J. Phys. Chem. 1994,98, 5943-5952
5943
Thermodynamics of Interactions between Ethidium Bromide and Poly (A)-Poly (U) Mixtures in Dilute and Concentrated Solutions Akihiro Kagemoto,' Atsushi Yoshii, Satoshi Kimura, and Yoshihiro Baba Laboratory of Chemistry, Department of General Education, Osaka Institute of Technology, Asahi- ku, Osaka 535, Japan Received: February 14, 1994"
In order to obtain information about the thermodynamic interactions for the poly(A).poly(U) duplex (duplexI)-ethidium bromide (EB) system in dilute solution, the heats of mixing, AmixHl,and UV spectra for the duplex-I-EB system were measured. From UV spectral measurements, the binding constant, K1, and the number of duplex-I base pair occluded by EB, nl, were estimated to be about 2.0 X lo5 dm3 mol-' and 2.5, respectively. The change in net enthalpy, M I ' , obtained using AmixHl,K1, and nl values was estimated to be about -28 kJ (mol of EB)-l. Using AH1' and K1 values, the changes in free energy, AGIO, and entropy, ASl, based on an interaction of EB with duplex-I were estimated from AGIO = -RT In K Iand A S 1 = ( M I ' AGlO)/T, respectively. AGIO and A S 1 were estimated to be-30 kJ (mol of EB)-' and 7.0 J K-l (mol of EB)-l, respectively, demonstrating that the duplex-I-EB system forms thermodynamically the stable complex by contribution of AH(. To elucidate the thermal stability of the duplex-I-EB complex, the duplex-I-EB complex was studied from the dissociation process using an adiabatic differential scanning calorimeter. The change in net enthalpy, AHp-lO, based on dissociation for the duplex-I-EB complex was estimated to be about 30 kJ (mol of EB)-'. It was suggested that the reverse sign of the AHp-1' value was in fairly good agreement with AHl' (-28 kJ) estimated from AmixHl. However, since most such studies have been done in dilute solutions, a more realistic understanding can be reached by inserting poly(A)-poly(U) mixture intovesicles of phosphatidylcholine (PC) as a mainly constitutive component of the cell membrane. The poly(A)-poly(U)-PC mixtures (abbreviated as duplex-11) containing EB in the concentrated solutions were prepared by mixing 10.0 wt % ' poly(A)-poly(U) and 5 wt 7% PC. It was found that duplex-I1 showed the phase states with the spherical and layer liquid crystals in the concentrated solutions. In order to analyze exactly duplex-11, duplex-I1 was dissolved by Ringer solution. The layer liquid crystal surrounding the spherical liquid crystal was dissolved and transformed into an isotropic phase, but not the spherical liquid crystal. Duplex-I1 was divided into two components such as the supernatant liquid (isotropic phase) and the condensed duplex-I1 (spherical liquid crystal) using a centrifuge. DSC measurements for the supernatant liquid were carried out under the same experimental conditions as duplex-I in dilute solution. The results were similar to those of duplex-I in dilute solutions. The difference between the change in enthalpy of duplex-I1 and that of the supernatant liquid gives the change in enthalpy, AHw2, based on dissociation of condensed duplex-11-EB complex. The change in net enthalpy, A H p - 2 O , converted per mole of EB based on dissociation for the condensed duplex-11-EB complex was estimated to be about 4.0 kJ (mol of EB)-l. AHw2O for condensed duplex-11-EB system does not agree with AHD-~"for the duplex-I-EB system in dilute solution. On the other hand, as one approach to elucidate the conformation of a poly(A)-poly(U) mixture in the condensed duplex-11, an interaction between the poly(A).2poly(U) triplex and EB in dilute solutions was also studied by means of microcalorimetry and spectrophotometry. From UV spectral measurement, the binding constant, K3, for the poly(A).2poly(U) triplex and a number of base triplets, n3, occluded by EB were estimated to be about 9.7 X lo4 dm3 mol-' and 3.5, respectively. Using these values, the change in net enthalpy, AH,', based on complex formation was determined to be about -3.0 kJ (mol of EB)-l, suggesting that the absolute value of A H 3 ' was in fairly good agreement with that of A H p - 2 ' (4.0 kJ) for the condensed duplex-11-EB systems. In order to confirm the molecular conformation in the condensed duplex-11, absorbances at the poly(A)-poly(U) mixture in the concentrated solutions were measured a t wavelengths of 260 and 280 nm. The poly(A)-poly(U) mixtures with the concentration above 7.0 wt 7% are possible of forming a poly(A)-2poly(U) triplex. That is, the spherical liquid crystal enclosed by phosphatidylcholine may be made up by poly(A).Spoly(U) with a triple-stranded helical structure in concentrated solutions. It is worth noting that an equimolar poly(A)-poly(U) mixture in concentrated solutions forms poly(A).2poly(U) triplex, and the EB molecule interacts with an internal poly(A).2poly(U) triplex (condensed duplex-11) inserted into a vesicle of phosphatidylcholine as the mainly constitutive component of the cell membrane.
1. Introduction
One of the most interesting problems in biopolymer areas is the intermolecular interaction between a nucleic acid and small molecule. In particular, the relationships between intercalation and inhibition of the transcription or replication of DNA are an important problem for understanding gene expression. The ~~~
~
~
~
~~~~~~~
* To whom correspondence should be addressed. @
Abstract published in Aduance ACS Abstracts, April 15, 1994.
