Langmuir 2006, 22, 3273-3279
3273
Simple Method for the Preparation of DNA-Poly(o-methoxyaniline) Hybrid: Structure, Morphology, and Uncoiling of Poly(o-methoxyaniline) on the DNA Surface Arnab Dawn and Arun K. Nandi* Polymer Science Unit, Indian Association for the CultiVation of Science, JadaVpur, Kolkata, 700 032 India ReceiVed September 16, 2005. In Final Form: January 23, 2006 The DNA-poly(o-methoxyaniline) (POMA) hybrid is prepared by mixing aqueous solutions of POMA [emeraldine salt (ES), doping level [Cl]/[N])0.52] and sodium salt of DNA (Na-DNA) and is then freeze-dried after 4 days. Three different compositions (WDNA ) 0.25, 0.5, and 0.75, WDNA is the weight fraction of DNA) of the hybrids are prepared. The SEM pictures show a gradation in morphology; for example, for WDNA ) 0.75, fibrils are present but, at lower DNA concentration, a fibrillar network structure of the hybrid is observed. The circular dichroism (CD) spectra of the hybrid solutions indicate unchanged DNA conformation and WAXS patterns indicate intact crystal structure of DNA in the hybrid. The UV-vis spectra suggest no denaturation of DNA during the blending process. The UV-vis spectra of the hybrids in aqueous medium show a gradual red shift of the π band to polaron band transition with time. The plots of these wavelengths with time are sigmoidal, indicating the autocatalytic nature of the process. With an increase in the temperature, the rate of the red shift of the above peak is faster. Arrhenius analysis of the rate (1/τ700 where τ700 is the time required to reach the absorption maximum at the wavelength 700 nm) yields straight lines for the three hybrid compositions with activation energy values of 13-15 kcal/mol. These values are almost equal to the activation energy values of conformational transition of other polymers, supporting the red shift in UV-vis spectra occurs from conformational transition. DNA offers a surface where POMA gets adsorbed and the stable conformational transition resulting in the uncoiling of the POMA chain occurs through repulsive interaction among bound radical cations of POMA (ES) on the DNA surface. The conductivity of the hybrids is on the order of 10-6 S/cm, and the I-V characteristic curves indicate the semiconducting nature of the hybrids.
Introduction The immobilization of DNA on a conducting polymer is presently of great interest for its application in the study of DNA hybridization,1,2 gene therapy,3,4 genoelectronics,3,4 and various biosensing applications.4-6 Also DNA has a unique secondary structure where a stack of π electrons in the base pair promoted the possibility of DNA to form a “molecular conduit”.5 Wrapping of a conducting polymer on the DNA surface might be useful to connect the molecular wires,7 and these hybrids might play an important role to bridge the human-machine interface.4 There have been many attempts to prepare conducting polymer-DNA hybrids in the past few years, and polypyrrole has been mainly used for its good thermal and mechanical properties.2,3,8-10 There are only few reports for the polymer polyaniline (PANI)11-15 which is also environmentally stable and cheapest among the others. Also the conductivity of PANI * To whom correspondence should be addressed. E-mail: psuakn@ mahendra.iacs.res.in. (1) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896. (2) Thompson, L. A.; Kowalik, J.; Josowicz, M.; Janata, J. J. Am. Chem. Soc. 2003, 125, 324. (3) Wang, J.; Jiang, M. Langmuir 2000, 16, 2269. (4) Wallace, G. G.; Kane-Maguire, L. A. P. AdV. Mater. 2002, 14, 953. (5) Lassalle, N.; Mailley, P.; Vieil, E.; Livache, T.; Roget, A.; Correia, J. P.; Abrantes, L. M. J. Electroanal. Chem. 2001, 509, 48. (6) (a) Wilson, E. K. Chem. Eng. News 1998, 51, July. (b) Liu, B.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 1942. (7) Heeger, A. J. Angew. Chem., Int. Ed. 2001, 40, 2591. (8) Saoudi, B.; Jammul, N.; Chehimi, M. M.; McCarthy, G. P.; Armes, S. P. J. Colloid Interface Sci. 1997, 192, 269. (9) Saoudi, B.; Jammul, N.; Abel, M. L.; Chehimi, M. M.; Dodin, G. Synth. Met. 1997, 87, 97. (10) Wang, J.; Jiang, M.; Fortes, A.; Mukharjee, B. Anal. Chim. Acta 1999, 402, 7. (11) Nagarajan, R.; Liu, W.; Kumar, J.; Tripathy, S. K.; Bruno, F. F.; Samuelson, L. A. Macromolecules 2001, 34, 3921.
