J. Phys. Chem. 1987, 91, 4621-4625
4621
Dependence of Intramolecular Proton Transfer on Solvent Friction M. Lee,t J. T. Yardley,$ and R. M. Hochstrasser*t The Department of Chemistry, The University of Pennsylvania, Philadelphia, Pennsylvania 19104, and Allied Signal Corporation, Morristown, New Jersey 07960 (Received: April 7 , 1987) Fluorescence lifetimes, T ~and , rotational diffusion times, T ~of, 2-(2’-hydroxy-5’-methylphenyl)benzotriazole (HMPB) were measured in alcohol and ether solvents. In ethers the fluorescenceemission decays exponentially (1.5 f 0.1 ns) and is invariant to the nature of solvents and the experimental temperatures. In 1-alkanols (propanol-undecanol), the fluorescence decays are nonexponential but consist mainly of two exponentials whose parameters depend on the solvent viscosity and temperature. The long component lifetimes vary from 100 to 500 ps in these solvents and have strong correlations with the measured T ~ We propose, therefore, that the excited-state.proton transfer of the molecule in alcohol solvents is controlled by solvent friction. The measured proton-transfer rates were very close to the sum of the Stokes-Einstein-Debye diffusion rates for the solute in various solvents and the solvent dielectric relaxation rates.
I. Introduction A common way of describing reaction kinetics in solution is to use the concept of encounter complex. For example, if the bimolecular reaction A + B C is to occur, the reacting molecules, A and B, must first come sufficiently close together. According to classical theory, originally formulated by Smoluehowski, the rate constant for this step is diffusion-controlled and k = 47rrD, where r is the sum of the radii of A and B and D is the sum of translational diffusion coefficients of A and B.’ The Smoluchowski theory usually overestimates the reaction rate constant.2 One contribution to this discrepancy concerns the fact that, even if the two molecules are in close contact, the encounter complex is still required to achieve the structure appropriate for reaction.*s3 It is therefore expected that rotational diffusion of solvent and solute will contribute to the overall rate of the reaction. There were several attempts to clarify the bimolecular kinetics for translationally and rotationally diffusing reactants, but the resulting formulas are not easily compared directly with experiments! When one of the reactants is a solvent, the overall process is altered and can be regarded as pseudounimolecular. The reaction rate constant in this case may be mainly determined by the rotational diffusion of the molecules. However, the overall rate is still complicated by the inevitable intervention of the dissociation process, which requires the treatment of the fragment dynamics5 A more straightforward analysis can be applied when excited-state intramolecular processes are studied because the nonequilibrium ensemble created by absorption of short light pulses can be probed and the importance of rotational effects on the rate constant can be directly assessed. In this work we present a study of the intramolecular excited-state proton-transfer (ESPT) reaction of 2-(2’-hydroxy-5’methylpheny1)benzotriazole (see structure below) (HMPB) in 1-alkanols.
-
H -0,
The spectroscopy and excited-state kinetics of HMPB have been extensively studied in crystals, low-temperature glasses, and solutions.610 These studies were often motivated by the fact that such structures can undergo rapid, efficient radiationless transitions from their first excited singlet states, making them important as UV stabilizers. The absorption and fluorescence spectra of HMPB are sensitive to the nature of the environment. In solutions the normal Stokes-shifted fluorescence occurs near 400 nm whereas t University of Pennsylvania.
$AlliedSignal Corp.