0022-365419412098-5943$04.50/0
common intercalators are characterized by heterocyclicaromatic chromophores and included dyes as mutagenic substances.1-5 It is well-known that an interaction between DNA and dye brings about so-called intercalation in which a flat aromatic dye molecule is inserted into adjacent base pairs of DNA.6 Many studies concerning the intercalation of DNA with dye have been carried out by means of various methods.'-11 In our previous papers,12J3the change in enthalpy based on the 0 1994 American Chemical Society
5944 The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
intercalation of DNA with dye and thermal properties of the DNA-dye complexes was studied by means of microcalorimetry and spectrophotometry. From the results obtained, we reported that the helical structure of DNA and its physical properties were considerably influenced by intercalating dye, demonstrating that an intercalation seems to cause the mutation or carcinogen. Recently, in order to obtain information about an influence of a base specificity of DNA on dyes, the interaction modes between DNAs containing 26.5,42.0, and/or 72.0% of guanine-cytosine base pairs and ethidium bromide (EB) as the dye were studied by means of differential scanning calorimetry. It was reported that the change in enthalpy based on the interaction between the d(ApA) sequence of DNA and EB was slightly larger than that of the d(GpG) one.l4 However, since most such studies have been done in dilute solutions, it seems to be very difficult to exactly understand the physiological function in living cell from the behavior of DNA in dilute solution. For a more realistic understanding, DNA in concentrated solutions should be studied, because DNA in vivo exists as solutionsof 70~01%concentration.15 From thestandpoint mentioned above, the behaviors of DNA in concentrated solutions were previously studied by means of spectrophotometry and microcalorimetry.16 It resulted that DNA in concentrated solutions formed a cholesteric liquid crystal with a double-stranded helical structure having a right-handed helical sense as pointed out by E. Iizuka et al.,*7 and the change in enthalpy based on formation of the cholesteric liquid crystal was estimated to be about -3.0 kJ using a microcalorimeter. The interactions between the cholesteric liquid crystal of DNA and small molecules were also studied using the same method.lS However, there have been no studies on the behaviors of ribonucleic acid (RNA) in dilute and concentrated solutions. In this paper, in order to obtain further information about an interaction of R N A with dye, a binding mode between R N A and EB in dilute and/or concentrated solutions was studied by means of microcalorimetry and spectrophotometry. Since the R N A molecule has a complicated structure, one approach is to use a simple model such as the poly(A).poly(U) duplex having a double-stranded helical structure similar to that of DNA. Thus, an interaction mode between EB and poly(A).poly(U) duplex was studied in dilutesolutions. Furthermore, for a more realistic approach, the interaction modes between EB and the condensed poly(A)-poly(U) mixture inserted into vesicle of phosphatidylcholine as a major component of the cell membrane were also investigated by means of microcalorimetry. We will discuss the binding modes of EB for R N A as a ground of the changes in enthalpy accompanying the interactions between EB and poly(A)-poly(U) duplex in dilute solution and also between the condensed poly(A)-poly(U) mixture and EB. 2. Experimental Section Materials. The poly( riboaden ylic acid).poly(ribouridylic acid) duplex [poly(A).poly(U) duplex] and poly(A) and poly(U) samples were purchased from Yamasa Shoyu Co. Ltd., Japan. To remove the contaminants such as protein containing the poly(A)-poly(U) duplex and the poly(A) and poly(U) samples, these samples were purified according to a usual method19 such as phenokhloroform extraction. The absorbances at 260 nm, &0, and at 280 nm, &0, for the purified poly(A).poly(U) duplex were measured at room temperature. From the results, the ratio of A260 to ,4280 is about 2.2, demonstrating that this value is reasonable in comparison with that of the purified RNA.I9 The characterizations of the poly(A).poly(U) duplex were in good agreement with thoseof the poly(A)-poly(U) duplex formed by an equimolar mixture of poly(A) and poly(U). CD spectra of the poly(A).poly(U) duplex were in fairly good agreement with those of duplex prepared by an equimolar poly(A)-poly(U) mixture as shown in Figure 1.
Kagemoto et al. L
I
5
5.0
-0
*\
2
& -5.0 cb Y
h/nm Figure 1. CD spectra of poly(A) (-), poly(U) (---), an equimolar mixture of poly(A) and poly(U) (- -), and poly(A).poly(U) duplex
-
(-1.
Ethidium bromide (EB) as fluorochrome and phosphatidylcholine extracted from egg yolk as the major constitutive components of the cell membrane used in this study were purchased from Sigma Chemical Co. Ltd. Solvents used in this study were physiological saline solution (Ringer solution) at pH 7.00 and 0.01 mol dm-3 phosphate buffer solution at pH 7.00. Water used to prepare the Ringer and buffer solutions was passed through an inverse osmotic membrane, deionized by the ion-exchange resin, and finally distilled. The concentrations of poly(ribonuc1eotides) were determined by the phosphorus analysis method.20 Apparatus and Procedure. The apparatus used in this study was the calorimeters such as a differential scanning calorimeter (DSC; DASM-4) and flow microcalorimeter. For the DSC measurements, the solutions with various concentrations of EB at a given concentration of 0.05 wt 5% of the poly(A).poly(U) duplex were measured by the DSC. The heating rate employed for DSC measurements was about 1.0 K min-1. A flow microcalorimeter used in this study was similar to that reported by Wadso et al.,zl except for the special pump systems for the mixing process shown in Figure 2A (a) and the computer systems for data treatment and regulation of the pumps in the mixing process. As seen in Figure 2A, the pump used for mixing consists of a 180-mm-long glass cylinder (a) with a 20-mm i.d. and a 190-mm-long stainless steel piston (b) with a 16-mm 0.d. having a definite volume exactly. An EB solution and a poly(ribonucleotide) duplex solution filled in the glass cylinder of each pump were mixed by driving a stainless steel piston rod by a dc motor (c) (MM-A4-L1-300, Kyoei Tsusin Kogyo Co. Ltd., Japan) controlled by mode of feedback of a counter electromotive force using a computer (NEC-9801, Nipon Electric Co. Ltd., Japan) system. Figure 2B shows the vertical sectional view of the mixing cell which is made of a coiled 50-mm-long stainless steel pipe with a 1.00-mm i.d. sandwiched between two stainless steel plates. For the calibration of this microcalorimeter, the heats of dilution, &-, of an aqueous sucrose solution were measured at 298.15 0.005 K. &-was in good agreement with that in the literatureZ2 (accuracy *2.0%). Using this flow microcalorimeter, the heats of mixing of the poly(A).poly(U) duplex solution with a concentration of 1 X 10-3 mol dm-3 at a given flow rate of 3.0 X 10-3 cm3 s-1 and an EB solution with various flow rates were measured at 298.15 f 0.005 K. In order to confirm the molecular conformations of the poly(A)-poly(U) duplex and to obtain binding parameters between the poly(A).poly(U) duplex and EB, the absorption spectra were measured at room temperature using a spectrophotometer (Hitachi 220 A, Japan). The cell used for UV spectral measurements in concentrated solutions was a demountable quartz cell with path length of 0.05 mm (GL Science, Inc., Japan).