may be tuned by changing the oxidation properties.16 Using enzymatic polymerization condition with horseradish peroxidase (HRP), Nagarajan et al.11 synthesized PANI-DNA hybrids in situ, whereas Lokshin et al.12 reported the PANI-DNA complexes by mixing a chloroform solution of PANI-dodecyl benzene sulfonic acid (DBSA) complex with a dioctyl dimethylammonium-DNA solution. The electropolymerization of aniline on the DNA template13 and photopolymerization of aniline (dimer) in the presence of DNA14 are the other two methods reported so far. Recently, we reported the formation of DNA-conducting polymer hybrid by mixing a poly(o-methoxy aniline) (POMA) solution and the protonated form of DNA in water.15 The unique characteristics of this method are that both the conformation and crystal structure of double stranded DNA remain intact. However, it is a lengthy process as the protonated DNA is prepared by mixing dilute HCl solution with a solution of the sodium salt of DNA (Na-DNA) followed by dialysis to remove the sodium and chloride ions. Care was taken such that DNA was not denatured under the protonating condition. Here we report a method where the Na-DNA solution is directly used to prepare the hybrid without going through the above lengthy process. The emeraldine salt of PANI has numerous radical cation centers, and these centers can react easily with the anions of DNA to form the complex (cf. Scheme 1). However, the emeraldine salt (ES) form of PANI is insoluble in water. So alternate routes were attempted by various workers11-15 as (12) Lokshin, N. A.; Sergeyev, V. G.; Zezin, A. B.; Golubev, V. B.; Levon, K.; Kabanov, V. A. Langmuir 2003, 19, 7564. (13) Shao, Y.; Jin Y. D.; Dong, S. J. Electrochem. Commun. 2002, 4, 773. (14) Uemura, S.; Shimakawa, T.; Kusabuka, K.; Nakahira, T.; Kobayashi, N. J. Mater. Chem. 2001, 11, 267. (15) Dawn, A.; Nandi, A. K. Macromol. Biosci. 2005, 5, 441. (16) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1986, 82, 2385.
10.1021/la052536+ CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006
3274 Langmuir, Vol. 22, No. 7, 2006 Scheme 1
mentioned earlier. Here we have attempted to solubilize PANI (ES) in water by incorporating a substituent (methoxy) in the ortho position, and it is soluble in water having colloidal dimensions (average diameter ) 668 nm).15 By mixing the NaDNA solution (average diameter ) 715 nm) with POMA, the complex is prepared at three different compositions. In this manuscript, we report the morphology and structure of the hybrids studied by scanning electron microscopy (SEM) and wide-angle X-ray diffraction (WAXS), respectively. The chain conformation of DNA in the hybrid structure is also studied by circular dichroism. The dc conductivity and the I-V characteristic curves of the hybrids at different compositions are also reported. In our previous work, the acid form of DNA was mixed with POMA to form the complex and a slow doping was observed as the red shift of π band to polaron band occurred slowly.15 Three possibilities, e.g., (i) steric hindrance of DNA colloid to POMA colloid for interaction, (ii) orientation of rigid DNA chain before the interaction, and (iii) orientation of POMA chain in the reaction were proposed for the slowness. Among these the second factor was neglected as the DNA chain conformation and structure remained intact after the reaction. In the present system, there would be no reaction in the doping process, and consequently, the first factor may also be neglected. If the slow red shift of the π band to polaron band transition occurs in this system, the orientation of the POMA chain with time after the complexation with DNA might be the only cause. The slow conformational change of synthetic polymers and biopolymers is not uncommon in the literature. The higher rate of helix-coil transition in biopolymers usually occurs by increasing temperature and vice versa.17-21 In synthetic polymers, such a transition, namely called a coil-to-ordered conformer transition, is observed during the gelation study.22-24 In all of these cases, the coil-to-helix transition occurred in the solution state. In this paper, we put evidence of conformational transition of POMA on DNA surface from UVvis spectral data. Apart from the above study, the conductivity and I-V curves characterizing semiconducting behavior of the system have been presented. (17) Flory, P. J.; Weaver, E. S. J. Am. Chem. Soc. 1960, 82, 4518. (18) Zimm, B. H.; Bragg, J. K. J. Chem. Phys. 1959, 31, 526. (19) Buhot, A.; Halperin, A. Macromolecules 2002, 35, 3238. (20) Ciszkowska, M.; Osteryoung, J. G. J. Am. Chem. Soc. 1999, 121, 1617. (21) Kemp, J. P.; Chen, Z. Y. Phys. ReV. Lett 1998, 81, 3880. (22) Mal, S.; Nandi, A. K. Polymer 1998, 39, 6301. (23) Dikshit, A. K.; Nandi, A. K. Macromolecules 1998, 31, 8886. (24) Malik, S.; Jana, T.; Nandi, A. K. Macromolecules 2001, 34, 275.
