J. Phys. Chem. 1984,88, 5709-5713
5709
Electric Dichroism Evidence for Stereospecific Binding of Optically Active Tris Chelated Complexes to DNA Akihiko Yamagishi Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: October 14, 1983; In Final Form: June 14, 1984)
Electric dichroism measurements have been performed on an aqueous solution containing calf thymus DNA and optically active metal complexes such as Ru(bpy):+, Ru(phen)32+,and Fe(phen):+ (bpy = 2,2'-bipyridyl and phen = 1,lo-phenanthroline). Comparing the electric dichroism spectra in the visible region between the enantiomers of the complexes, we coqcluded that a A-isomer is bound to DNA with one of its ligands intercalated between the base pairs of DNA. Contrary to this, no intercalation occurred in the binding of the A-isomers. A A-isomer is likely to be attracted by DNA mainly due to electrostatic forces.
chelates spectrophotometrically, using the reported extinction coefficients'0 ~ ~ ~ ~ ( R u ( b p y= ) ?2.1 + ) X lo4, ~ , ~ ~ ( R u ( p h e n ) ,=~ + ) 2.2 X lo4, and csm(Fe(phen),2+) = 1.0 X lo4. The optical purities of the above resolved compounds were estimated to be better than 95% on the basis of the measured optical rotations.8-10 Calf thymus D N A was purchased from Worthington Biochemical Corp. and used without further purification. A stock solution was prepared by dissolving 3.0 mg of DNA in 5 mL of water containing 10 mM hexamethylenetetramine as a buffer agent (pH 6.5 at 20 "C). This amount of added salt was sufficient to prevent denaturation according to the literature.6c The concentration of DNA was measured on the basis that the molecular weight per base pair was 700.& Poly(styrenesu1fonicacid) (HPSS) was synthesized by sulfonating poly(styrene) with the molecular weight of 3 X lo5. HPSS was converted to the potassium salt (KPSS) by neutralizing with KOH. For the measurement on a solution of KPSS, no buffer was added so that the pH of the solution was varied in the range of 5.5-6.5. Other materials used were of reagent grade and used without further purification. Instruments. The visible absorption spectra were recorded on a EPS-3T spectrophotometer at 20 OC. The electric dichroism Experimental Section was measured with a Teflon cell having quartz windpws and two Materials. Ru(bpy),C12 and Ru(phen),C12 were prepared by gold electrodes separated by 0.4 cm. The incident monochromatic reacting RuC1,.nH20 with bpy.H20 and phen.H20, respectively. light was polarized with a linear polarizer (Nihon Kogaku (Japan)) The solid mixture of RuC1, and 10-times excess of ligand was at the angle of either 0 or 90" with respect to the electric field heated a t 150-170 "C for 10 h. Racemic Ru(bpy),C12 was redirection. An electric field pulse of 7.5 kV cm-l was applied for solved by adding KI and potassium antimonyl tartrate (KSbO1 ms on a sample solution with an electric field pulse generator Tart) to its aqueous solution.s The perchlorate salt of A-Ru(Denkenseiki (Japan)). Measuring the dependence of the electric (bpy)j2+ was obtained by adding NaC104 to an aqueous solution, dichroism amplitude on field intensity, we also applied a pulse dissolving the above precipitate (A-R~(bpy),.(SbOTart)~*A-Ru- with maximum height of 34 or 62 kV cm-' by discharging the (bpy),12.18H20). The perchlorate salt of A-Ru(bpy)32+was electric energy from a coaxial cable of 0.02 pF. The pulse decayed obtained by adding NaC104 to the above filtrate. The resolution with a half-life time of about 100 ps. The dichroism signals were of racemic Ru(phen),C12 was performed by adding KSbOTart stored in a transient memory (Kawasaki Elec. Co. (Japan)) and to its aqueous solution. The perchlorate salts of the enantiomers displayed on either an oscilloscope or a recorder. were obtained as in the case of R~(bpy),.