*
Interactions between EB and Poly(A)-Poly(U) Mixtures
The Journal of Physical Chemistry, Vol. 98, No. 23, I994
5945
TABLE 1: Enthalpy Change AH, per Mole of EB Accompanyiong the Interaction Process P A h H 1 (kJ mol-L)b r’c AH1 (kJ mol-l)d 0.04 -1.2 f 0.1 3.97 x 10-2 -30 0.05 -1.4 f 0.0 4.94x 10-2 -28 0.07 0.09 0.11 0.15 0.30 0.47 0.60 0.80 1.20 1.60 2.00
-1.9 f 0.1 -2.2 f 0.1 -2.8 f 0.2 -4.1 f 0.1 -7.8 f 0.0 -9.6 f 0.1 -10.2 f 0.1 -ll.OfO.l -12.2 f 0.1 -12.2 f 0.0 -12.4 f 0.2
-2 1 -26 -26 -26 -28 -27 -27 -28 -3 1 -3 1 -3 1
6.93 X 8.45 X 10.6 X 14.7 X 21.1 x 10-2 35.7 x 10-2 38.2 X 38.9 X le2 39.4x 10-2 39.5 x 10-2 39.6 X
r denotes the molar ratio of EB to Duplex-I base pair. mol means mole of Duplex-I base pair. r’ denotes the molar ratio of bound EB to Duplex-I base pair. mol means mole of EB.
TABLE 2: Enthalpy change AH&, per Mole of EB Accompanying the Interaction Process
tj
kl Figure 2. (A) Schematic diagram of the pump used in the mixing process: (a) glass cylinder, (b) stainless steel piston, and (c) dc motor. (B) Schematic diagram of the mixing cell: (a) coiled stainless pipe, (b) heating coil with resistance of 47.8Cl, and (c) Wood’s metal. The space between thecoiled stainless pipeand thestainless pipeis filled with Wood‘s metal to obtain the best conditions of the thermal conduction.
The microscope used to observe the phase states of the poly(A)-poly(U) mixture in concentrated solutions was a polarization microscope (XTP-11, Nikon Co., Ltd., Japan). The centrifuge used to separate the supernatant liquid and condensed poly(A)-poly(U) mixture was a LC-120 TOMY Seiko, Japan. The abbreviations used in this paper are presented as follows: duplex-I is the poly(A).poly(U) duplex in dilute solution, duplexI1 is the mixtures of poly(A)-poly(U) mixture with phosphatidylcholine after diluting Ringer solution, and the condensed duplex-I1 is condensed poly(A)-poly(U) mixture inserted into a vesicle of phosphatidylcholine separated by a centrifuge after diluting Ringer solution.
3. Results and Discussion 3.1. InteractionsbetweenDuplex-I and Ethidium Bromide (EB). Heat of Mixing of Duplex-I-EB System. In order to obtain information about the binding mode based on an interaction of the poly(A).poly(U) duplex (duplex-I) with ethidium bromide (EB), the heats of mixing of duplex-I and EB solutions were measured at 298.15 f 0.005 K using a flow microcalorimeter. The heats of mixing obtained proved to be exothermic, demonstrating that an interaction between duplex-I and EB exists. The results obtained are listed in Table 1 and shown in Figure 3, where the heat of mixing, AmixHlper mole of base pair, is plotted against the molar ratio, r, of EB to duplex-I base pair. As seen in Figure 3, the absolute value of AmixH1increases at first and then is an asymptote to AmixHl = 12 kJ at r = 0.8, indicating that this definite value means the termination of an interaction between duplex-I and EB. The behaviors of AmixHl are identical with those of the poly(A)-poly(U) duplex-9-
0.08 0.10 0.20 0.40 0.60 0.80 1 .oo 1.20 1.40 1.60 1 .80 2.00
2.3 3.3 10.6 15.0 19.8 20.6 20.8 19.0 19.0 22.6 20.6 20.2
7.90 X 10-2 9.87X 10-* 19.3 X 34.7 x 10-2 38.6 X 10-2 39.2X 1t2 39.5 x 10-2 39.6 X 39.7 x 10-2 39.7 x 10-2 39.8 X 39.8 X 1W2
29 33 55 43 51 53 53 48 48 57 52 51
r denotes the molar ratio of EB to Duplex-I base pair. b mol means mole of Duplex-I base pair. r’denotes the molar ratio of bound EB to Duplex-I base pair. d mol means mole of EB. r
7
0.0
1.0
E
2.0
I
0 ,0‘1
7 -.-e-.
I
x
2
-20
Figure 3. Plots of the heat of mixing, A ~ H Ifor, duplex-I-EB system against r, where r is the molar ratio of EB to duplex-I base pair.
aminoacridine dye as reported p r e v i o ~ s l y .From ~ ~ the result as mentioned above, it is considered that EB molecule is possible of intercalating into adjacent base pairs of duplex-I.
On the other hand, the absorption spectra for solutions with various r to obtain information for an interaction process of the duplex-I-EB system were measured a t room temperature using as an absorption spectrophotometer. The result obtained is shown in Figure 4. As seen in this figure, absorption spectra show an isosbestic point at 5 11 nm similar to those for the DNA-EB system.24 An interaction of duplex-I with EB seems to bea binding mode which is called an intercalation based on the insertion of EB into adjacent base pairs of duplex-I although it is unequivocally not able to be determined due to a lack of some information at the present work. In order to determine exactly the change in enthalpy based on an interaction between duplex-I and EB, the percentage, A, of EB bound to duplex-I was calculated according to the treatment of Peacocke and Skerrett25 as follows
5946 The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
O.*
Kagemoto et al.
~
-L
0.5 -
.\.\ ‘e
._..._./
0.0I 450
\a
*
L
1
I
500 550 h/nm Figure4. Typical absorption spectra of EB solutions with theconcentration of 2.5 X 10-5 mol dm-3 containing various concentrationsof duplex-I as (-10, (-...-I 1.3 x 10-5, (-..-) 3.1 x 10-5,(-.-) 6.1 x 10-5, (---I 1,l X 10-4, 2.9 X 1 P mol d m 3 where mol is mole of duplex-I base
Figure 5. Scatchard’s plot of duplex-I-EB system. Since Scatchard’s plot does not give a straight line, an analysis of the data was carried out according to McGhee and Hippel (see eq 2).
(e-)
pair.
