J . Phys. Chem. 1984, 88, 6350-6353
6350
The microscopic origin of multiexponential decay of species embedded in micelles’ has been discussed in terms of cooperative processes.16 In this picture the movement of one surfactant molecule needed to allow reaction of the embedded species depends on the configuration and dynamics of other surfactant molecules forming the micelle. As in polymer dynamicsI6 it is primarily the mean field due to other molecules, not the properties of isolated molecules, that leads to cooperative processes and hence to nonexponential decay. The differences between the distributi40nsof TMB’ lifetimes in micelles of SlOS and S12S, either\in HzO or DzO, are small compared with that caused by substitution of D 2 0 for HZO.In a giwp medium the increase of alkyl chain length increases only the effective lifetime of TMB cation radicals in micelles in agreement with conclusions on asymmetric localization of the solute within the micelle^.^ As the alkyl chain length increases the TMB cation radicals have decreased water interactions at the micellar interface and decay more slowly. Thus the maximum probable lifetime changes as denoted by T~ but the internal cooperative processes in the micelles are not changed and hence the lifetime distribution width of TMB cation radicals is not changed. Substitution of D 2 0 for H 2 0 not only increases the effective TMB+ lifetime but also causes a significant broadening of the (16) Skinner, J. L. J. Chem. Phys. 1983, 79, 1955.
TMB+ lifetime distribution. This lifetime broadening effect, similar for both kinds of micelles, can arise frbm tighter molecular packing inside the micelle in D 2 0 than in H 2 0 due to stronger hydrophobic interactions in DzO which has been demonstrated for a number of lipid aqueous dispersion^.^ Tighter molecular packing makes the micelle dynamics more cooperative and leads to a wider distribution of lifetimes.16 This tighter molecular packing in DzO also explains why the effect of alkyl chain length on the effective lifetime of TMB cation radicals is slightly greater in H 2 0than in DiO as shown in Table I. Additional support for coopejative processes iA micelle dynamics leading to nonexponential decay of embedded species comes from the fact that their effective lifetimes are many orders of magnitude greater than the time scale of events for solute exits from micelles or for the exchange of surfactant molecules between the micelle and the bulk sol~tion.’~J~
Acknowledgment. This research was supported by the US. Department of Energy under contract DE-AS05-80ER10745. Registry No. TMB+, 21296-82-2; S12S, 151-21-3; SIOS, 142-87-0; deuterium, 7782-39-0. (17) Thomas, J. K.; Grieser, F.; Wong, M. Ber. Bunsenges. Phys. Chem. 1978, 82, 937. (18) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Inr. Ed. Eng. 1980, 19, 675.
Changes in Partial Molar Volumes and Isentropic Partlal Molar Compressibilities of Stacking of Some Nucleobases and Nucleosides in Water at 298.15 K Harald Hailand,*+ Arne Skauge,f and Ingrid Stokkelandt Department of Chemistry, University of Bergen, N-5014 Bergen-U., Norway, and Norsk Hydro Research Center in Bergen, N-5013 Nygardstangen, Norway (Received: May 17, 1984)
The partial molar volumes and isentropic partial molar compressibilities of some nucleobases, purine and caffeine, and some nucleosides, cytidine, thymine, and uridine, in aqueous solution have been measured at 298.15 K. These data have been used to calculate the changes in partial molar volume and compressibility due to stackihg (self-association). The results show that the volume changes are negative for the nucleobases and positive for the nucleosides. The compressibility changes are positive for both types of solutes, but they are a magnitude larger for the nucleosides. It appears that the stacks exhibit an intrinsic compressibility and that the distance between the associated base units are pressure dependent. The ribose unit of the nucleosides causes a far less efficient packing of the stacks than for the pure bases.
Introduction Nucleic bases, nucleosides, and nulceotides associate in aqueous solution as demonstrated by various experimental techniques: vapor pressure osmometry, sedimentation equilibria, ultrasound measurements, NMR, and other spectroscopic The field has been reviewed by Ts’023924and more recently by Siege1 et aLZ5 It is generally accepted that this self-association is of a vertical stacking type and that it proceeds beyond the dimer stage. In spite of great effort the nature of the associating forces is not fully understood. However, there seems to be little doubt that dipole-induced dipole forces play a significant part in this type of interaction. Thermodynamic data for this self-association (stacking) process have mostly been derived from osmotic coefficients or spectroscopic work, though Gill et al.,26*27 Cesaro et a1.,28and Sakurai et al.29 have provided calorimetric data. Little is known about the changes in partial molar volume (AV,) or compressibility (AK,) as these compounds associate. Density measurements have been carried ~
‘University of Bergen. *Norsk Hydro Research Center in Bergen.
