1396
J. Phys. Chem. 1981, 85, 1396-1401
Kinetics of Complexation of 18-Crown-6 Ether with LIC104 in Solvents of Low Permittivity. 1,3-Dioxolane and 1,2-Dlmethoxyethane Herman Farber and Serglo Petruccl” Department of Chemistry and Electrical Engineering, Polytechnic Institute of New York, Brooklyn and Farmin&& (Received: September 22, 7980)
Campuses, New Yo&
This work deals with the kinetics and mechanism of the coordination of macrocyclic ligands with alkali metals in media of low permittivity, a topic relevant to the understanding of ionic transport processes during nerve impulses. To this end ultrasonic absorption data in the frequency range 3-300 MHz at 25 “C for LiC104added to 18-crown-6ether (18C-6),at a molar ratio of N 1,in the solvents 1,3-dioxolane (DXL) and 172-dimethoxyethane and for the concentration range 0.025-0.25 M, have been collected and are reported. For 1,3-dioxolane,a single Debye relaxation with a relaxation frequency showing concentration dependence can describe the ultrasonic data. 1,2-Dimethoxyethanesolutions of LiC14show a single relaxation which was interpreted as due to ion pair quadrupole conversions. Upon addition of 1843-6,another relaxation at lower frequency appears. These data can then be described by the sum of two Debye relaxation processes. The results for 1,3-dioxolaneare interpreted in accordance with Chocks mechanism as one of the two forms of the crown ether reacting with the ion pair Li”C104. The results for 1,2-dimethoxyethaneare interpreted semiqualitatively by a coupled Chock’s mechanism, the ion pair reacting to form quadrupoles, but also reacting with 184-6 to form complexes. Approximate values of the forward rate constants are calculated for the two processes. An alternate two-step mechanism (without postulating two forms of the crown ether) is discussed. It is shown that this mechanism (already proposed for the complexation of valinomycin with alkali metal cations) is kinetically indistinguishable, using the availabledata, from Chocks mechanism. To investigate the validity of the assumed Chocks mechanism, which postulates a much faster equilibrium between two forms of the crown ether, we collected microwave dielectric data for 0.1 M 18-C-6in both solvents for the frequency range 1-85 GHz at t = 25 “C. No substantial difference from the permittivity (real part) and loss coefficientof the solvent is measurable for the solutions. Static permittivities measured at 4.0 and 0.5 MHz confirm that solutions and solvents are indistinguishable. The above would indicate either the absence of an “open” polar form of the crown ether or its presence in such low concentrations as to be undetectable. Presence of an equilibrium in 18-C-6has finally been revealed in dioxolane at -20 “Cby ultrasonic relaxation, sustaining the assumption of Chocks mechanism for the complexation of LiC104with 184-6 in this solvent.
Introduction Coordination of metal cations by ring-structured ligands is ubiquitous in nature. Notable examples are the coordination of iron and (for some reptiles) of copper by the heme in blood, coordination of magnesium by porphyrin in plants, and selective coordination of alkali metals by antibiotics as valinomycin, thought to be responsible for transport processes during conduction of nerve impulses. The development of synthetic monocyclic and bicyclic ligands as crown ethers and cryptands, respectively, has brought intense activity1 in the study of the stability and structure of complexes between these ligands and various metal cations. For these systems, kinetic studies are far less numerous2and are limited to aqueous solutions, or in general to media of high permittivity. Up to the present, kinetic investigations in media of low permittivity (resembling the membrane of biological cells) between electrolytes and macrocyclic ligands as crown ethers are lacking. Still the work is relevant because of the need for knowledge of the processes occurring in nonaqueous medium during cation transport. To this end we have investigated, by ultrasonic relaxation techniques, the complexation of LiC104with 184-6 ether in two ethers with (1) Nelson; G. A. Ed.; “Coordination Chemistry of Macrocyclic Compounds”, Plenum Press: New York, 1979. Chapter 3 by Lamb, J. D.; Izatt, R. M.; Christensen, J. J.; Eatough, D. J. Chapter 9 by Popov, A. I.; Lehn J. M. (2) See the following for an excellent review and quoted literature therein. Liesegang, G. W.; Eyring, E. M. In “Synthetic Multidentate Macrocyclic Compounds”;Academic Press: New York, 1978. Cox, G. G.; Schneider, H.; Stroka, J. J.Am. Chem. SOC. 1978,100,4746. Cox, B. G.; Knop, D.; Schneider, H. Ibid. 1978,100, 6002. 0022-3654/81/2085-1396$01.25/0
static permittivities of about 7. Previous work on the electrolyte alone (done to have a dynamic picture of this simpler system) has been r e p ~ r t e d . ~ During the development of the work we have also used resulta obtained by microwave dielectric relaxation, as shown below.