A = -‘f - ‘x ‘f
(1)
- ‘b
where cf is the extinction coefficient at 477 nm of free EB which does not bind to duplex-I, e, the extinction coefficient at 477 nm of a mixture, and cb the extinction coefficient at 477 nm of EB bound to duplex-I. Here e b was determined as the extinction coefficient which does not change at a given concentration of EB regardless of an increase of the concentration of duplex-I in solution. From the results obtained, Scatchard’s plot was carried out and shown in Figure 5, where r’/Cfis plotted against r’. Here, Cf is the concentration of free EB which does not bind to DuplexI, and r’is the molar ratio of bound EB to base pair of duplex-I. It is seen from Figure 5 that this plot does not give a straight line. In order to obtain the exact binding parameters such as the binding constant and number of duplex-I base pairs occluded by EB, the binding parameters which give the best fit for the experimental data can be determined according to the neighborexclusion binding model of McGhee and HippeP as follows:
r’ = Kl(l - n,r?
(2)
Cf
where K I and nl are the binding constant and number of base pairs occluded by the dye molecule, respectively. According to eq 2, K 1and nl values which give the best fit for the experimental data were calculated according to the nonlinear least-squares treatment. K I and nl values are estimated to be about 2.0 X 105 dm3 mol-’ and 2.5, respectively. Using K 1 and nl values, the calculated curve (solid line) according to eq 2 gives the best fit for the experimental data as seen in Figure 5. On the other hand, the heat of interaction, AHI, per mole of EB can be determined from the relationship between AmixHIand c b / P ( = r ? as
AmixH, AH, = -
(3)
cb/p
where P is the molar concentration of duplex-I base pair, and the concentration of the bound EB, c b was determined using K1 and nl values obtained above. Under the assumption that the complex between duplex-I and EB is formed according to the following reaction process as duplex-I
+ EB * (duplex-I-EB)
complex
The binding constant, K1, can be expressed as
(4)
where C is the total molar concentration of EB. From eq 5, c b / P can be expressed as
(6)
AH13 obtained according to eqs 3 and 6 are listed in the last column of Table 1 as shown in Figure 6, where AHl is plotted against r. As seen in Figure 6, the absolute value of AH1 shows a definite value which is nearly independent of r. The average value of AH1 for each r corresponds to the net heat of interaction, AHl‘, and its value was estimated to be about -28 kJ (mol of EB)-1. It is suggested that the AH’’ value is the net heat of interaction based on the complex formation between duplex-I and EB. 3.2. Thermodynamic Quantities for Duplex-I. Using AH’’ and K1 values, the thermodynamic quantities based on an interaction of duplex-I with EB were estimated from AG10 = -RT In K 1and ASI = (AHl’- AGlO)/T. AGIO and AS1 were estimated to be about -30 kJ mol-’ and 7.0 J K-l mol-’, respectively, although the change in entropy based on the complex formation is, in general, negative, where mol is moles of EB, suggesting that the interaction between duplex-I and EB forms thermodynamically a stable complex from the viewpoint of AGIO value. In addition, the contribution of AHl’ concerning the stability of the complex formation is larger than AS’. The reason why AS1 based on the complex formation is a positive value is very difficult to explain due to a lack of some information such as the effects of dehydration surrounding duplex-I accompanying the interaction and/or an electrostatic interaction between the Pod- group with a negative charge in duplex-I and the N+-C*Hs group with a positive charge of EB. Further study is in progress to clarify various problems as mentioned above. 3.3. Thermal Properties of Duplex-I-EB Complex. In the preceding section, it has been suggested from the results of the calorimetric and spectral measurements that the interactions between duplex-I and EB have formed thermodynamically the stable complex. In order to obtain information about the behaviors of the duplexI-EB complex, the thermal properties of solutions with various concentrations of EB at a given concentration of duplex-I were studied by a differential scanning calorimeter (DSC). Typical DSC curves are shown in Figure 7a-c, together with the results of the temperature dependence of the absorbance of UV spectra on duplex-I solutions containing EB. As seen in Figure 7a, the
Interactions between EB and Poly(A)-Poly(U) Mixtures
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5941
r
-
1
0
E
2.0
1.0
0.0
d
330 310
Figure 6. Plots of heat of interaction, A H I ,against r where A Z f l is heat ofinteractionpermoleofEBconverted byusingK1 andnl valuesestimated according to McGhee and Hippel. (a)
A
80
(b)
60
t
I
0.0
Figure 8. Plots of (a) transition temperature, Tt-l and (b) the observed heat, A H 0 b l , per mole of nucleotide for duplex-I-EB system against r.
/
...
,------
,
3
323
.
333
2.G
r
..................
................ .....................
...........
1.0
/
343
,
I
353
4
?
3
T/K Figure 7. Typical DSC curves (-) and the temperature dependence of UV spectra at 260 nm (- - -) and 280 nm for duplex-I-EB systems: (a) r = 0, (b) r = 0.02, and (c) r = 0.4. (-e)
DSC curve for duplex-I at r = 0 exhibits thermograms with two endothermic peaks at 325 and 342 K. The results of the temperature dependences of the absorption (broken line) at 260 nm, A260,and that (dotted line) at 280 nm, ,4280, for the duplexI-EB complex are also shown in the same figure. From the results, the hypochromic effect at ,4280 appearing in the vicinity of 324 K corresponding to start an endothermic peak at 325 K of DSC curve demonstrates the formation of the poly(A).Zpoly(U) triplex. Therefore, an endothermic peak at 325 K of DSC curve corresponds to transform into the poly(A).2poly(U) triplex with a triple-stranded helical structure from the poly(A).poly(U) duplex. This tendency is the same that thepoly(A).poly(U) duplex in higher ionic strength transforms into the poly(A)-Zpoly(U) triplex with increasing temperature as reported previ0usly.2~ On the other hand, an endothermic peak at 342 K corresponds to the helix-coil transition of the poly(A)-2poly(U) triplex from the results of the hyperchromic effects a t A260 and A280 although an endothermic peak temperature of DSC curve is slightly higher than a temperature of the hyperchromic effect estimated from the temperature dependence of the absorption. The typical DSC curve for the duplex-I solution with r = 0.02 was shown in Figure 7b, together with the results of the temperature dependences of the absorbances at 260 and 280 nm under the same experimental conditions. The thermal behaviors at r = 0.02 are identical to those at r = 0, except that an endothermic peak temperature corresponding to that a t 325 K and r = 0 shifts to higher temperature.