out by Cesaro et a1.28and K a ~ a r d a .The ~ ~ latter calculated AV, by taking the difference between the apparent molar volume at (1) Ts’o, P. 0. P.; Melvin, I. S.; Olson, A. C . J. Am. Chem. SOC.1963,85, 1289. (2) Ts’o,P. 0.P.; Chan, S.I. J . Am. Chem. SOC.1964, 86, 4176. (3) Chan, S.I.; Schweizer, M. P.; Ts’o, P. 0. P.; Helmkamp, G. K. J. Am. Chem. SOC.1964,86, 4182. (4) Schweizer, M. P.; Chan, S.I.; Ts’o, P.0.P.J. Am. Chem. Soc. 1965, 87, 5241. ( 5 ) Broom, A. D.; Schweizer, M. P.; Ts’o, P. 0. P. J. Am. Chem. SOC. 1967,89, 3612. (6) Schellman, J. A. C. R. Trav. Lab. Carlsberg 1956, 29, 223. (7) Van Holde, K. E.; Rosetti, G. P. Biochemisty 1967, 6, 2189. (8) Rosetti, G. P.; Van Holde, K. E. Biochem. Biophys. Res. Commun. 1967, 26, 717. (9) Solie, T. N.; Schellman, J. A. J. Mol. Biol. 1968, 33, 61. (10) Ts’o, P. 0. P.; Kondo, N. S.;Robins, R. K.; Broom, A. D. J. Am. Chem. SOC.1969, 91, 5625. (11) Bugg, C. E.; Thomas, J. M. Biopolymers 1970, 10, 175. (12) Poerscke, D.; Eggers, F. Eur. J. Biochem. 1972, 26, 490. (13) Evans, F. E.; Sarma. R.H. J. Biol. Chem. 1974, 249, 4757. Lohmann, W. Biophys. Sirucr. Mech. (14) Schimmack, W.; Sapper, H.; 1975, 1, 113. (15) Heyn, M. P.; Bretz, R. Biophys. Chem. 1975, 3, 35.
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Nucleobases and Nucleosides in Water at 298.15 K
The Journal of Physical Chemistry, Vol. 88, No. 25, 1984 6351
0.1 and 0.025 m, a method that does not seem satisfactory. Cesaro et aLZ8calculated AVs of caffeine by assuming dimerization only.28 AV, of 9-methylpurine has been derived from spectroscopic measurement^.^' The available data show that A 6 of nucleobases are negative as opposed to the volume changes of micelle formation or the formation of a separate phase from aqueous solution. Data are scarce, though, and since partial molar volumes and compressibilities have proved useful in the study of aqueous solutions, we found it worthwhile to undertake an investigation of the volume and compressibility changes of stacking of some nucleobases and nucleosides. Experimental Section Purine, caffeine, thymidine, cytidine, and uridine were obtained from Sigma at their best quality. They were dried in evacuated desiccators and used without further purification. Aqueous solutions were prepared in the range 0.01-0.2 m, and density measurements were carried out by a Paar density meter, DMA 601. The temperature at the oscillator was controlled to within f0.005 O C . The densities could thus be determined within f5 X 10" g ~ m - ~The . ultrasonic velocity was measured by the "sing-around" method.32 The apparatus was slightly modified by controlling the return signal so that the amlitude of the signal could be kept at a constant level at all times. The ultrasonic cell was made of glass with piezoceramic transducers in both ends. The cell length was 5 cm. The ultrasonic velocities could thus be determined within f0.05 m s-l. Osmotic coefficients at 25 O C were determined for caffeine by vapor pressure osmometry. A Knauer vapor osmometer was used with KCl as standard.33
Models
TABLE I: Limiting Partial Molar Volumes and Isentropic Partial Molar Compressibilities of Some Nucleobases and Nucleosides at 298.15 K
purine caffeine cytidine thymidine uridine
v2Qf 0.2, cm3 mol-' 84.40 145.20 153.50 167.55 151.45
Here K is the equilibrium or stacking constant and ml the monomer concentration. The AK model assumes that the enthalpy change is independent of i but that the entropy may vary by
ASi = R In
Ki = e-AGiIRT
(1)
The SEK model now assumes that the entropy change for all such reactions is constant and that the enthalpy change is independent of the size of the aggregate. This leads to the following expression for the total content of solute: m = ml/(l
- Km1)2
(2)