Dawn and Nandi
Experimental Section Samples. Calf thymus DNA (type 1; sodium salt) was purchased from Sigma Chemicals, USA. POMA was synthesized from the monomer o-methoxy aniline using ammonium peroxydisulfate by a process described earlier.15 It was then deprotonated by stirring in a 0.1 M ammonium hydroxide solution for 48 h and was then dried in a vacuum at 30 °C for 5 days to get the emeraldine base (EB) form. The measured molecular weight of the polymer from the intrinsic viscosity measurement in 97% H2SO4 was 20 900.15,25 The emeraldine base form was converted into the emeraldine salt form by mixing 6 mL (N/1000) of an HCl solution with 30 mL of a 0.005% (w/v) POMA (EB) solution for 30 min. The solution of POMA-emeraldine salt (0.0042%, w/v) was transparent and greenish blue. The dopant concentration was calculated by drying the resultant solution, and the nitrogen/chlorine ratio was found from elemental analysis using a Perkin-Elmer series II, CHNS/O analyzer (model 2400). Blending with DNA. DNA (Na salt) solution (0.02%, w/v, pH ) 7.13, source water pH ) 6.88) was made by mixing the required amount of DNA in sterilized double distilled water for 2 h with occasional shaking. The blends of different compositions were prepared by mixing the above DNA solution with the POMA emeraldinesalt solution (0.0042% w/v, pH ) 5.05) in required proportions and left for 4 days at 30 °C. The pHs of the resultant solutions for the compositions (WDNA) 0.75, 0.5, and 0.25 are 6.4, 5.8,and 5.3, respectively. In this pH range, DNA does not denature.15 The mixtures were then frozen dried and were finally dried in a vacuum for 3 days. The samples were characterized by SEM, WAXS, etc. Characterization. For scanning electron microscopy (SEM) study, one drop of the above 3-day-aged solution was dropped on a microscopic cover slide and was dried in air at 30 °C and finally in a vacuum at 30 °C for 5 days. The SEM micrographs of the samples were made through a Hitachi SEM apparatus (S-2300). The wide-angle X-ray scattering (WAXS) study was performed on the above dried samples in a powder diffractometer (Philips, PW 1830). Nickel filtered Cu KR radiation was used in the work. The samples were taken in powder form, and the scan was made at a step size of 0.02° 2θ with 0.5 s per step. Spectroscopy. To characterize the doping behavior, the UV-vis spectroscopy of the sample solutions were performed in a quartz cell of thickness 1 cm from 190 to 1100 nm using a UV-vis spectrophotometer (Hewlett-Packard, model 8453) at 30 °C. A pure water spectrum was subtracted from the solution spectra. For the kinetics measurement, the experiment was started after 15 min of mixing and the spectra was taken for different aging times. The CD spectra of the aqueous solutions of POMA (ES), DNA, and POMA-DNA were made using a spectropolarimeter (JASCO, J-600) in 1 cm quartz cuvette. The POMA-DNA solutions were aged for 4 days before the experiment. Conductivity. The dc conductivity of the samples was measured by two-probe method in a nitrogen atmosphere. The sample was sandwiched between two indium-titanium oxide (ITO) conducting strips of 1 mm width placed perpendicularly (area, a ) 1 mm2), and the thickness (l) of the sample was measured using a screw gauge. The resistance of the sample was measured from a Keithley Electrometer (model 617) and the conductivity was calculated from the relation σ ) 1/R × 1/a.............