At finally, the DSC curve of the duplex-I-EB complex a t r = 0.4 is shown in Figure 7c, together with the temperature dependences of the absorbances a t 260 and 280 nm for the duplexI-EB complex. As seen in Figure 7c, an endothermic peak at higher temperature of the DSC curve a t r = 0.4 shifts to higher temperature compared with that a t r = 0; however, an endothermic peak of the DSC curve appearing in the vicinity of 325 K at r = 0 disappears, while the hyperchromic effects at ,4260 and ,4280 corresponding to the start of the thermal changes reflect a helixcoil transition of duplex-I in the duplex-I-EB complex. It is a very interesting problem that, by adding EB, duplex-I does not form a triple-stranded helical structure with increasing temperature. From the results above, an endothermic peak of the DSC curve at 343 K seems to correspond to a sum of the change in enthalpy based on a helix-coil transition of duplex-I and the heat of dissociation based on dissociation of EB from the duplex-I-EB complex. Under the assumption that an endothermic peak and area of the DSC curve correspond to the transition temperature, Tt-I, and the change in enthalpy, A H o b l ,observed accompanying the phase transition, the plots of Tt-l and AHo&, against r a r e shown in Figure 8, a and b, respectively. As seen in Figure 8, Tt-l increases at first and then reaches a definite value with increasing r, while M o b s - , increases by r = 0.6 and reaches a constant value with increasing r. The constant value of AHob-l at r > 0.6 is comparable with that of AmixHI estimated from the heat of mixing in the preceding section. It is suggested that the complex based on the interaction between duplex-I and EB forms thermally a stable complex. 3.4. Dissociation Process for Duplex-I-EB Complex. Assuming that the duplex-I-EB complex at r > 0.08 is dissociated according to the following reaction processes as [duplex-I (helix) - EB]
-
Aff&l
[duplex-I (coil)]
+ EB
(7)
Affl-lO
[duplex-I (helix)]
[duplex-I (coil)]
(8)
The difference between reactions 7 and 8 gives the heat of dissociation, AfZdis-1, as
-
AfGtkl
[duplex-I (helix)-EB]
[duplex-I (helix)]
+ EB
(9)
Thus, AHdiJs-1 (reaction 9) can be estimated by subtracting AHt-lo at r = 0 (reaction 8) from AH0bs-l (reaction 7): m d i r s - 1 = A H o b l - AHt-lo. Since A H d i s k l estimated by such a way is
5948 The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
7 Y
1
Pa
0.0
1.0
2.0
r
Figure 9. Plots of the heat of dissociation,A&&, duplex-I-EB system against r.
per mole of EB for
the heat of dissociation per mole of base pair, it will be needed to convert to the heat of dissociation, AHw1, per mole of EB. Under the assumption that the temperature dependence of cb for eq 6 may be negligibly small, AHbl per mole of EB can be estimated according to the same analysis as shown in eq 3. AHdiss-l
AHH, = cb/p
The results obtained are shown in Figure 9, where AHw1 is plotted against r. As seen in Figure 9, A H ~shows I a definite value which is nearly independent of r although the calculated data are somewhat scattered. From this figure, the calculated curve which is given the best fit for the experimental data was calculated by nonlinear least-squares treatment, and its net heat of dissociation, A H b l O , per mole of EB was estimated to be about 30 kJ by extrapolation to be the limit of r 0. The reverse sign for this value (30 kJ) is in fairly good agreement with -28 kJ estimated from AmixH1. 3.5. Thermal Behaviors of the Duplex-11-EB System. Since most such studies are carried out usually in dilute solutions, it seems to be very difficult to exactly understand physiological functions in living cell from these behaviors in dilute solutions. For a more realistic understanding, the mixtures of the poly(A)-poly(U) mixture with phosphatidylcholine as a major constitutive component of the cell membrane in concentrated solutions should be studied. Preparation of Duplex-II. The poly(A)-poly(U) mixture condensed in concentrated solutions was prepared at room temperature by mixing of phosphatidylcholine as a major constitutive component of the cell membrane with the concentration of 5.00 wt %, showing an isotropic phase (see Figure lob), and a poly(A)-poly(U) mixture with the concentration of 10.0 wt %, showing the layer liquid crystal2*(see Figure loa). The phase states for the poly(A).poly(U) mixture with phosphatidylcholine at 2 weeks after preparation were observed at room temperature using a polarization microscope. From the results, the phasestates with thespherical and layer liquid crystals were observed as shown in Figure 1Oc. In order to obtain further information about the spherical liquid crystal, the solutions containing the spherical and layer liquid crystals were diluted by Ringer solution. From the results, the layer liquid crystalline phase was dissolved by Ringer solution and transformed into an isotropic phase. But the spherical liquid crystalline phase was not dissolved and remained as the spherical liquid crystal in Ringer solution, suggesting that the liquid crystal of the poly(A)-poly(U) mixture inserted into vesicle of phosphatidylcholine forms densely packed spherical liquid crystal similar to that of the DNA-phosphatidylcholine system as reported previously.’* It is worth noting that the liquid crysta) of the poly(A)-poly(U) mixture was inserted into a vesicle of phosphatidylcholine as a major constitutive component of the cell membrane. In addition, a spherical liquid crystal which does not dissolve by adding Ringer solution as shown in Figure 1Od is presented by the abbreviation of duplex-I1 here.
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Kagemoto et al.