(16) Garland, F.; Christian, S . D. J . Phys. Chem. 1975, 79, 1274. (17) Plesiewicz, E.; Stepien, E.; Bolewska, K.; Wierzchowski, K. L. Biophys. Chem. 1976,4, 131. (18) Egan, W. J . Am. Chem. SOC.1976, 98, 4091. (19) Maevskii, A. A.; Sokhorukov, B. I. Russ. J . Chem. 1976, 50, 153. (20) Lam, Y. F.; Kotowycz, G. Can. J . Chem. 1977, 55, 3620. (21) Matthies, M.; Zundel, G. J . Chem. SOC.,Perkin Trans. 2 177, 14, 1824. (22) Neurohr, K. J.; Mantsch, H. H. Can. J . Chem. 1979, 57, 1986. (23) Ts'o, P. 0. P. In "Fine Structure of Proteins and Nucleic Acids"; Fasman, G. D., Timasheff, S . N.; Eds.; Marcel Dekker: New York, 1970. (24) Ts'o, P. 0. P. "Basic Principles in Nucleic Acid Chemistry"; Academic Press: New York, 1974. (25) Scheller, K. H.; Hofstetter, F. H.; Mitchell, F. H.; Prijs, B.; Siegel, H. J. Am. Chem. SOC.1981, 103, 247. (26) Gill, S . J.; Downing, M.; Sheats, G. F. Biochemistry 1967, 6, 272. (27) Farquhar, E. L.; Downing, M.; Gill, S. J. Biochemisrry 1968, 7, 1224. (28) Cesaro, A.; Russo, E.; Crescenzi, V. J . Phys. Chem. 1976,80, 335. (29) Sakurai, M.; Morimoto, S.; Inoue, Y. J. Am. Chem. SOC.1980,102, 5572. (30) Kasarda, D. D. Biochim. Biophys. Acta 1970, 217, 53 (31) Gaarz, U.; Ludemann, H.-D. Ber. Bunsenges. Phys. Chem. 1976,80, 607. (32) Garnsey, R.; Boe, R. J.; Mahoney, R.; Litovitz, T. A. J . Chem. Phys. 1969, 50, 5222. (33) Stokes, R. H.; Levien, B. J. J . Am. Chem. SOC.1946, 68, 333.
ai + constant
(3)
Here 9, is the number of ways the solute monomers can be distributed between the aggregates. From eq 1 one thus obtains an attenuated stacking constant:
Ki = e-lnie-M/RT(constant)= K/i
(4)
The total content of solute can be expressed as
m = ml$cml
(5)
The total content of aggregates including monomers is independent of the model and can be found from vapor osmometry:
ma = mcp = m l 4- m2 +
... + mi
(6)
Here cp is the osmotic coefficient. The stacking constant can now be calculated for both models since ma = m l / ( l
Two main models for the stacking (self-association) of bases or nucleosides have been described in the literature: 1 , 2 ~ 1 5 ~ 1the 6 sequential equal constant or isodesmic model (SEK model), and the attenuated constant model (AK model). In both models the equilibrium constant for forming an aggregate of i monomers from an aggregate of i - 1 monomers has been given as
(K,Of 0.4) x 104, cm3 mol-' bar-l 1.5 5.2 -18.2 -4.0 -17.0
- Kml)
ma = ( l / K ) ( $ c m- 1)
(SEK model)
(7)
(AK model)
(8)
ma can be calculated from osmotic coefficients. Combining eq 2 and I , or 5 and 8, means that ml and K can be determined from the SEK model or the AIS model. In the latter case it is necessary to use an iteration procedure to find ml.
Apparent Molar Volumes and Compressibilities The apparent molar volumes and isentropic apparent molar compressibilities have been calculated in the usual way.34 It is possible to divide the apparent molar volumes or compressibilities into two parts, the contributions from the monomers and the contribution from the molecules in the aggregates: fi = XI'Pv,mono
+ XZfi.stack
(9)
amma and fipack are the apparent molar volumes of free monomers and monomers in the aggregates (stacks), respectively. x1 and x2 are the corresponding mole fractions. At infinite dilution only free monomers should be present in solution. Accordingly cpv,mono can be taken as equal to apparent molar volume at infinite dilution. The same argument can be used for the apparent molar compressibilities. Since the content of monomers in the aggregates is m - m l , the apparent molar volume of the aggregated molecules can be calculated by rearranging eq 9:
Pv,stack can be calculated at each measured concentration and extrapolated to infinite dilution to obtain V20stack.The changes in partial molar volumes due to stacking can then be calculated as
(34) Harned, H. S.; Owen, B.B. "The Physical Chemistry of Electrolytic Solutions", 3rd ed.; Reinhold: New York, 1958.