(ClO~)~. The perchlorate Results salts of the enantiomers of Fe(phen)32+were obtained by the established m e t h ~ d . ~We determined concentrations of these Curve a in Figure 1 shows the electronic spectrum of A-Ru(bpy)32+in an aqueous solution. When DNA was added, the spectrum varied as shown by curves b and c, keeping the isosbestic (1) Dwyer, F. P.;Gyarfas, E. C.; Koch, J.; Rogers, W. P. Nature (London) point at 465 nm. In the inset of Figure 1, the decrease of ab1952, 170, 190. sorbance at 450 nm, -AA450, was plotted against the ratio of base (2) Kindani, Y.; Noji, M.; Tashiro, T. G a m 1980, 71, 637. pair to metal complex, P/M. -AA450 increased almost linearly (3) Kuramochi, H.; Takahashi, K.; Takita, T.; Umezawa, H. J. Antibiot. 1981, 34, 994. with P/M below P/M = 2 until it leveled off above P/M = 2.5. (4)Srivastava, R. C.; Froehlich, J.; Eichhorn, G. L. Biochimie 1978, 60, There was no difference observed in the electronic spectra between A79 the A- and A-enantiomers of Ru(bpy)32+. On the basis of the (5) Barton, J. K.; Dannenberg, J. J.; Raphael, A. L. J. Am. Chem. SOC. above results, it was concluded that both of the enantiomers of 1982, 104, 4967.
Introduction A difference in biological activities between the enantiomers of an optically active metal complex has been noted in several examples such as toxicity and drug efficiency.'g2 In order to understand such effects at the level of the molecular interactions, it is of vital importance to study the chirality effects on the binding structures of a metal complex. For that purpose, various kinds of spectroscopic methods have been applied so far such as circular dichroism and magnetic resonance Few reports, however, succeeded in obtaining direct evidence for the stereospecific binding of a metal complex in biological systems. The present paper describes the initial electric dichroism studies on the binding of an optically active metal chelate to DNA. The work was motivated by the recent report by Barton et al. that there existed a remarkable difference in the binding strength to DNA between the enantiomers of tris( 1,lO-phenanthroline)zinc(II) (Zn(~hen)?+).~We intended to obtain direct structural evidence for such stereospecificity by measuring the electric dichroism spectra of a DNA-metal chelate
(6) (a) Ding, D.; Rill, R.; Van Holde, K. E. Biopolymers 1972, 11, 2109. (b) Hogan, M.; Dattagupta, N.; Crothers, D. M. Proc. Natl. Acad. Sci. U.S.A. 1978,75, 195. (c) Stellwagen, N. C. Biopolymers 1981, 20, 399. (7) The same method is already applied to other kinds of biopolymers (Yamagishi, A. Biopolymers 1981, 20, 201). (8) Dwyer, F.P.; Gyarfas, E. C. J. Proc. R . SOC.N.S.W. 1949, 83, 170. (9) Dwyer, F. P.;Gyarfas, E. C. J. Proc. R. SOC.N.S.W. 1949, 83, 832.
(10)McCaffery, A.J.; Mason, S. F.; Norman, B. J. J. Chem. SOC.A 1969, 1428. Ill) Yamagishi, A. J. Phys. Chem. 1981, 85, 2129. (12) Dourlent, M.; Hogrel, J. F.; Helene, C. J. Am. Chem. SOC.1974, 96, 3398.
0022-3654/84/2088-5709%01.50/0 0 1984 American Chemical Society
Yamagishi
5710 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 1
0
I
'O0
1
I
Wave Ieng t h/n
500
Figure 1. The electronic spectra of an aqueous solution of A-Ru(bp~),~+ M. The"ratio of the base pairs of and DNA; [A-Ru(bpy)?+].= DNA to a Ru(I1) chelate IS 0 (a, -), 1.5 (b, -*-), and 2.5 (c, ---). Inset is the plot of the decrease of the absorbance at 450 nm (-A&,,) against the above ratio. A and 0 denote the results for Aand A-isomers of Ru(bpy),Z+,respectively. (b\
-Time
i
m 2aJ
1
Figure 2. The electronic spectra of an aqueous solution of (a) A-Ru(phen)?' or (b) A-Fe(~hen),~+ and DNA. The ratios of the base pairs of DNA to a metal(I1) chelate are 0 (-) and 2.5 (---).