(i)
The I-V characteristics of the samples were studied using the same sample by applying voltage from -1 to +1 V, and the current was measured at each applied voltage.
Result and Discussion A. Morphology. In Figure 1a-c, the SEM pictures of the three compositions of the blends are presented. There is a gradation in morphology with increasing POMA concentration. As for (25) Jana, T.; Nandi, A. K. Langmuir 2000, 16, 3141.
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Figure 2. CD spectra of Na-DNA and POMA-DNA blends at indicated weight fractions of DNA and POMA (ES) solutions at 30 °C.
Figure 1. SEM micrographs of POMA-DNA hybrids (a) WDNA ) 0.75; (b) WDNA ) 0.5; (c) WDNA ) 0.25.
example in WDNA ) 0.75, the fibrils are clearly observed. On the other hand, in the micrographs of the sample WDNA ) 0.5, a fibrillar network structure is observed, and it is difficult to distinguish between DNA and POMA entities. The morphology is therefore due to the hybrid. For WDNA ) 0.25, the hybrid also exhibits fibrillar network morphology. Thus, in the system, a gradation of morphology from fibril to fibrillar network structure occurs due to addition of POMA. B. Circular Dichroism (CD). The CD spectra of the samples at different compositions are presented in Figure 2. From the figure, it is clear that DNA has positive peaks at the wavelengths 219 and 278 nm, but it also has a negative peak at the wavelength 248 nm. This type of CD spectra corresponds to the B-polymorphic structure of DNA.26 On the other hand, in the emeraldine salt (ES) form of POMA, there is no such characteristic peak in the CD spectra indicating the absence of any helical structure in it. So the Na-DNA has a double-helical structure, and POMA has a coil structure in the solution. In the hybrids of different (26) Sprecher, C. A.; Basse, W. A.; Johnson, W. C. Biopolymers 1979, 18, 1009.
compositions, the CD spectra of DNA is retained, indicating that the double helix structure of DNA remains intact in the hybrids.27 C. WAXS Pattern. In Figure 3, the WAXS patterns of the pure Na-DNA, POMA (ES), and hybrids are presented. It is apparent from the figure that the X-ray pattern of DNA is retained in all of the three hybrids, indicating the crystal structure remains intact during the hybrid preparation. In POMA rich hybrids, the diffraction peaks corresponding to POMA (ES) are also observed. It indicates that there are some unreacted POMA and they retain their own structure. However, it is evident that at WDNA ) 0.75 the characteristic peaks of POMA (ES) disappeared indicating that the POMA crystal structure might be lost but the DNA peaks are retained even in 25% DNA content hybrid. This observation certainly points out that the final structure of DNA remains intact throughout the whole compositions of the blend. D. UV-vis Spectra. The UV-vis spectra in aqueous medium of POMA-EB, POMA-ES, and the POMA-ES/Na-DNA blend (WDNA) 0.5) at different aging times are shown in Figure 4 (for other compositions see Figure 1a,b in the Supporting Information). From the figure, it is apparent that POMA-EB has two peaks at 321 and 599 nm. The former corresponds to the π-π* transition band of the benzonoid structure, and the later corresponds to excitation band of quinonoid ring of POMAEB.28-31 Converting POMA (EB) into POMA (ES) by adding HCl, the excitation band at 599 nm disappears and a new peak appears at 628 nm that corresponds to the π band to localized (27) Freifelder, D. Physical Biochemistry: Applications to Biochemistry and Molecular Biology, 2nd ed.; W. H. Freeman and Company: New York, 1982. (28) Magidi, M. R.; Ashraf, S. A.; Kane-Maguire, L. A. P.; Norris, I. D.; Wallace, G. G. Synth. Met. 1997, 84, 115. (29) Stejskal, J.; Kratochvı´l, P.; Radhakrishnan, N. Synth. Met. 1993 61, 225. (30) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995, 7, 443. (31) Ruokolainen, J.; Eerikainen, H.; Torkkeli, M.; Serimaa, R.; Jussila, M.; Ikkala, O. Macromolecules 2000, 33, 9272.