0022-36541871209 1-462l$Ol SO10
this emission is not observed in crystalline or low-temperature glass environments. In these rigid media the HMPB excited state is apparently able to undergo rapid intramolecular proton transfer and a new fluorescence around 600 nm is seen which has been attributed to the tautomeric form with the hydroxyl proton shifted to a triazole nitrogen. The efficient radiationless transition that occurs in certain HMPB derivatives is usually attributed to this rapid hydrogen transfer. Numerical calculations of the groundand excited-state structures support this tautomerization picture.7b Scott and co-worker~’~ reported a 14-ps decay time for the blue fluorescence. The fluorescence decay times of other structurally related molecules, such as hydroxyphenylbenzothiazole,’] hydroxyflavone,I2 ~alicylidenaniline,’~ methyl s a l i ~ y l a t e , ’and ~ benzotroponolI5 undergoing intramolecular ESPT, have been reported to be less than a few picoseconds. However, HMPB and hydroxyflavone have much longer lifetimes in alcohol^.^^^" The increase in lifetime was explained by invoking hydrogen bonding between the solute and solvent which slows down the intramolecular proton transfer. It follows that both solute and solvent structural changes are expected to be part of the proton-transfer process. The overall proton-transfer process might therefore be expected to depend systematically on the solvent friction. We show in this paper that the proton-transfer rates in HMPB are in fact (1) Smoluchowski, M. V . Z . Phys. Chem. 1917, 92, 129. (2) See, for example: North, A. M. Collision Theory of Chemical Reactions in Liquids; Methuen: London, 1961. (3) (a) Noyes, R. M. Prog. React. Kine?. 1961, I , 733. (b) Solc, K.; Stockmayer, W. H. J. Chem. Phys. 1971,54, 2981; In?.J. Chem. Kine?. 1973, 5 , 733. (c) Lee, S.; Karplus, M. J . Chem. Phys. 1987, 86, 1883, 1904, and
references therein. (4) (a) Ivin, K. J.; McGarvey, J. J.; Simmons, E. L.; Small, R. J . Chem. SOC.,Faraday Trans. 1 1973,69, 1016. (b) Burfoot, C. G.; Caldin, E. F. Zbid. 1976,72,963. (c) Crooks, J. E.; Robinson, B. M. Faraday Symp. Chem. SOC. 1975, No. 10. (5) Eigen, M. Angew. Chem., Intl. Ed. Engl. 1964, 3, 1. (6) Werner, T. J . Phys. Chem. 1979, 83, 320. (7) (a) Huston, A. L.; Scott, G. W.; Gupta, A. J. Chem. Phys. 1982, 76, 4978. (b) Bocian, D. F.; Huston, A. L.; Scott, G. W. Ibid. 1983, 79, 5802. (c) O’Connor, D. B.; Scott, G. W.; Coulter, D. R.; Gupta, A.; Webb, S. P.; Yeh, S. W.; Clark, J. H. Chem. Phys. Lett. 1985, 121, 417. (8) Flom, S. R.; Barbara, P. F. Chem. Phys. Lett. 1983, 94, 488. (9) Woessner, G.; Goeller, G.; Kollat, P.; Stezowski, J. J.; Hauser, M.; Klein, U. K. A.; Kramer, H. E. A. J. Phys. Chem. 1984,88, 5544; 1985,89, 3629. (10) Ghiggino, K. P.; Scully, A. D.; Leaver, I. H. J . Phys. Chem. 1986, 90, 5089. (1 1) (a) Elsaesser, T.; Kaiser, W. Chem. Phys. Lett. 1986,128,231. (b) Barbara, P. F.; Brus, L. E.; Rentzepis, P. M. J . Am. Chem. SOC.1980, 102, 563 1. (12) (a) Topp, M. R., private communication. (b) Woolfe, G. J.; Thistlethwaite, P. J. J. Am. Chem. SOC.1981, 103, 6916. (c) McMorrow, D.; Kasha, M. J. Phys. Chem. 1984, 88, 2235. (13) Barbara, P. F.; Rentzepis, P. M.; Brus, L. E. J . Am. Chem. Soc. 1980, 102, 2786. (14) Smith, K. K.; Kaufmann, K. J. J. Phys. Chem. 1978, 82, 2286. (15) Jang, D.-J.; Brucker, G. A.; Kelley, D. J . Phys. Chem. 1986,90,6808. (16) Choi, K.-J.; Boczer, B. P.; Topp, M. R. In Ultrafast Phenomena I K Auston, D. H., Eisenthal, K. B., Eds.; Springer: New York, 1984; p 368. (17) Stranjord, A. J. G.; Barbara, P. F. J . Phys. Chem. 1985, 89, 2355, 2362.
0 1987 American Chemical Society
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Lee et al.