Ru(bpy)$+ were bound to DNA almost quantitatively, occupying about 2.5 base pairs per chelate. Similar behaviors were observed for the electronic spectra of the DNA-Ru(phen)?+ and DNA-Fe(phen)$+ systems as shown by parts a and b of Figure 2, respectively. Thus, these chelates were also bound to DNA as Ru(bpy)$+ did. A square pulse of an electric field with the intensity of 7.5 kV cm-l was applied on a solution of A-Ru(bpy)$+ and DNA at P/M = 3. Parts a and b of Figure 3 exhibit the signals at 425 nm when the monitoring light was polarized at 0 and 90°, respectively. Transient change in transmittance occurred within 50 f i s after the onset of the electric field. After attaining the stationary value, the transmittance decayed unexponentially in the absence of the electric field. The stationary amplitudes of the signal obeyed well the following expression for the orientational dichroism1° A A / A = ( p / 6 ) ( 1 3 COS 28) (1)
+
in which 0 is the angle between the electric field and the polarization of the incident light; i.e. 0 = 0 and 90' for parts a and b of Figure 3, respectively. AAIA in eq 1 denotes the relative absorbance change and p the reduced linear dichroism defined by (2) P = (€1, - el)/€ in which e, ell, and t L are an isotropic extinction coefficient, an extinction coefficient for parallel polarized light, and an extinction coefficient for vertically polarized light, respectively. The observed dichroism was, therefore, concluded to be caused by the alignment of a bound Ru(bpy)32+ion which had followed the orientational motion of DNA. Under the electric field, double-stranded DNA is known to orientate its helical axis in the direction of the electric field.6 Curves a and b in Figure ? show the wavelength dependences of the dichroism amplitudes at 0 = Oo, AA, for the solutions of DNA-A-Ru(bpy)32+ and DNA-A-Ru(bpy)32+, respectively. Apparently the enantiomers of Ru(bpy)$+ gave different spectra in the electric dichroism, depending on their absolute configu-
-Ti me Figure 3. The transient change of transmittance when a square pulse of an electric field with intensity 7.5 kV cm-I and duration 1 ms was imposed on a solution of DNA and A-Ru(bpy)?+. The monitoring light was polarized at (a) 0" or (b) 90' with respect to an electric field.
*1
1/71 b
4
l
l i
t w
0
0
0
0
Wavelengt h/n rn
Figure 4. The dependence of the dichroism amplitude, AA, on the wavelength when the monitoring light was polarized parallel with an M Aelectric field (0 = O O ) . The solution contained DNA and Ru(bpy)?+ (A) or A-Ru(bpy)32+(0);[DNA]/[Ru(II)] = 3.
