 70,86 (1979).
Figure 8. UV spectra of (a) PCP, of complexes with (b) N-methylmorpholine and (c) triethylamine, and (d) of tetrabutylammonium salt, ail In silicone grease suspension.
grounds of dipole moment measurements. With increasing ?& PT, the band of the HB state becomes shifted somewhat toward longer wavelength (HB). It is well-known that with increasing hydrogen-bond energy the proton-donor band is shifted toward longer wavelength. Also in the solid state (silicone grease suspension) in some cases one could find the HB and PT forms coexisting with the Characteristic bands of position analogous to that of solutions. As expected, the equilibrium region is shifted toward weaker bases. Trialkylamines with PCP form in the solid state only the ion pairs. In Figure 8 the spectra of solid N-methylmorpholine complex where a fairly clear PT equilbrium occurs are compared with those of the EhN complex which shows no HB form. Figure 8 also shows the band of the tetrabutylammonium salt where disturbances of the PCP anion by the ammonium cation is par-
J. Phys. Chem. 1982, 86, 406-413
406
ticularly insignificant and the position of that band is shifted to the greatest extent (338 nm in CHCls and 346 nm in the crystalline state). All data collected for various complexes are summarized in Table 111. Because of band interference the above-mentioned values of ,A, include an error of 1-2 nm. A t the present stage of studies, it has not been possible to get quantitative data on PT equilibria
(apart from very rough estimates). On the other hand, it was possible to investigate-in parallel to IR studies-the effect of the physical state and solvent activity on one selected complex, viz., that with EhN. We have already seen how dramatically the proton absorption bands in the infrared have been changed on passing to more polar solvents.
Reversing-Pulse Eiectrlc Blrefrlngence Study of Helical Poly(a-L-glutamic acid) In N,N-Dimethyltormamide wfth Emphasis on a New Data Analysis for the Polydisperse System
'
Klwamu Yamaoka' and Kazuyorhl Ueda FacuRy of Scbnce, Hkoshlm University, Hbashlsende-machi,Naka-ku, Hiroshima 730, Japan (Received: June 22, 198 1; I n Final F m : September 8, 198 1)
The electrooptical properties and the conformation of poly(a-L-glutamicacid), (Glu),, in Nfl-dimethylformamide (DMF) were studied at concentrations of 2.5,4.9, and 12.6 mM and at 20 OC in the electric field strength region 0-20 kV/cm by means of reversing-pulse electric birefringence (RPEB). The RPEB signal showed a deep minimum, revealing that both the permanent dipole moment and the polarizability anisotropy contribute to the field orientation of (Glu),. The polydispersity of the (Glu), sample, expressed with the weight-average length and the ratio of weight- to number-average lengths, could be estimated from the reverse transient by a new analytical method. The axial translation of (Glu), in DMF was 1.5 A per residue, indicating the a-helical conformation. The steady-state signal (An) obeyed the Kerr law at low fields (0-ca. 4 kV/cm), showing a saturation at high fields. The field strength dependence of An could not fit theoretical orientation functions derived for the monodisperse system, but it fitted one of the functions over an entire field strength range, when a continuous length distribution of (Glu), was considered. From field-free decay curves, the relaxation times for overall rotation (7-l were evaluated. A considerable dependence of 7- on field strength could be reproduced with the polydispersity parameters obtained from the reverse transients.
Introduction Transient or square-pulse electric birefringence (EB) techniques have been employed in studies of the electrooptical and hydrodynamic properties of synthetic polypeptides to obtain an insight into their conformations in s o l u t i ~ n . ~ -For ~ a given polypeptide sample of known molecular weight, ita conformation may be characterized by such factors as the axial ratio, the length per residue, the electric permanent and induced dipole moments, and the relative orientation of the peptide and side-chain chromophores. A number of experimental EB data have been accumulated for a variety of polypeptides; yet, reported values of the above physical quantities for a given polypeptide species are not always in good agreement even if they were determined under nearly the same conditions. For example, values of the axial translation per residue, which is believed to be 1.5 A,B and the permanent dipole moment, which ranges from 4 to 6 D/residue,' for the
a-helical polypeptides are scattered rather widely. Since theories of electric field orientation for the nonconducting rigid, monodisperse polypeptide of axial symmetry have been well formulated,9i8the above disagreement is mostly due to the neglect of the molecular weight (or length) distribution of a sample. For example, the transient EB of poly(a-L-glutamic acid), (Glu),, was measured in Nfl-dimethylformamide (DMF), but a precise analysis of the data was hampered because of polydispese lengths of (G~U),,.~ Several methods have since been proposed for determining the average relaxation time of a polydisperse system from a field-free decay c 1 m 7 e . l ~ ~ ~ Matsumoto et al. obtained a value of 1.67 A for the axial translation per Glu residue for (Glu), in DMF,lBa value larger than 1.5 A for the a helix. Their elegant method requires a highly accurate measurement of the decay curve, but such accuracy is ettainable only at concentrations of (Glu), where the formation of aggregates is unavoidable. Since (Glu),, together with poly(L-lysine),has been used
(1) Part 2 of Reversing-Pulee Electric Birefringence of (Glu),. For part 1, see ref 24. (2) Tinoco, I., Jr. J. Am. Chem. SOC.1967, 79, 4336. (3) O'Koneki, C. T.;Yoehioka, K.; Orttung, W. H. J. Phys. Chem. 1969, 63, 1558. (4) Boeckel, G.; Genzling, J. C.; Weill, G.; Benoit, H. J. Chim. Phys. Phys.-Chim. Biol. 1962,69,999. (5) Yoehioka,K. 'Molecular Electro-Optice"; OKoneki, C. T., Eds.; Marcel Dekker: New York, 1978; Part 2, Chapter 17, pp 601-43. (6) Pauling, L.; Corey, R. B.; Branson, H. R.Proc. Natl. Acad. Sci. U.S.A. 1961,37,205. Pauling, L.; Corey, R. B. Zbid. 1961,37,235. (7) Wada, A. J. Chem. Phys. 1969, 30, 328. In 'Polyamino Acids, Polypeptides, and Proteins"; Stahmann,M. A., Ed.; University of Wisconsin Press: Madison, WI, 1962; p 131.
(8) Shah, M. J. J. Phys. Chem. 1963,67, 2215. (9) Yamaoka, K. Ph.D. Dissertation, University of California, Berkeley, CA, 1964. (10) Ycehioka, K.; Watanabe, H. Nippon Kagaku Z w h i 1963,84,626. (11) Mateumoto, M.; Watanabe, H.; Yoshioka, K. Kolloid 2.2.Polym. 1972, B O , 298. (12) Tsuji, K.;Watanabe, H.; Yoshioka, K. Adu. Mol. Relaxation Processes 1976,8, 49. (13) Kobayaai, S . Biopolymers 1968, 6, 1491. (14) Schweitzer, J.; Jennings, B. R. Biopolymers 1972, 1 1 , 1077. (15) Coles, H. J.; Weill, G. Polymer 1977,18, 1235. (16) Mataunoto, M.; Watanabe, H.; Yoshioka, K. Biopolymers 1970, 9,1307.
0022-3654/82/2086-0406$01.25/0
0 1982 American Chemical Society