The Journal of Physical Chemistry, Vol. 91 No. 17, 1987 ~
1-alkanol
viscosity," CP
methanol ethanol propanol butanol pentanol hexanol heptanol octanol nonanol decanol undecanol
0.6 1.2 2.1 2.9 3.5 4.6 6.4 8.2 10.2 12.8 15.8
T,,
T ~ ,
TR,
TD,b
PS
PS
PS
PS
26 35 39 39 43 50 56 73 66
20c 53d 103 145 183 241 301 370 417 463 506
116 180 229 314 385 515 619 770 858
53 198 430 668 927 1210 1465 1780 2150 2019 1538
"Viscosities are from ref 30. bThe references for the T,, data are as follows: methanol (ref 25a); ethanol (ref 25b); propanol-decanol (ref 25c); undecanol (ref 25a at 25 " C ) . CReference 10. dReference 7a.
well correlated with the rotational diffusion rates of the solute. The proton-transfer process is found to be describable in terms of conventional stochastic dynamics (Stokes-Einstein-Debye theory). 11. Experimental Section
The fluorescence lifetimes, T F , of HMPB in 1-alkanols (propanol through undecanol) were measured at constant temperatures by time-correlated single photon counting. A broad range of excitation wavelengths were obtained with Rh560, Rh6G, and DCM dye lasers and frequency-doubling crystals. The emission signal was collected over the range 380-480 nm with an analyzer set at the magic angle with respect to the vertically polarized excitation beam. The position of the analyzer was computer controlled so that the fluorescence anisotropy could be readily measured. The rotational diffusion times, rR, were obtained from the anisotropy decays in the same set of solvents. The instrument function necessary for the deconvolution was obtained from the Raman scattering signal of pure n-pentane solvent. With a microchannel plate tube (Hammamatsu Model R1645U-07), an instrument function of 120 ps was easily maintained during the experiments. HMPB was kindly donated by the Ciba-Geigy Corp. and zone-refined 50 times before use. The best quality of solvents were chosen. 111. Results
The fluorescence lifetimes (T F ) and the fluorescence anisotropy ) HMPB were obtained by deconvoluting the obdecays ( T ~ of served signals from the instrument function using software developed by G . R. Holtom in this laboratory. In the l-alkanol solvents the decay was distinctly nonexponential. Double-exponential decay fits were applied to these data, and acceptably small weighted residuals (x2 1.1) were obtained. In the previous experiments which used a streak camera, only single decay times in methanol'O and ethan01'~J~were reported. The short fluorescence lifetime components we have observed (see Table I) range from ca. 30 ps in propanol to ca. 70 ps in undecanol. The long components vary from 100 ps in propanol to 500 ps in undecanol. Reliable values of 7F for the short components could not be determined with this instrument but the long components were reproducible to within f10 ps. The two components have approximately the same amplitudes for 305-nm excitation and 400-nm detection and both lifetimes increase as solvent viscosity increases. We also carried out experiments in which the fluorescence detection wavelength was varied at a fixed excitation wavelength (305 nm). At each wavelength setting the observed decays were the sum of two exponentials, corresponding to the short and long components discussed above. However, the ratios of the two amplitudes at t = 0 were found to depend on detection wavelength indicating the presence of two emission spectra. These spectra, shown in Figure 1, were evaluated by measuring the amplitudes at a variety of detection wavelengths, with the same laser intensity
I
1
TABLE I: Fluoreseence Lifetimes ( T ~ T, Z ) , Rotational Diffusion Times ( ~ p of ) HMPB, and Debye Relaxation Times ( T ~ of ) Normal Alcohols at 20 OC
,
1-
1
'
7-
,
1 I 1 lkl 1 1 380 400 420 4 4 0 4 6 0 4 8 0 5 0 0 DETECTION W A V E L E N G T H ( n m )
I
Figure 1. Resolved emission spectra of HMPB in I-pentanol at 20 OC. Relative amplitudes of the two decay components are drawn as a function o f f = 0 of detection wavelength. X and 0 correspond to the short and long decay components, respectively. The decay times are 1 5 % for the entire detection wavelength. The broken line is the sum of two amplitudes, resembling the structureless, steady-state spectrum. 25i-r-r---
T
-
7
--
I I
_L__LI_32 33 3 4 35 l/T K-'1
2 2 b > T
Figure 2. Temperature dependence of the fluorescence decay rate of HMPB at a constant viscosity of 2.1 cp. Both short- and long-lifetime components are plotted. Least-squares fits give the same positive slope of 2 kcal/mol. The solvents used are propanol, butanol, pentanol, hexanol, and octanol.