rations. In Figures 5 and 6, the similar comparison of AA at 0 = Oo was made for the systems of DNA-Ru(phen)?+ and DNA-Fe(phen)$+. These enantiomers also gave the different dichroism spectra, depending on their absolute configurations. As for the latter system, the solutions of the A- and A-enantiomers turned out to exhibit the same dichroism spectra at about 5 h after preparing them (curve c in Figure 6 ) . The results indicated that the enantiomers of the bound Fe(phen)?+ ions racemized to result in the same bound states of the racemic mixtures. Table I lists the results of Figures 4, 5, and 6 in terms of the values of the reduced linear dichroisms, p, at the electric field strengths of E = 7.5,34,and 62 kV cm-l. The wavelengths were chosen the peak positions in the dichroism spectra of eitwr the A- or A-enantiomers. p at the infinite electric field strength, po, was obtained by extrapolating p against the inverse of the electric field strength ( 1 / E ) to 1 / E = 0.6c As a comparison, the electric dichroism was measured for a solution containing an optically active metal complex and po-
The Journal of Physical Chemistry, Vol. 88, No. 23, 1984 5711
Optically Active Metal Complexes
TABLE I: The Reduced Linear Dichroism Values (p) for the Enantiomers of Ru(bpy),zt, Ru(phen),l+, and Fe(phen)32+Bound to DNA
P*O a
Plea 'p? 'p3
A
A
A
h=425
h=460
h=425
h=460
h=425
h=535
A=480
nme
nrnf
nme
nrnf
nme
nrnf
nme
nrnf
nme
nrnf
.me
nmf
-0.27 -0.41 -0.43 -0.45
0.0
+0.06
+0.19 +0.28
-0.36 -0.55
+0.10 +0.10
+0.30
0.0 0.0 0.0
t0.36
+0.9
-0.36 -0.55 -0.57 -0.60
-0.15
-0.01 -0.01
-0.22 -0.24 -0.25
+0.58
bides
91 p
A
A
h = 4 7 5 h = 4 2 5 h=475 p a t 7.5 kV cm-' p a t 34 kVcm-' p a t 62 kVcm-'
F e(p hen) ,z+
Ru(phen),zt
Ru(bpy),'+ A
62 55 48
eg /deg
-0.01
48 41 66
+0.32
0.0
+0.28
+OS8
-0.01
+0.44
-0.01
+0.46 +0.47
+0.90 +0.94
-0.60
-0.01
64 55 46
-0.58
52
+0.97
64
0 .o
h=480
+OS5
+0.60 59 46 64
55
50 88
h=535
46
extrapolated to the infinite field strength. The extrapolation was made by plotting p against the inverse of electric field strength. The angle between the transition The angle between the transition moment w1 and the helical axis of DNA obtained according to eq 4. The angle of a C, axis with respect to the helical axis of DNA. e Peak moment p z and the helical axis of DNA obtained according to eq 4. 11. f Peak I. ap
I
350
-+
-3
I
400
450 Wavelength/nrn
500
550
Figure 7. The wavelength dependence of AA at B = Oo for the KPSSR ~ ( b p y ) , ~system. + The solution contained KPSS and M A-Ru( b p ~ ) , ~(A) + or A-R~(bpy),~+ (0) [KPSS]/[Ru(II)] = 12.
400 Wave1eng t h/n rn
500
Figure 5. The wavelength dependence of AA at B = 0' for the DNAR ~ ( p h e n ) , ~system. + The solution contained DNA and 10" M A-Ruhen),^' (A) or A-Ru(phen)," (0);[DNA]/[Ru(II)] = 3.
Figure 8. The wavelength dependence of AA at B = Oo for the KPSSF e ( ~ h e n ) , ~system. + The solution contained KPSS and A-Fe(phen),2+ (A)or A-Fe(~hen),~+ (0); [KPSS]/[Fe(II)] = 4.
TABLE II: The Reduced Linear Dichroism Values ( p ) for Ru(bpy)q2+,Ru(phen)?+, and Fe(phen)**+Bound to KPSS R~(bpy),~'e Ru(phen),*' Fe(phen)32+d p at 7.5 kV cm-I +0.54 +0.55 +0.45 p at 34 kV cm-I +0.61 +0.61 +0.56 +0.64 +0.58 p at 62 kV cm-' +0.63 PO
'pab
4 0
03 Wavelength % n
45
Figure 6. The wavelength dependence of AA at -9 = Oo for the DNAFe(phen),(CIO.& system. The solution contained 1.6 X M A-Fehen),^+ (A) 6 A-Fe(phen)3zt (0) or racemic F e ( ~ h e n ) , ~ (+0 ) ; [DNA]/[Fe(II)] = 3.