Experimental Section The equipment and procedure for the ultrasonic work has been described pre~iously.~7~ For the dielectric work, in addition to the equipment already described: we have extended our measurement to 85 GHz. The new reflectometer with the 60-90 waveguide consists of a 8620C Hewlett Packard sweep oscillator, Hughes Co. 44415H main frame adapter, and 44425 modulator/ leveler, a 44714H Impatt plug in, two 47325H-1100 detectors, a 45725H-1000direct reading variable attenuator, and two 44345H-310 10-dB directional couplers. Further a wavemeter, Hughes 45715H-1000, was used to measure the frequency of the radiation. The cell was similar to the ones described for the other bands. The authors are grateful to Professor A. Oliner, Director of the Microwave Research Institute of our school, for allowing the use of the above instrumentation. The instrumentation for the static permittivity consisted of a Bontoon resonator and a thermostated two-terminal cell of capacity Co = 5.08 pF. This value was obtained at (3) Onishi, S.; Farber, H.; Petrucci, S. J.Phys. Chem. 1980,84,2922. (4) Fanelli, A,; Petrucci, S. J.Phys. Chem. 1971, 75, 2649. Saar, D.; Brauner, J.; Farber, H.; Petrucci, S. Ibid. 1980,84, 841. (5) Farber, H.; Petrucci, S. J.Phys. Chem. 1975, 79, 1221. Saar, D.; Brauner, J.; Farber, H.; Petrucci, S. Ibid. 1980, 84,841.
@ 1981 American Chemical Society
The Journal of Physical Chemistty, Vol. 85, No. 70, 1981 1397
Complexation of 18-C-6 with LiC10, so0
(%)
1017
( cm-1 sz )
400
ISOr
I
0 LiCfO, 0 . 0 5 2 M t 18-C-6 0 . 0 5 7 M IN 1-3 DIOXOLANE:
160
0 IS-C-6 0.05M IN 1-3 DIOXOLANE: t.25.C
LiCPO,
2ool
sot
0.05M
+ 15-C-5
0.0% IN 1-3 DIOXOLANE:
loot
t s25OC
\
60!
2
5
IO
20
50
IO0 200
500 I O 0 0
f(MH2)
+
Flgure 2. a / f 2vs. ffor 0.046 M LiCiO, 0.051 M 15-crown-5 ether in doxolane. The solid line is the sum of two Debye relaxation functions cm-I s2, fR = 50 MHz, A , = 122 with parameters A , = 15 X X cm-I s2, f, = 725 MHz, B = 30 X ld-I7 cm-' s2. The value of B is tentative. grrors in 9 will be reflected in fR2.
\
6oo/
0
IN OME 0 0 5 5 M 18-C-6
400
f(MHz)
Flgure 1. a/P vs. the frequency f(MHz) and excess sound absorption per wavelength p (= ,yc A) vs. f(MHz) for LiCIO, 18-C-6 ether at Cow, = 0.052 M and CSlm = 0.057 M in 1,3dioxolane; t = 25 "C.
200
+
f = 0.5 and 4.0 MHz by calibration with both solvent DME and DXL, using the static values eo = 7.05 and eo = 6.95 at t = 25 "C, respectively. For the materials, 18-C-6ether was an Aldrich product. This was subjected to prolonged vacuum mm) followed by recrystallization from distilled acetonitrile and vacuum up to constancy in weight. The melting point of the final product, measured in a hot plate connected to a microscope, was 39-41 "C. This value is in good agreement with literature results.6 LiC104, 1,3-dioxolane (DXL), and 1,2-dimethoxyethanehave been purified as already de~cribed.~ The solutions were prepared by weighing both crown ether and LiC104,keeping the molar ratio of crown ether and LiC104of the order of 1. Contact of the solvents with the atmosphere was minimized to 20-30 s during preparation of the solutions and filling of the cells.