and irradiation period being used for each spectral point. The results of Figure 1 were achieved only when the whole laser system was run under conditions of excellent long-term stability. The separated fluorescence spectra peak at 395 and 415 nm corresponding to the fast and slow emitting species, respectively. The temperature dependence of the fluorescence lifetime was studied at constant viscosity by varying the solvents and their temperatures. The plot of fluorescence decay rate vs. 1/ T (see Figure 2) would have given a negative slope if there was any potential barrier involved in the process. However, a slightly positive slope (2 kcal/mol) was obtained, suggesting the absence of any significant barrier. Such behavior in other cases was attributed to solvent polarity effects.'* However, the lifetimes of the long components show a strong correlation with a / T (see Figure 3), where 7 is the zero frequency shear viscosity. This suggests that the intermolecular ESPT has a stochastic component, because it has the functional form of the Stokes-Einstein-Debye re1ation.lg The proton transfer is a nonradiative decay process with rate constant kpTand thus is extracted from the measured lifetimes T f from 1/~= f k,
+ kfn,+ kpT
(1)
(18) Hicks, J.; Vandersall, M.; Babarogic, Z.; Eisenthal,K. B. Chem. Phys. Letr. 1985, 116, 18. (19) Berne, B. J.; Pecora, R. Dynamic Light Scarrering: Wiley: New York, 1976; Chapter 7 .
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987 4623
Proton-Transfer Reaction of HMPB I
I
a
1000,
'
I 9001
CI
400 a
I-
a a
/
700-
L
W J
STICK
800;
a
'/I
/
1
600
1
400
300-
J
/
a
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VISCOSITY
Figure 4. Rotational relaxation times of HMPB ih 1-alkanolsvs. viscosity. At 20 OC the straight line is a least-squares fit for eight data points (propanol-decanol). Also shown are two lines obtained from the
Stokes-Einstein-Debye equation using different boundary conditions. structure is labeled S M (see below) and corresponds to the long lifetime component in HMPB.
,...
k,tk,
M-lj
-
II
S-H
M
H
"'H
-*-...
/
-
PRODUCTS
SM S'M' The rearranged, reactive structure is denoted as S'M' to emphasize that local motions of both solvent (S S') and solute ( M M') are needed in order to bring about a form which can react rapidly. If there are no internal barriers restricting these angular motions, the rate constant relating S M and S'M' would be the sum ( k s + kM)of the appropriate rotational diffusional rates for the solvent (S) and molecule (M). In order to obtain these rates it would be necessary to know the details of the internal motions which generate the needed structures. However, the overall rotational motions of the solute and solvent molecules, for which the rotational diffusion coefficients can be measured, may give a reasonable estimate of the value of ks + k, in the form kR kD where kR is the solute rotational relaxation rate and kDthe Debye relaxation of the solvent. In any event, the assertion that the rate is given by ks kM is already an idealization since the solvent and solute motions cannot be independent as a result of the hydrodynamic effects. The rotational relaxation times iR = l / k R of HMPB in 1alkanols (propanol-undecanol) at 20 OC are shown in Figure 4 plotted against the solvent viscosities. The observation of rotational relaxation times in the range of hundreds of picoseconds for a a ?r* transition of a nearly planar triazole indicates that the transition dipole whose reorientation is occurring is directed along the longest in-plane axis. The two dashed lines were calculated from the modified Stokes-Einstein-Debye equationL9
-
-
+
+
-
where Vis a molecular volume and kBis the Boltzmann constant. The parameter f depends upon the shape of the molecule and can be calculated from the Perrin equation2' for prolate ellipsoids ( p < 1): 1 - p4
2
--
f= -
'I
L
P2
(2 - p 2 ) ( 1 - /)2)1/2 In
1
+ (1 - p 2 ) ' / 2
-
(3)
P
where p is the axial ratio of the ellipsoid of HMPB ( p = a / b ) . These equations apply to the rotation about the a axis. The parameter Cis specified by the hydrodynamic boundary conditions. For stick boundary conditions, C = 1 while, for slip boundary ( 2 0 ) Strickler, J.; Berg, R. A. J . Chem. Phys. 1962, 37, 814.
(21) Perrin, P. F. J . Phys. Radium 1934, 5 , 497; 1936, 7, 1.
4624
The Journal of Physical Chemistry, Vol. 91, No. 17, 1987
- -r'
~
-7-
I4
-
-
1
Im
Lee et al.
2
--
12-
m
.