6352 The Journal of Physical Chemistry, Vol. 88, No. 25, 1984
Hailand et al.
TABLE II: Stacking Constants, Changes in Partial Molar Volumes of Stacking, and Changes in Isentropic Partial Molar Compressibilities of Stacking from the SEK Model and from the AK Model at 298.15 K SEK model AK model A V:, AK: x io, W, AK: x 104, K cm3 mo1-l om3 mol-' bar-' K om3 mol-I cm3 mol-' bar-I
purine caffeine cytidine thymidine uridine
2.1' 9.0 0.87" 0.91* 0.61"
-4.7 -6.6 5.3
7.6 8.8 19.0
0.0
30
20.4
81
6.1b 23.0 1.996
-3.3 -5.4 4.4
5.8 7.0 17.0
1.42b
16.7
71
'Data from ref 24. bData from ref 16.
okXlo4 crn3rnol-'bar-'
0.05
I
1
I
I
0.05
0.1
0.15
I
* 0.2
Figure 1. Apparent molar volumes of some nucleobases and nucleosides as functions of their molalities in water at 298.15 K: ( 0 )purine, (0) caffeine, (m) uridine, ( 0 ) cytidine, (A) thymidine.
The changes in isentropic partial molar compressibilities can be derived by a similar set of equations: AK,o = KZQstack - KzQmono
(12)
Results and Discussion Figure 1 shows the apparent molar volumes of nucleobases and nucleosides as functions of their molalities. The curves are not linear, and the observed curvatures can be regarded as additional evidence for self-association of these solutes. The curvatures differ for the two types of solutes, the slopes being negative for the bases and positive for the nucleosides. Such differences in the slopes do not appear when the isentropic apparent molar compressibilities are plotted in the same way (Figure 2). Here, however, the values at infinite dilution distinguish the two types of compounds, as seen
0,1
0,15
* 0,2
Figure 2. Isentropic apparent molar compressibilities of some nucleobases and nucleosides as functions of their molalities in water at 298.15 K: ( 0 ) purine, (0) caffeine, (m) uridine, (0)cytidine (A)thymidine.
from Table I. The bases exhibit positive limiting isentropic partial molar compressibilities of the same order of magnitude as ordinary uncharged organic nonelectrolytes in water, for instance, amines and amide^.^^,^^ The data suggest that interactions between water and the nucleobases are nonspecific, i.e., hydrophobic hydration. The nucleosides, on the other hand, exhibit negative values of the limiting isentropic partial molar compressibilities. For cytidine and uridine the values are of the same order of magnitude as that Preof ribose, the latter being -12.5 X lo4 cm3 mol-' sumably the ribose unit is the determining factor. The deoxy form in thymidine apparently leads to a much higher partial molar
(35) LoSurdo, A,; Chin, C.; Millero, F. J. J. Chem. Eng. Data 1978, 23, 191. (36) Kaulgud, M. V.; Shrivastava, A.; Awode, M. R. Ind. J. Pure Appl. Phys. 1980, 18, 864. (37) Hailand, H.; Holvik, H. J . Solution Chem. 1978, 7 , 587. (38) Kauzmann, W. A d a Protein Chem. 1959, 14, 1.
J. Phys. Chem. 1984,88, 6353-6356
6353
This model of part dehydration of the bases is not valid for the nucleosides. A large part of the nucleosides, the ribose unit, is hydrophilic. This part of the molecule will probably be hydrated even in the stacks. The bulky, fully hydrated ribose unit probably requires much space, leading to less efficient packing of the nucleosides when they form stacks. The nucleosides are amphiphilic molecules containing a hydrophilic part, the ribose unit, and a hydrophobic part, the base. In this respect they resemble micelle-forming surfactants. Both types of compounds also exhibit positive AVvalues when they form aggregates, micelles or stacks. Perhaps this is a general feature of amphiphilic aggregation in water as opposed to purely hydrophobic compounds. Normally, negative volume differences for a process lead to negative compressibility changes as well and positive volume changes to positive compressibility change^.^'^^^ This is not observed for the nucleobases. They exhibit negative AV, and positive AK, values. Further the AK, values of the nucleosides are unusually large, especially for uridine. Normally the solutes are considered incompressible; Le., the intrinsic compressibility is zero. The observed compressibilities are ascribed to the compressibility of the hydration sheath. For the stack as an entity the intrinsic compresibility need not be zero. The distance between the monomers may well be reduced by pressure. This will explain the large partial molar compressibilities obtained for the nucleobases and especially the nucleosides of the stacks.