tassium poly(styrenesu1fonate) (KPSS). KPSS bound one metal complex of the present type per two styrenesulfonate residues as deduced from the spectral measurements similar to Figures 1 and 2. When a n electric field was imposed on a solution, the electric
+0.65
+0.66
78
78
+0.60 15
a p extrapolated to the infinite strength. The extrapolation is made by plotting p against the inverse of electric field strength. bThe angle of a C, axis with respect to the helical axis of DNA obtained according to eq 3. ' A = 450 nm. d A = 500 nm.
dichroism appered in the wavelength region of 400-600 nm. In this case, the dichroism rose and decayed within 100 ps, indicating t h a t the persistent length of a PSS chain was shorter than that of DNA. Figures 7 and 8 show the wavelength dependences of AA at 6 = Oo for the KPSS-Ru(bpy)t+ and KPSS-Fe(phen)?+ solutions, respectively. Both of the A- and A-enantiomers gave
5712 The Journal of Physical Chemistry, Vol. 88, No. 23, 1984
Yamagishi
7
Y
fr
i Figure 9. The electronic spectra of an aqueous solution of Cu(phen)zC1 and DNA under an N, atmosphere. The solution contained 5 X 10" M Cu(phen)*+ and DNA; [DNA]/[Cu(I)] = 0 (a), 1 (b), and 2 (c). identical dichroism spectra. Their shapes were almost identical with the absorption spectra of bound chelates at zero electric field (e.g. Figure 2a,b), implying that p in eq 1 was independent of the wavelength. The p values for these chelates at 7.5, 34, and 62 kV cm-I are also listed in Table 11. p o in the same table is the value when p is extrapolated to the infinite intensity of the electric field. The additional electric dichroism measurements were performed on a solution of DNA and C ~ ( p h e n ) ~ +C.~ ( p h e n ) ~was + known to cleave DNA in the presence of O2I3 Thus, if electric dichroism was observed for the solution, it might provide evidence for the binding structures of Cu(phen),+ in the course of its attacking DNA chains. Curve a in Figure 9 shows the electronic spectrum of C ~ ( p h e n ) ~under + an N2 atmosphere. When DNA was added, the spectrum changed to curves b and c. The results confirmed the binding of Cu(phen)2+ to DNA. When air was introduced to the system, the solution became colorless, implying that the D N A - c ~ ( p h e n ) ~complex + decomposed. An electric field pulse of 7.5 kV cm-I was imposed on a solution of spectrum c. The transient decrease of the transmittance was observed at 8 = 0' as already observed in Figure 3. The amplitude of the signal was well coincident to the static spectrum of a bound Cu(phen),+. It was, however, found that the signal was isotropic because it did not depend on the polarization angle, 8. In other words, the change was not due to the orientational motion of a D N A - c ~ ( p h e n ) ~ ~ + but to some chemical or physical change caused by the electric field. At present, no further experiments were performed on this system.
Discussion The A- and A-enantiomers of the present tris chelated complexes gave the different electric dichroism spectra when they were bound to DNA. This was direct evidence that these chelates interacted with DNA in a stereospecific manner. The electronic spectra of the present chelates exhibited two peaks in the visible region: one in the shorter wavelength (denoted by peak I) and the other in the longer wavelength (denoted by peak 11). In order to interpret the electric dichroism results, it is necessary to clarify the nature of the transitions for peaks I and 11. According to McCaffery et a1.I0 peaks I and I1 are assigned to the charge-transfer trapsitions from the d orbital of a central metal ion to the r*-orbitals in the ligands. Felix et al. conclude that the excited states for peaks I and I1 are antisymmetric ($) and symmetric (X) with respect to the C2 ~ y m m e t r y . ' ~On the basis of these assignments, the transition moments for peaks I and 11, jii and &, respectively, are concluded to orient in the directions of the Y and X axes in Figure 10, respectively. It is apparent that these X and Y axes are chosen along the three equivalent directions on an X-Y plane for a free complex with C3 symmetry. The Structures of the Chelates Bound to PSS. In the PSS systems, po for peaks I and 11, pio and p2', respectively, take the (13) Que, B. G.; Dowvey, K.M.; So, A. G. Biochemistry 1980,19, 5987. (14) Felix, F.; Ferguson, J.; Gudel, H. U.; Ludi, A. Chem. P h p . Lett. 1979, 62, 153.