/
i
X
/
/
-
/
k
z a c v)
z
0 0 W
c a n
0
50
100
150
200
250
T/T (KCP-I )
Figure 5. Comparison of proton-transfer rates a t 20 OC ( 0 )and 40 O C (X) with rotational relaxation rates (0) for HMPB in 1-alkanols (propanol-undecanols) as a function of T / T . The solid line is drawn with the inverse of the slope obtained in Figure 4.
conditions the appropriate value can be obtained from the tabulation in Hu and Zwanzig.22 The viscosities of the alcohols are large enough that a small value of T R at 7 = 0 (Le., the intercept in the SED plot) is not detectable so the data were fitted as if there were no intercept. Considering the van der Waals radii and molecular volumes of the composite atoms,23the calculated Vwas 196 A3, corresponding to a = 2.8 and b = 6.0 A, assuming a prolate ellipsoid. A least-squares fit of the measured T R to eq 2 gives the slope of 61 ps/cP or VfC = 241 A3. From eq 2 and 3 VfC is calculated to be 314 A3 using stick and 88 A3 for slip boundary conditions. Therefore, the rotational motion of HMPB in I-alkanols is more closely described by stick boundary conditions. Similar conclusions were reached for the rotational diffusion of other hydrogen-bonded species.24 Although the proton-transfer times are predicted qualitatively from the rotational relaxation times, we in fact obtain better agreement if the solvent motions are also incorporated. Figure 5 shows the measured proton-transfer rate constants and solute rotational relaxation times as a function of T / s and it is apparent that the values of kPTare consistently larger than the values of kR for the same values of T / q . This suggests that the effective diffusional rate constant determining the proton transfer should be larger than kR. This in turn suggests that the solvent motion be incorporated to speed up the proton transfer. In order to test these ideas we show in Figure 6 a plot of kPT,calculated from k R + kD, compared with the measured values as a function of viscosity. The values of k,, being a measure of the solvent reorientation correlation rate, were obtained from experimental values of the Debye relaxation frequencyzsbecause it is well-known that strongly associated solvents like alcohols do not follow simple hydrodynamic equatiomz6 The values and viscosity variations of kPI.and (kR kD) are qualitatively the same. The fit is worst in the high-viscosity regime where the values of kR + kDexceed kPTbut it is generally good enough to provide a case for the model we have proposed. The measurements of kR refer to rotations of the transition dipole of the fluorescence process (along the long molecular axis) and are probably not exactly the proper rotational diffusion rates to use for kM. Similarly the values of k , correspond to rotations of the permanent dipole moment of the solvents which may not give the correct interpretation for ks. Furthermore, the boundary conditions and hence the viscosity dependence of ks and k M are expected to be somewhat different from those of kR and
+
(22) Hu, C. M.; Zwanzig, R. J. Chem. Phys. 1974, 60, 4354. (23) (a) Bondi, A. J . Phys. Chem. 1964, 68, 441. (b) Edward, J. T. J . Chem. Educ. 1970, 47, 261. (24) (a) Waldeck, D. H.; Fleming, G. R. J. Phys. Chem. 1981,85, 2614. (b) Philip, L. A.; Webb, S. P.; Clark, J. N. J . Chem. Phys. 1985, 83, 5810. (25) (a) Maryott, A. A. NBS Circ. 1958, No. 589. (b) Sagal, M. W. J. Chem. Phys. 1962.36, 2437. (c) Garg, S. K.; Smyth, C. P. J. Phys. Chem. 1965.69, 1294. (26) Smyth, C. P. In Molecular Interactions; Ratajczak, H., OrvilleThomas, W. J., Eds.; Wiley: New York, 1980; Vol. 2.
( V I S cos I T Y j ? c p-1) Figure 6. Comparison of km and kR kD of HMPB vs. viscosity a t 20 'C. Open circles (0) are kPTfor propanol to undecanol. The A is for ethanol data taken from ref 7a and X for methanol data from ref 10. The straight line is the same as in Figure 5. The curve is not a fit but is obtained by adding the Debye relaxation rates ( l / i D ) to the solvent reorientation rate for each solvent.