Results Figure 1 reports a representative plot of the quantity a/f (cm-l s2) vs. the frequency f (MHz) and of the corresponding excess sound absorption per wavelength, p = X a, vs. f . The systems shown are LiC104and 18-C-6ether at concentrations of CoLicl0= 0.052 M and c01sC6 = 0.057 M at t = 25 "C in the solvent 1,3-dioxolane. a is the absorption coefficient of sound (Np cm-l). The solid lines are the fitted functions for the single Debye relaxation:
f/fR
with pmax= [ ( A / 2 ) u f Rp] , = aeXJ= ( a - B f ) X , the wavelength X = u / f , u is the sound velocity (cm/s), and B the quantity a/? for the solution at f >> f w 1,3-Dioxolane shows a relaxation at several hundred megahertz, therefore B is the extrapolated value of the background absorption ignoring such solvent relaxation. Figure 2 shows the quantity a/f" for 0.046 M LiC104+ 0.051 M 15-crown-5ether in 1,3-dioxolaneat 25 "C. Two ~
~
~~
~~~
(6) Greene, R. M. Tetrahedron Lett. 1972, 1793. Pederson, C. J. J. Am. Chem. Soe., 1967,89, 7017.
t _ _ _ _41 __
01
1
0 13M + 18-C-6 -IN DUE: t.25-C
LICPO,
r
0 15H
1
i 6
LICIO,
I b h 100 i l o500b o
0 0 4 8 M + 18-C-6 IN OM€: l:25'C
0052M
r
f(MHz)
f(MHz)
Flgue 3. (alp)vs. the frequency f(MHz) and excess sound absorpth per wavelength p vs. f (MHz) for LiC10, and 184-6 ether at the concentrations Indicated in 1,2dimethoxyethaneat t = 25 "C. The solid line Is the calculated function, sum of two Debye relaxation processes (dashed lines). B is the background absorption.
relaxation processes are visible, one pertaining to the solute and the other (the high frequency) to the solvent. The solid line is the sum of two Debye relaxation processes:
with parameters AI = 15 X cm-' s2, f~~ = 50 MHz, A2 = 122 X 1O-l' cm-' s2, fb = 725 MHz, and B = 30 X cm-l s2. Because of limitations in frequency the values of B and therefore of f R 2 are tentative. This plot shows, however, both Al and f R 1 , pertaining to the solute, to be vastly different from the solutions of LiC104 + 18-C-6 in dioxolane. Figure 3 shows representative plots for the quantity (alp)vs. the frequency f for LiC104and 184-6 at the concentrations indicated in 1,2-dimethoxyethaneat t = 25 "C. The solid line is the calculated function, sum of two Debye relaxations (eq 11). B is the background absorption of the solvent. Also 1.1 is reported for the same systems. The solid line corresponds to the function
(111)
The dashed lines correspond to the single Debye contributions to p. Table I reports the calculated parameters according to eq 1-111 for the concentrations and solvents investigated at 25 "C.
The Journal of Physical Chemistry, Vol. 85, No. 10, 1981
1398
Farber and Petrucci
TABLE I: Results of the Ultrasonic Parameters in Accord with the Functions 1-111 and Sound Velocity u for LiClO, and 1 8 4 - 6 in the Solvents Investigated at 25 "C 1,3-Dioxolane lO"A, 101'B, io-su, CoLic104,M C'laCsr M cm-' sa em-' s1 f R , MHz cm/s 105p 0.196 0.211 572 160 13 1.344 500 0.144 0.163 7 80 155 12 1.343 629 0.115 782 152 12 1.343 630 0.0919 0.057 611 155 10 1.341 410 0.052 0.0245 0.0284 400 155 9 1.345 24 2 0.05 133., 1,2-Dimethoxyethane C'LiClo., M C'iscs, M 1OSr2 fR2, MHz losri fR1, MHz B, cm'' sa 1 0 3 ,cm/s 0.23 0.26 4 80 19 6 20 140 50 1.210 0.18 0.20 440 16 520 120 45 1.201 0.13 0.15 500 12 440 100 40 1.186 0.089 0.11 427 8.5 284 80 40 1.182 0.048 0.052 250 4.5 170 40 39 1.172 0.055 32., TABLE 11: Dielectric Parameters eo, ,E and f~ in Accordance with a Single Debye Relaxation Describing the Dielectric Relaxation of 1 8 4 - 6 Ether in both 1,3-Dioxolane and 1,2-Dimethoxyethaneat 25 "C system '0 'eo 1,3-Dioxolane Solutions 7.0 3.35 0.11 M 1 8 4 - 6 pure solvent 6.95 3.35 1,2-Dimethoxyethane Solutions 7.05 2.75 0.10, M 1843-6 pure solvent 7.05 2.75
13 5
8.