Figure 10. A tris chelate complex viewed in the direction parallel with its C, axis. It is shown that the transition moments for peaks I and 11,
PI and PZ, respectively, lie in the directions of Y and X,respectively. same values as given by in Table 11. We regard these results as an indication that a chelate retains its C3 symmetry on a PSS chain. Under these situations, the orientations of jil and jiz are not determined uniquely, but they are distributed on an X-Y plane almost in a uniform way. As a result, pio and pZo are both expressed by the following equation' p i o = pzo = y8(1 - 3 cos 2p3)
(3)
in which cp3 is the angle between the polymer axis of PSS and the C3axis of a chelate. The p3 values thus calculated are given in Table 11. The results show that p3is close to 90' or the chelates are bound to PSS with their C3axes perpendicular to the orientating polymer chain. For the PSS systems, the chelates might be attracted mainly due to the electrostatic force between a central metal ion in a chelate and the negatively charged styrenesulfonate residues of PSS. Under the conformation of p3= go', it is seen that the metal ion is located as closely as possible to the styrenesulfonate residues. The Structures of the A-Isomers Bound to DNA. For the DNA systems, the A-isomers take very small values of pl' and large negative values of pzo (Table I). We propose that a bound Aisomer undergoes some structural distortion to lose its C3 symmetry. Most probably, a A-isomer is bound to DNA with one of its three ligands intercalated between the base pairs. If the bond between a metal ion and an intercalated ligand is stretched, the chelate loses the C3symmetry but possesses the Cz symmetry around the bond. Based o ~the i above intercalation model, the angle between (or ji,) and the helical axis of DNA, pi (or cpz), is calculated from p i o (or p z o ) according to the following equation.I2 pIo
= Y4(1
+ 3 cos 2p,)
(4)
cpl and cp2 thus obtained are tabulated in Table I. As for the orientation of ji2, cp2 takes an angle of about 63' with respect to the helical axis. The obtained cpz values are, however, regarded as the lower limits of the true v a l ~ e s . ' ~It is noted that if one of the ligands of a bound chelate is intercalated between the neighboring base pairs of DNA, the short axis of the ligand, which coincides with the direction of ji2, is vertical to the helical axis of DNA. In this sense, the present results of pz> 63' assist the above model of the intercalation of the A-isomer. The values for pi are in the range of about 55'. cpl would not be very much different from this value, even if one takes into account the errors in extrapolating the observed values to the infinite field. When a bound chelate takes these values of pl,the long axis of the intercalated ligand declines by about 20' from (15) We only utilized three experimental values of p at the field strength of 7.5, 34, and 62 kV cm-I. Usually a more detailed dependence of p on the field strength is necessary to obtain pccurate values of po (cf. ref 6 ) . Another ambiguity in assuming po as the value at the complete orientation of DNA lies in the possibility that the present sample of DNA is partially entangled within an individual chain. As a result, a part of a bound chelate does not orientate even at the infinite strength of electric field. This may result in lowering an observed dichroism amplitude per chelate or specifically in making the values of q2 smaller than the real values.
J. Phys. Chem. 1984, 88, 5713-5720
Figure 11. The possible orientations of the (a) A- and (b) A-isomers
bound to DNA. the horizontal level. Since the intercalated ligands are closely faced with the base pairs, the above conclusions imply that molecular planes of the base pairs themselves also decline by this magnitude of the angle.16 (16) The previous electric dichroism results also suggested that the molecular planes of the base pairs of DNA incline relative to the helical axis (see ref 6b).