DSC Measurements for the Duplex-11-EB System. An interaction between EB and duplex-I1 enclosed by phosphatidylcholine was also studied by the DSC under the experimental conditions similar to that of duplex-I in dilute solutions. The typical DSC curves at r = 0.4 and 0.8 for the duplex11-EB mixtures are shown in Figure 1l a (see DSC curves 2 and 3), together with DSC curve at r = 0 of duplex-I1 (DSC curve 1). As seen in Figure 1 la, with increasing r, the endothermic peaks of DSC curves 2 and 3 shift to higher temperature in comparison with curve 1 of duplex-11, demonstrating that an interaction between duplex-I1 and EB exists similar to that of duplex-I in dilute solutions. The transition temperature, T,2, and the change in enthalpy, AHOb2, observed for the duplex-11-EB systems were also estimated from the endothermic peak temperature and area of DSC curve, respectively. The plots of T1-2 and A H o b l against r are shown in Figure 12a,b, together with Tt-l and AHobl of duplex-I. Here, r is the molar ratio of EB to duplex-I1 base pair. As seen in Figure 12, Tl-2 and A H 0 b 2 increase with increasing r and then are an asymptote to Tt-2 = 356 K and A H 0 b 2 = 60 kJ at r = 0.6. The behaviors of Tt-2 are similar to that of T1, for the duplexI-EB system in dilute solutions. However, U 0 b 2 a t r > 0.6 is larger than AH0bl of duplex-I. It is very difficult to make clear for this result of ~iH~bs-2 due to a lack of the some information for the complicated systems. However, as possible factors, it is expected that this large value of A H 0 b - 2 is the effect of EB for the spherical liquid crystal and an isotropic phase surrounding the spherical liquid crystal dissolved by Ringer solution. In order to obtain information about the behavior of EB for an isotropic phase dissolved by Ringer solution, the duplex-I1 solution was divided into two components such as a spherical liquid crystalline phase (condensed duplex-11) and an isotropic phase (supernatant liquid) as shown in Figure 10e,f by centrifugation. The concentration of the condensed duplex-I1 separated by centrifugation was estimated as basics of the concentration of the supernatant liquid determined by UV spectral measurement. DSC measurement of the supernatant liquid was carried out. The typical DSC curves for the supernatant liquid at r = 0.4 and r = 0.8 are shown in Figure 1l b (curves 2’ and 39, together with DSC curve at r = 0 for the supernatant liquid (curve 1’). The transition temperature, T,1’ and the change in enthalpy, AHobl’, observed for the supernatant liquid are also estimated from the endothermic peak and area of DSC curve, and they are plotted against r as shown in Figure 12a,b, together with and/or AH0b-l for duplex-I and Tt-2 and/or A H 0 b 2 for duplex11. As seen in Figure 12a,b, AH0bl‘ for the supernatant liquid is in good agreement with A H 0 b l for duplex-I in dilute solutions although T,l’is slightly higher incomparison with Tt-l for duplexI. That is, AHo~-l‘ = AH0bl (=AHt-lo AHdisS-,). 3.6. Heats of Dissociationof Duplex4 and Condensed DuplexIEEB Systems. A H 0 b 2 for duplex-I1 is considered as the sum of the heat of helix-coil transition, AHt-20,a t r = 0 and the heat of dissociation, Md-2, based on dissociation process of EB for the duplex-11-EB complex as
+
According to eq 11, a plot of M d i - 2 for duplex-I1 against r is shown in Figure 13, together with AHdi-l of the supernatant liquid and/or duplex-I. As seen in Figure 13, each AHdaez for duplex-I1 and both systems (Wd-1 for duplex-I and A H d i ~ r - 1 ’ for the supernatant liquid) increases a t first and then is an asymptote to M d - 2 = 28 kJ, htid-1 = 20 kJ, and AHd-1’ = 18 kJ a t r > 0.6, respectively, although the experimental data are somewhat scattered. As seen in Figure 13, as AHdiSs-1’ for the
Interactions between EB and Poly(A)-Poly(U) Mixtures
The JourMl of Physical Chemistry, Vol. 98, No. 23, I994 5949
f-
Dihtion by ringer solution
-
Stpdration t o t w o phases by centrifuge
Supernatant liquid
Condensed Duplex-ll
Figure10. Polarizing micmphotographsof variousphasestates as (a) poly(A).poly(U) duplex with the concentrationof 10.0wtW. (b) phosphatidylcholine with the concentration of 5.0 wt %, (c) poly(A).poly(U) duplex-phosphatidylcholine mixture by mixing of solution with the concentration of 10.0 wt 55 of poly(A).poly(U) duplex and that with concentration of 5.0 wt W phosphatidylcholine,(d) poly(A).poly(U) duplex-phosphatidylcholine mixture
diluted by Ringer solution (duplex-11). (e) supernatant liquid separated from duplex-11, and (0condensed duplex-I1 separated from duplex-11.
supernatant liquid is equal to that of duplex-I obtained in dilute solutions, however, AHdiu2 for duplex-I1 is larger than AHdbl for duplex-I (and/or the supernatant liquid). From the results,
it is suggested that the difference between A H b 2 for duplex41 and AHdbl for duplex-I (and/or the supernatant liquid) gives the change in enthalpy, M w 2 , based on the dissociation process
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The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
Kagemoto et al.
r
I
323
333
,
I
,
343
I
,
353
Figure 14. Plots of (b) AHbi for duplex-I-EB ( i = 1, 0)and for the condensed duplex-11-EB (i = 2,O) systems against r where AHbi ( i = 1 and 2) is heat of interaction per mole of EB converted by using K Iand nl values estimated according to McGhee and Hippel.
I
363
T/K Figure 11. Typical DSC curves for (a) duplex-11-EB system: curve 1, r = 0; curve 2, r = 0.4; and curve 3, r = 0.8. (b) Supernatant liquid separated from duplex-11-EB system curve l’, r = 0; curve 2’, r = 0.4; and curve 3’, r = 0.8. 370
330
r
i
I
310 1
,
L
d
0
E 7
Y
\ m
n
f a
1
I
4
0.0
I 1.0
2.0
r Figure 12. Plots of (a) transition temperature, Tt-i, for duplex-I (i = l), for duplex-I1 ( i = 2), and for supernatant liquid (i = 1’) and (b) the observed heat, AHHobi, for duplex-I (i = l), for duplex-I1 ( i = 2), and for supernatant liquid ( i = 1’) against r: (0) duplex-I-EB system, (0) duplex-11-EB system, (0) supernatant liquid separated from duplex11-EB system. ,-
E
r Figure 13. Plotsof AH** for duplex-I-EB (i = 1,0),and/or supernatant liquid ( i = l’, 0)and for duplex-11-EB (i = 2,O) systems against r. Here, L i & i e i is estimated by subtracting at r = 0 from AHobi (i = 1, l’, and 2).