43.0 €8
120
'i1.0 I
/
3-
3 05 I
,
8
2
5
35
45
35 35 35 35
3.0-
8r
1
0. Retaining Chock's mechanism in this solvent leads to kz > 5 X lo8 M-' s-l. On the other hand, for LiC104 X
Supplementary Material Available: Appendices I, 11, and 111, leading to eq VI, XIII, and XIV (8 pages) are available as supplementary material. Ordering information is given on any current masthead page.
+
Studies on the Phase Transition in the Single Lamellar Liposomes. 3. Kinetic Behavior of the Phase Transition Shoshl Inoue, Mlklo Nishlmura, Tatsuya Yasunaga, Faculty of Science, Hiroshima University, 1- 1-89, Hipshkenda-machi, Nakaku, Hiroshima 730, &pan
Hlroyukl Takemoto, and Yoshlnorl Toyoshima" Faculty of Infegrafed Arts and Sciences, Hiroshima Unlversky, 1- 1-89, Higashlsenda-machi, Nakaku, Hiroshima 730, &pan (Received: April 30, 1980; In Final Form: January 19, 198 1)
The kinetic process of phase transition of uniformly sized single lamellar dipalmitoylphosphatidylcholine liposomes was studied by using the fluorescence temperature-jump method as a function of liposome size. The relaxation of the fluorescence intensity of distearyloxacarbocyanine incorporated in the liposome bilayers consisted of a rapid process with a relaxation time of less than 1M S and a slower process. The former was observed over the whole temperature range studied, but its relaxation amplitude exhibited a maximum at the midpoint of the phase transition, while the latter was observed only in the phase transition region and was characterized as a single relaxation. The relaxation time and amplitude of the slow process exhibited maxima at the midpoint of the transition and the maximum relaxation time increased with the liposome size up to 0.1 s in the large liposome which consists of 33 X l@lipid molecules. These results were compared with the equilibrium properties of the phase transition.
Introduction Aqueous dispersions of neutral phospholipids such as phosphatidylcholine readily form a multilamellar liposome structure composed of many concentric bilayer sheets of various sizes interspersed with water, exhibiting a thermotropic phase transition with regards to the chain ordering of the lipid molecules.'-s The phase transitions of the multilamellar liposomes are characterized by a highly cooperative main transition and an additional somewhat broader pretransition with a small latent heat. It has been qualitatively known that the change of a liposome from the multilamellar to single structure results in the alternation of the transition behavior, e.g., a decrease in the latent heat and in the cooperativity and a lowering of the
transition temperature.4i6 In part 2 of this series: we suggested that, because of the lack of interbilayer interaction within each liposome, the single lamellar liposomes have their own particular characteristic phase transition which is essentially different from those of multilamellar liposomes, showing the temperature profiles of the fluorescence intensity of an amphiphilic dye molecule incorporated in the bilayers. The results clearly demonstrated that in the single lamellar liposomes the pretransition which is observed in the multilamellar structure disappears and the midpoint temperature and van't Hoff enthalpy of the main transition decrease with decreasing size of the liposome. The results obtained in the single lamellar liposomes also suggested a higher order transition
(1) A. D. Bangham and R. W. Home, J. Mol. Biol., 8, 660 (1964). (2) D. Chapman, R. M. Williams, and B. D. Ladbrooke. Chem. Phvs. Lipids, 1, 445 (1967). (3) S. Mabrev and J. M. Sturtevant. Proc. Natl. Acad. Sci. U.S.A.. . 73.. 3962 (1976).
(4) M. P. Sheetz and S.I. Chan, Biochemistry, 11, 4673 (1972). (5) J. Suurkuusk, B. R. Lentz, Y. Barenholz, R. L. Biltonen, and T. E. Thompson, Biochemistry, 16, 1393 (1976). (6) H. Takemoto, S.Inoue, T. Yasunaga, M. Sukigara, and Y. Toyoshima, J.Phys. Chem., 85, 1032 (1981).
~
0022-3654/81/2085-1401$01.25/0
0 1981 American Chemical Society
~
~~