5713
Figure 11 shows the possible binding structure of a A-isomer to DNA. It is notable that the two remaining ligands which are not intercalated between the base pairs are located in the same direction as the helical groove of DNA. Under this conformation, no steric hindrance exists between the chelate and DNA. These situations would be a main factor in realizing the intercalation of the A - i ~ o m e r . ~ The Structures of the A-Isomers Bound to DNA. In contrast to the A-isomers, there are no distinct negative peaks observed at peaks I1 for A-Ru(bpy)?+ and A - R ~ ( p h e n ) ~ ~On + .the basis of the foregoing discussion, the results mean that no intercalation occurs for the A-isomers. Supposing that a A-isomer has one of its three ligands intercalated between the base pairs, the remaining two ligands lie in a direction opposite to the helical groove of DNA. This might result in the high instability of the bound chelate due to the interference of the ligands with the main chain of DNA. For the A-isomers, the orientation angles of cpl and cpz for ,?il and ,?iz, respectively, are obtained from p I o and pzo according to eq 4. The orientation angles of a C3 axis are also calculated as given by cp3 in Table I. cp3 values are found to be close to 90’. Most probably the A-isomers are bound to DNA due to the electrostatic forces between the central metal ions and the phosphate groups of DNA as similarly concluded for the PSS systems,
Acknowledgment. Thanks are due to Prof. Helge Johansen (The Technical University of Denmark, Denmark) for the discussion in interpreting the results. Registry No. A-Ru(bpy),2t, 24162-12-7; A-Ru(bpy)?+, 52389-25-0; A-Ru(phen)?’, 19368-51-5; A-Ru(phen)?+,24162-09-2; A-Fe(phen)3z+, 24324-09-2; A-Fe(phen)3zt,47836-89-5.
Large Micelles in Concentrated Solutions. The Second Critical Micellar Concentration G. Porte,*+ Y. Poggi,* J. Appel1,t and G. MaretS Centre de Dynamique des Phases CondensEes, associZ au C.N.R.S.LA233, US.T.L., F-34060 Montpellier Cedex, France; Laboratoire d’Electrostatique, C.N.R.S.,F-38042 Grenoble Cedex, France; and Hochfeld-Magnetlabor des Max-Planck-Instituts fur Festkorperforschung, I66X, F-38042 Grenoble Cedex, France (Received: February 2, 1984; In Final Form: May 14, 1984)
Measurements of magnetic birefringence and viscosity are reported for concentrated aqueous micellar solutions of a cationic surfactant. They clearly show two separate concentration ranges with different mean size and shape of the micelles, supporting the idea of a “second critical micellar concentration” for the globule-to-rod transition in concentrated solutions. A phenomenological mean-field description of the micellar growth in “nonideal” solutions is presented. It reveals that a well-defined second critical micellar concentration cannot be accounted for by considering only deviations from ideality due to intermicellar interactions but appears by including some energy barrier involved in the early steps of the elongation process and related to intramicellar specific features.
Introduction The elongation of the micelles of ionic surfactants in aqueous solution is now quite well understood in the special case where the micelles in solution can be considered as noninteracting, that is to say, when the micellar elongation is induced by large addition of well-chosen salts (>0.1 M) in a dilute (few 10-3 M) aqueous solution of surfactant. rn such “ideal so~utions~, the equations accounting for the between micelles of different aggregation numbers can be easily written down, provided that the +Centrede Dynamique des Phases Condenshs. t Laboratoire d’Electrostatique. (IHochfeld-Magnetlabor des
Max-Planck-Institut fur Festkorperforschung.
0022-365418412088-5713$01.50/0
asymptotic form of the standard chemical potential I . L O ~of the monomer embedded in a N-micelle is given as a function of N. This form (1) .u0jv .. = u. O , + k T a / N ., (where a is the excess free energy in the ends of large rodlike micelles in units of kT; a does not depend on N) is determined’ by the unidimensional structure of the large micelles. The predictions of this simple description have been quite well verified by the systematic experimental investigations of several groupsF4 (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
0 1984 American Chemical Society