compressibility of this compound. Table I1 shows AV,O and M: for the stacking process calculated on the basis of both the SEK and AK models. There is only a very moderate model dependence. The AK model yields systematically lower values, but the differences are practically within the experimental error. Cesaro et aL2*have calculated AVg for caffeine by assuming only dimer formation, and even then the result remains the same. It thus seems safe to conclude that the AV: and AK: values are real and not model dependent. The difference between the bases and the nucleosides also shows in the AV: and AK: values. Purine and caffeine exhibit negative AV: values, and the nucleosides positive values (zero for thymidine). All AK: values are positive, but they are 1 order of magnitude larger for the nucleosides. Stacking of the bases leads to a volume contraction. The monomer is more efficiently incorporated in the stacks than in water. If hydrophobic solutes are dissolved in water, either they form a separate phase above a certain concentration or, if they are amphiphilic, they form micelles. A volume increase is observed in both cases. This suggests that hydrophobic solutes occupy interstitial positions in the water structure, requiring little extra volume.37 If this is also the case for the nucleobases, it is difficult to explain the negative volume changes of stacking of the nucleobases. Kasarada30 suggested that AV, still can be negative for very small aggregates. Gaarz and L ~ d e m a n , on ~ ' the other hand, have argued that this comparison of the association into stacks with the formation of a separate phase is questionable. The process to be considered as a parallel to stacking is the approach of two monomeric hydrophobic solutes from infinity to close contact. Such a process must involve removal of only part of the hydration sheath of both molecules. Such a process may well be connected with a volume decrease as seen for the nucleobases. It has been observed that the excess volumes of many hydrophobic solutes in water do in fact decrease with concentration up to a minimum value, the minimum occurring a t mole fractions between 0.1 and 0.2.39*40
Registry No. Purine, 120-73-0; caffeine, 58-08-2; cytidine, 65-46-3; thymidine, 50-89-5; uridine, 58-96-8. (39) Hvidt, A, J. Theor. Biol. 1975, 50, 245. (40) Franks, F. In "Water: A Comprehensive Treatise"; Franks, F., Ed.; Plenum Press: New York, 1975; Vol. 4. (41) H~iland,H.; Vikingstad, E. J . Chem. SOC.,Faraday Trans. 1 1976, 72, 1441. (42) H~iland,H.; Ringseth, J. A.; Brun, T. S.J . Solution Chern. 1979, 11. 719.
Cation Charge Effect on the Rate of Complexation of Crown Ethers: Ba(CIO,), In DMF
+ 18C6
William Wallace, Edward M. Eyring, and S. Petrucci" Department of Chemistry, Polytechnic Institute of New York, Brooklyn, New York 11201, and Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: June 1 1 , 1984; In Final Form: August 6, 1984)
Ultrasonic absorption spectra in the frequency range 1-500 MHz for barium perchlorate-18-crown-6 solutions in dimethylformamide in the concentration range 0.1-0.5 M, at various temperatures, are reported. The ultrasonic spectra are described by two Debye relaxation processes. Independence of the relaxation frequencies on concentration and the linearity of the excess maximum sound absorption coefficients per wavelength with concentration lead to interpretation of the data in terms of the Eigen-Winkler mechanism k
k
k-2
k-3
Me2+.-C 1, MeC2+
(MeC)2+
where the species Me2+-.C, MeC2+,and (MeC)2+symbolize three different metal ion-crown ether complexes. The forward rate constants k2 and k3 are larger for Ba2+than for K+ in DMF despite the fact that the two ions have similar ionic radii. Differences are tentatively rationalized in terms of an ion-dipole charge-enhanced potential affecting the rate constants. The situation is different in water where the reported complexation rate constant is larger for K+ than for Ba2+ and the rate-determining process of the complexation appears to be removal of water from the first coordination sphere of the cations. The same Eigen-Winkler mechanism applied to the removal of water dipoles from the first coordination sphere of the ions seems to account for the findings in water.
Introduction The kinetics of complexation of alkali metal ions with the crown ether 18C6 in the solvent dimethylformamide (DMF) have been *Department of Chemistry, Polytechnic Institute of New York.
recently studied'J by ultrasonic relaxation techniques with the results corroborated by ancillary Raman spectra' and backed by (1) K. J. Maynard, D. E. Irish, E. M. Eyring, and S . Petrucci, J . Phys. Chem., 88, 729 (1984).
0022-3654/84/2088-6353$01.50/00 1984 American Chemical Society