+
k,. In addition, it may be necessary to incorporate the inertial motion of S and M to achieve S'M'. Given these factors, the qualitative predictions illustrated in Figure 6 are very satisfactory. The relation between the proton-transfer rate and ( k R k,) might also be applicable in other systems that were previously studied. Solvent influences were detected for the excited-state lifetime of hydroxyflavone and attributed to polarity effects.I6J7 The rates varied from 30 ps in methanol to 60 ps in octanol, which is a similar change to that found in this work for the short-lived components of HMPB. The absence of the long component and its associated stochastic dynamics could be due to the bulky phenyl ring next to the hydroxylic group prohibiting the existence of structures having the characteristics of configuration I1 in alcohols. The occurrence of solvent-mediated proton transfer is clearly shown in the fluorescence of 7-azaindoleZ6which is rigid and has no such steric restriction on solventsolute structures. The increase in lifetime by a factor of 1.5 between methanol and butanol might be attributable to a viscosity effect.26 In presenting a model to predict the intramolecular proton transfer in such systems as HMPB, we have assumed, on the basis of the isoviscosity plots of the fluorescence lifetime, that there is no barrier to the internal rearrangement of HMPB which leads to the optimum proton tunneling configuration. If there were a barrier, it would be necessary to use a Kramers type theory2*to describe the proton transfer: The internal inertia would slow down the rate at which the reactive structure is formed and there should be a significant discrepancy between the solute rotational relaxation rate and the reaction rate for a given solvent.29 Further insight into this question is obtained from studies of the tautomer of HMPB which emits in the red spectral region. This red emission has an activation energy of about 300 cm-' in both hydrocarbon and crystal^.^ This result suggests that the whole proton-transfer reaction path is not energetically flat but that there are forces involved in the proton transfer which are separate from those arising from solvent-solute collisions.
+
Conclusions Experimental studies of the fluorescence lifetimes and fluorescence anisotropies of HMPB in alkanols and ethers have allowed us to obtain the proton-transfer times (intramolecular) (27) McMorrow, D.; Aartsma, J. J. Chem. Phys. Lett. 1986, 125, 5 8 1 . (28) Kramers, H. A. Physica 1940, 7, 284. (29) Lee, M.; Bain, A. J.; McCarthy, P. J.; Han, C. H.; Haseltine, J. N.; Smith 111, A. B.;Hochstrasser, R. M. J. Chem. Phys. 1986, 85, 4341. (30) (a) Landolt-BornsteinZehlenwerte umi Funktionen; Springer-Verlag: New York; Band 11, Teil 5. (b) Rauf, M. A.; Stewart, G. H.: Farhataziz J. Chem. Eng. Data 1983, 28, 324.
J . Phys. Chem. 1987, 91, 4625-4627 as functions of viscosity. The overall process was found to have little or no energy of activation around 300 K. The comparison of rotational relaxation and proton-transfer rates has revealed that they are strongly correlated, indicating that the proton transfer is mediated by solvent frictional forces. A reasonable fit of the data is obtained if it is assumed that the proton-transfer rate is the sum of the rotational relaxation rate for the solute and the Debye relaxation rate for the solvent at each viscosity. We believe
4625
these experiments constitute the first direct measurements of a reaction rate which implicates a sum of two rotational diffusion processes.
Acknowledgment. This research was supported by a grant from the Division of Materials Research of the National Science Foundation (DMR-85-07740). Registry No. HMPB,2440-22-4
Structures and Mechanisms of Llpid Phase Transitions in Nonaqueous Media: Dlpalmitoylphosphatidylcholine in Fused Salt W. Tamura-Lis, L. J. Lis,* Department of Physics and The Liquid Crystal Institute, Kent State University, Kent, Ohio 44242
and P. J. Quinn Department of Biochemistry, King’s College London, London W8 7AH, U.K. (Received: December 1, 1986; In Final Form: May 4, 1987)
The mechanisms of the single phase transition observed for L-dipalmitoylphosphatidylcholinedispersed in excess fused salt, ethylammonium nitrate (EAN), was examined with time-resolved X-ray diffraction. The samples were allowed to equilibrate at 0 OC for over 4 days to induce the subgel state. The subgel state characterized by an orthorhombic acyl chain subcell converts directly to the hexagonal subcell characteristic of the disordered acyl chain state. This transformation involves the continuous loosening of the subcell packing until the disordered state is induced. A concomitant change in the mesophase unit cell is observed which is characterized as a transition from a multilamellar array of bilayers to an hexagonal array of cylinders.