of the condensed duplex-11-EB complex as
AHD-~= AHc~iss-2- W d i s s - 1
(12)
AHp2 for the condensed duplex-11-EB complex seems to correspond to the change in enthalpy based on the complicated interactions as follows: AH,,
= MPC-EB + A~~(A-u)-EB + M(A-u)-Pc (13)
where AH~c-EB corresponds to the change in enthalpy based on dissociation of an electrostatic interaction between the N+-C2H5
group with a positive charge of EB and the PO4- group with a negative charge of phosphatidylcholine. However, the heat of mixing in this system was equal to be zero, which is independent of r (figure not shown in this paper). From this result, the heat of interaction, AH~c-EB, in eq 13 may be negligibly small, under the assumption that the heats of dilution of EB and/or phosphatidylcholine may be negligibly small. AH(A-u)-EB is the heat of dissociation based on the dissociation process between EB and an internal poly(A)-poly(U) mixture in condensed duplex-11. Finally, AH(~-u)-pccorresponds to the change in enthalpy based on an electrostatic interaction between the PO4- group with a negative charge of poly(A)-poly(U) mixture in the condensed duplex-I1 and the N+(CHs)3 group as a polar group of phosphatidylcholine with a positive charge. However, since the heat of mixing of duplex-I solution and phosphatidylcholine solution with various concentrations is equal to zero, which is nearly independent of the phosphatidylcholine concentration, AH(A-u)-Pc in eq 13 may thus be negligibly small. From these results, since AHPC-EB = 0 and AH(~-u)-pcN 0, then A H ( A - ~ ) - in E ~eq 13 is equal to AHw2. AH&*(=AH(A-~)-EB) converted per moleof EB was calculated for each r, under the assumption that the number of binding site of EB for the condensed duplex-I1 is nearly equal to that for duplex-I in dilute solutions described in the section 3.1. The results obtained are shown in Figure 14, where AH&* is plotted against r. From Figure 14, the net heat of dissociation, AHb20, for the condensed duplex-I1 is estimated to be about 4.0 kJ (mol of EB)-’ by extrapolation to be the limit of r 0 . The A H L ~ O value is not in accord with AH&,’ (=29 kJ (mol of EB-I)) for duplex-I estimated from Figure 9. The fact that A H w 2 O for the condensed duplex-I1 does not agree with AH0-l’ seems to demonstrate that the poly(A)-poly(U) mixture in the condensed duplex-I1 is possible of forming a conformation such as the poly(A).Zpoly(U) triplex with a triplestranded helical structure rather than the poly(A).poly(U) duplex. 3.7. Interaction between Poly(A)-2Poly(U)Triplex and EB. In order to confirm whether or not EB molecule interacts with the poly(A)-2poly(U) triplex, the heat of mixing, AmixH3,of EB and poly(A).2poly(U) triplex formed by an equimolar mixture of poly(A) and 2poly(U) in dilute solution was also measured at 298.15 i 0.005 K using a flow microcalorimeter. This system proved to be exothermic, demonstrating that an interaction between poly(A).2poly(U) triplex and EB exists. The results obtained is shown in Figure 15, where AmixH3is plotted against r. It can be seen from Figure 15 that AmixH3 decreases linearly with an increase of r although AmixH1(as shown in Figure 3) is an asymptote to AmixH1= 12 kJ at r = 0.8. The change in enthalpy based on an interaction of the poly(A).2poly(U) triplex with EB refers to analysis according to the same way a s that of poly(A).poly(U) duplex-EB system. The percentage of EB bound to poly(A).2poly(U) triplex was calculated according to Peacoke and Skerrett23 as described in section 3.1. The binding constant, K3, and the number of base triplets occluded by EB were 9.7 X 104 dm3 mol-’ and 3.5,
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Interactions between EB and Poly(A)-Poly(U) Mixtures
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5951
r
E
0
-
0.0
2.0
1.0
1\
ul
3.0
N
1 1
t
o
a, 0
K
o n
I
0
v,
n
c
-I
Q
Figure 15. Plots of the heat of mixing, A,kH3, for poly(A).2poly(U) triplex-EB system against r, where r is the molar ratio of EB to poly(A).Zpoly(U) triplex base triplet.
r -
0.0 i-
1.0
2.0
3.0 1 1
= a L t.;
-100
Figure 16. Plots of heat of interaction, AH,, per mole of EB, converted by using K3 and n3 values estimated according to MaGhee and Hippel for poly(A).Zpoly(U) triplex-EB system.
respectively. Using K3 and n3 values, the heat of interaction, AH,, per mole of EB was calculated for each molar ratio, r, of EB to base triplet of the poly(A).Zpoly(U) triplex.
xu
Figure 17. Plots of (a) absorbances at 260 nm and (b) at 280 nm against mole fraction of poly(U); XU in poly(A)-poly(U) mixture with (0)7.0, ( 0 ) 8.0, and ( 0 )9.0 wt %.
concentration above 7.0 wt % at these discontinuous point forms the poly(A).2poly(U) triplex. That is, a spherical liquid crystal enclosed by phosphatidylcholine is made up by poly(A).2poly(U) triplex with a triple-stranded helical structure in vesicle of phosphatidylcholine. It is worth noting that an equimolar poly(A)-poly(U) mixture in concentrated solutions forms poly(A)-2poly(U) triplex and EB molecule interacts with an internal poly(A)-2poly(U) triplex (condensed duplex-11) inserted into vesicle of phosphatidylcholine as a major constitutive component of the cell membrane.
4. Conclusion
s a solid line in Figure 16. The change in net enthalpy, AH3’, for thepoly(A).2poly(U) triplexcan be estimated by extrapolation to be the limit of r 0. The value of AH< estimated by such a method is -3.0 kJ (mol EB)-l. It is suggested that the interaction between the poly(A).2poly(U) triplex and EB was a side binding based on an electrostatic interaction between the =N+-CzH5 group with a positive charge of EB and the Pod- one with a negative charge of main chain of the poly(A).2poly(U) triplex. AH< (=-3.0 kJ) for the poly(A)-2poly(U) triplex was in fairly good agreement with the reverse sign of A H b 2 O (=4.0kJ) for the condensed duplex-11-EB system, suggesting that an equimolar poly(A)-poly(U) mixture in concentrated solutions is possible to form poly(A).2poly(U) triplex into vesicles of phosphatidylcholine and also that EB molecule interacts with poly(A)-2poly(U) triplex by a side binding based on an electrostatic interaction between the =N+-C*Hs group with a positive charge of EB and the PO,group with a negative charge of the main chain of the poly(A).2poly(U) triplex. 3.8. Molecular Conformation of Poly(A)-Poly(U) Mixture in Concentrated Solutions. In order to confirm the molecular conformation in an equimolar poly(A)-poly(U) mixture in concentrated solutions, the absorbances at 260 and 280 nm for an equimolar poly(A)-poly(U) mixture in concentrated solution for various mole fractions, XU,of poly(U) were measured under the experimental conditions as described in the Experimental Section. The results obtained are shown in Figure 17 at (a) 260 nm and (b) 280 nm. ’ As seen in Figure 17, the absorbance mixing curves at 260 and
-
In order to obtain information about the interaction modes between R N A and dye, the thermal and optical properties of duplex-I and EB in dilute solutions have been studied by means of UV spectrophotometry and microcalorimetry. From UV spectral measurement, an interaction between Duplex-I and EB in dilute solutions may be possible of forming the duplex-I-EB complex which is called an intercalation of EB inserted into adjacent base pairs of duplex-I. The thermodynamic quantities accompanying the complex formation weredetermined by combining the resultsof UV spectra and those of the calorimetric measurements. From the results, it has been suggested that the interactions between duplex-I and EB have thermodynamically formed the stable complex. The dissociation process for the duplex-I-EB complex has been studied using a differential scanning calorimeter, and the change in net enthalpy based on dissociation was estimated to be about 30 kJ (mol of EB)-l, and its reverse sign for this value was in good agreement with that estimated from the heat of mixing. From the results of the heat of mixing and the DSC, it has been concluded that the interaction between duplex-I with EB thermodynamically formed the stable complex in dilute solutions, and its change in enthalpy based on the complex formation was about -28 kJ (mol of EB)-l. Since most such studies have been done in dilute solutions, for a more realistic understanding, the poly(A)-poly(U) mixturephosphatidylcholine in concentrated solutions was studied under the same experimental conditions of duplex-I in dilute solutions. From the results, poly(A)-poly(U)-phosphatidylcholine mixtures in concentrated solutions have formed the spherical liquid crystal which is essentially different from an already known liquid crystal. From the fact that the spherical liquid crystal did not
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The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
dissolve by Ringer solution, it has been concluded that the poly(A)-poly(U) mixture formed the spherical liquid crystal by inserting into vesicle of phosphatidylcholine. Using the spherical liquid crystal, an interaction between the spherical liquid crystal and EB was also studied by the DSC under the same experimental conditions in dilute solutions. From the results, the spherical liquid crystal formed thermally a stable complex with EB, and its the change in enthalpy was determined to be about 4.0 kJ (mol of EB)-l from the dissociation process of the spherical liquid crystal-EB complex. In order to confirm the molecular conformation of poly(A)poly(U) mixture in the spherical liquid crystal enclosed by phosphatidylcholine, an interaction between EB and poly(A).Zpoly(U)triplex formed by 1:2 mixture of poly(A) and poly(U) was also studied by means of microcalorimetry and spectrophotometry. From the results, the change in enthalpy based on the interaction of poly(A).2poly(U) triplex with EB was determined to be about -3.0 kJ (mol of EB)-I, and this value is in fairly good agreement with the reverse sign of the change in enthalpy (4.0 kJ) obtained from the spherical liquid crystal-EB system. It has concluded that the molecular conformation of poly(A)-poly(U) mixture inserted into vesicle of phosphatidylcholine was a poly(A).2poly(U) triplex with a triple-stranded helical structure, and the interaction between poly(A).2poly(U) triplex and EB was a side binding based on an electrostatic interaction between the PO,group with a negative charge of main chain of polyribonucleotide and the =N+-C2H5 group with a positive charge of EB.
References and Notes (1) Waring, M. S. Annu. Rev. Biochem. 1981, 50, 159.
Kagemoto et al. Hartmann, G.; Behr, W.; Beissner, K.-A.; Honikel, K.; Sippel, A. Chem., Inr. Ed. Engl. 1968, 7, 693. Berman, H. M.; Young, P. R.Annu. Rev. Biophys. Bioeng. 1981,10, Heidelberger, C. Annu. Rev. Biochem. 1975, 44, 78. Fuller, W.; Waring, M. Ber. Bunsen-Ges. Phys. Chem. 1964,68,805. Lerman, L. S. J. Mol. Biol. 1961, 3, 18. Fredericq, E.; Houssier, C. Biopolymers 1972, 11, 2281. Lepecq, J. B.; Paoletti, C. J. Mol. Biol. 1967, 27, 87. Lee, C. H.; Chang, C. T.; Wetmur, J. G. Biopolymers 1973, 12, Blake, A.; Peacocke, A. R. Biopolymers 1967, 5 , 383. Lober, G.; Shutz, H.; Kleinwachter, V. Biopolymers 1972,11, 2434. Tanaka, S.; Baba, Y.; Kagemoto, A. Makromol. Chem. 1981,182, Tanaka, S.; Baba, Y.; Kagemoto, A.; Fujishiro, R.Polym. J. 1980, (14) Naruse, K.; Kimura, S.; Hosokawa, S.; Baba, Y.; Kagemoto, A. Rep. Prog. Polym. Phys. Jpn. 1991, 34, 611. (15) Earnshaw. W. C.: Casiens. S. R. Cell 1980. 21. 319. (16) Nakazaki; M.; Baba, t.;Kagemoto, A. Rep. h o g . Polym. Phys. Jpn. 1992, 35, 699. (17) Iizuka, E. Polym. J. 1978. 10, 293. (18) Kimura, S.; Baba, Y.; Kagemoto, A. Rep. Prog. Polym. Phys. Jpn. 1993, 35, 707. (19) Miwatani, T.; et al. The guide to biochemical experiment. 3. Separation and analysis methods of nucleic acid; 1988; p 120. (20) Chen, P. S.; Toribara, T. Y.; Warner, H. Anal. Chem. 1956, 28, 1756. (21) Wiidso, I.; Monk, P. Acta Chem. Scand. 1968,22, 1842. (22) Herrington, E. F. G. Pure Appl. Chem. 1979, 40, 399. (23) Kano, K.; Baba, Y.; kagemoto, A.; Beatty, C. L. Polym. J. 1983,15, 657. (24) Thomas, G.; Rcques, B. FEES Lett. 1972, 26, 169. (25) Peacocke, A. R.;Skerrett, J. N. H. Tram. Faraday SOC.1956,52, 261. (26) McGhee, J. D.; Hippel, P. H. J. Mol. Biol. 1974, 86, 469. (27) Tanaka, S.; Baba, Y.; Kagemoto, A. Polym. J . 1976, 8, 325. (28) Kagemoto, A.; Okada, Y.; hie, H.; Baba, Y. Thermochim. Acta 1991, 1 , 176.