Introduction The interaction between phospholipids and water is currently receiving a great deal of attention as we strive to understand how lipid structures and biological membranes are stable in this solvent. This interest has also spawned a renewed effort in the study of phospholipid structures and phase transitions in nonaqueous solvents. In particular, the replacement of water by organic solvents such as glycerol’-’ and “dry” solvents such as trehaloseM has been shown to alter the bilayer structure, packing, and phase-transition parameters for systems containing phosphatidylcholines. A uniquely different type of solvent used in previous studies7v8was ethylammonium nitrate (EAN), a fused salt of low melting temperature, which provides a purely ionic (although highly polar) medium. Thus the role of ionic bonding between head groups can be studied and contrasted to that of hydrogen bonding in water or similar polar solvents. The effect of replacing water with EAN was first reported,’ using bilayers made from distearoylphosphatidylcholine. It was found that the presence of EAN had little effect on the main (gel to liquid crystal) phase transition in DSPC, although there was some evidence of an increase in the pretransition temperature. A further observation was that the d spacing for the L, phase in a 1:l (by weight) mixture of EAN and DSPC was drastically reduced when compared with a fully hydrated DSPC bilayer. The acyl chain packing was found to be hexagonal subcells with a sharp ~
~ _ _ _
~~
~
~
(1) McDaniel, R. V.; McIntosh, T. J.; Simon, S. A. Biochim. Biophys. Acta 1983, 731, 97. (2) Rowe, E. S. Biochemistry 1983, 22, 5299. (3) Simon, S. A.; McIntosh, T. J. Biochim. Biophys. Acta 1984, 773, 169. (4) Crowe, J. H.; Crowe, L. M.; Chapman, D. Science (Washington, D.C.) 1984, 223, 701. (5) Lee, C. W. B.; Waugh, J. S.; Griffin, R. G. Biochemistry 1986, 25, 3731. (6) Finegold, L.; Singer, M. A. Biochim. Biophys. Acta 1986, 855, 417. (7) Evans, D. F.; Kaler, E. W.; Benton, W. J. J . Phys. Chem. 1983, 87, 533. (8) OLeary, T. J.; Levin, I. W. J . Phys. Chem. 1984, 88, 4074. ~~
0022-3654/87/2091-4625$01.50/0
reflection at 4.1 and a broad reflection at 4.0 A for the L, and La phases, respectively. The surface areas for the DSPC head groups in EAN of both the L, and L, phases were similar (ca. 80 A*) when calculated with the L, and L, d spacings. These results contrast with previous structural information determined for DSPC bilayers in water, in which, for example, a surface area of 52 A2 is reported for L,. However, it could not be determined whether these observations were influenced solely by the solvent character or whether the amount of solvent present also had some influence. A later study using Raman spectroscopy examined the dispersion of dipalmitoylphosphatidylcholine(DPPC) in EAN.* Only a single phase transition was observed at 59.5 “C, which was characterized as an acyl chain transition from an ordered orthorhombic subcell to a disordered hexagonal subcell, rather than the usual multiple phase transitions observed for DPPC dispersed in water. Another difference between DPPC dispersed in EAN vs. DPPC in water was the inferred presence of a micellar (in EAN) rather than a lamellar (in water) phase above the liquid crystal phase transition temperature. The basis for the assignment of the high-temperature micellar phase for DPPC in EAN was the degree of disruption of the Fermi resonance of the 1440-cm-’ Raman band as it influenced the relative intensities of symmetric and asymmetric C-H stretch modes. Recently, high-intensity X-ray beams at synchrotron sources have been used to determine lipid mesophase and acyl chain structures at high r e s o l u t i ~ n ,as~ ~well ~ ~ as the kinetics and mechanisms of a variety of phase transitions.”-I6 In this report, DPPC bilayers dispersed in EAN (20% w/v DPPC/EAN) were (9) Caffrey, M. J.; Feigenson, G. W. Biochemistry 1984, 23, 323. (10) Tenchov, B.; Lis, L. J.; Quinn, P.J. Biochim. Biophys. Acta 1987, 897, 143. (1 1) Caffrey, M.; Bilderback, D. H. Biophys. J . 1984, 45, 627. (12) Caffrey, M. Biochemistry 1985, 24, 4826. (13) Ranck, J. L. Chem. Phys. Lipids 1983, 32, 251. (14) Laggner, P. Top. Curr. Chem., in press. (15) Lis, L. J.; Quinn, P. J. Biochim. Biophys. Acta 1986, 862, 81. (16) Quinn, P. J.; Lis, L. J. J . Colloid Interface Sci. 1987, 115, 220.
0 1987 American Chemical Society