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Dawn and Nandi Table 1. UV-vis Peak Positions of POMA-DNA Hybrids at Different Blend Compositions for Different Aging Times at 30 °C polaron π bandband-π* polaron transition for band band π-π* aging base pair transition transition transition in DNA composition time (nm) (nm) (nm) (h) (nm) (WDNA) 0.25
0.50
0.75
Figure 3. WAXS patterns of Na-DNA and POMA-DNA hybrids at indicated weight fractions of DNA and POMA (ES).
Figure 4. UV-vis spectra of POMA (EB), POMA (ES), and POMA-DNA hybrid (WDNA ) 0.50) solutions at indicated aging times at 30 °C.
polaron band transition.30,31 The π-π* band also shifts from 321 to 325 nm for the emeraldine salt formation. The doping level was measured from the elemental analysis of the dried sample and was found to be [Cl]/[N] ) 0.518. It should be noted that the π-π* transition band of POMA (EB) shifts from 321 to 325 nm on 51.8% doping. The 4 nm red shift may be due to the formation of the benzonoid structure from the quinonoid structure of POMA (EB) for extending conjugation throughout
0.5 6 12 47 55 0.5 6 12 40 50 0.5 6 12 47 55
262 262 262 262 262 262 262 262 262 262 262 262 262 262 262
350 350 350 350 350 351 351 351 351 351 352 352 352 352 352
453 452 449 445 445 451 450 447 443 443 452 451 448 442 442
655 687 721 828 828 659 701 751 834 834 655 678 711 847 847
the chain.30 As POMA is in the coiled state, some kink may remain in the chain yielding lower conjugation length. Upon addition of Na-DNA to the POMA (ES) solution, the π-π* transition band increased from 325 to 351 nm instantaneously. No definite reason for this large red shift is known to us. Probably the stabilization of the π band of POMA during adsorption on DNA and decrease of kink structure of POMA chain in this process cause the π electron cloud to be more stable. It may also be probable that there is some type of interaction of the π cloud of DNA with the π cloud of POMA resulting in the formation of a stabler π cloud of POMA. This stabler π electron cloud has a lower π-π* transition energy showing a large red shift in the UV-vis spectra. The most important observation is that the π band to polaron band transition is a slow process in all three compositions. As for example, at room temperature, it takes about 40-47 h to attain a nonvariant peak for all of the compositions. The amount of red shift of the π band to polaron band transition peak depends on the composition of the blend. The nonvariance in the said peak position comes at 828, 834, and 847 nm for the blend compositions WDNA) 0.25, WDNA) 0.5, and WDNA) 0.75, respectively. In Table 1, the time variation of different peak positions for the three compositions of the blends at 30 °C are presented. It is apparent from the table that the transition band of DNA for its base pair at 262 nm and π-π* transition of benzonoid rings of POMA do not change with time. The former indicates that the DNA is not denatured under the mixing process, and it remained intact with aging time.27 The latter indicates that the π-π* band of the benzonoid rings of POMA (ES), though affected significantly with the addition of Na-DNA initially, do not change with aging time for all of the three blend solutions. The initial shift of the π-π* band of POMA (ES) on addition of Na-DNA is 25-27 nm for the three compositions of the hybrid. Such a large red shift may be attributed to the transformation of a significant amount of quinonoid structure into the benzonoid structure producing a large extended conjugation.30 Now we discuss the polaron band to π* band transitions which show a blue shift with aging time. The blue shift is about 8-10 nm for the three different compositions of the blend, and the red shift of π band to localized polaron band with time may be attributed to the blue shift of the polaron band to π* band transition. This has been clarified in Figure 5 where the energy levels of POMA (EB), POMA (ES), and POMA-DNA systems
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Figure 5. Schematic model of various energy states of POMA (EB), POMA (ES), and POMA (ES)/DNA.
are shown schematically. The downward arrow at the polaron band in the POMA-DNA system indicates that the polaron band shifts to lower energy with time until it is fixed after 47 h (dotted line). Here the polaron band is also wider to express the polydispersity in the conformational change of the POMA chain on the DNA surface. It is now reasonable to discuss the slowness of the red shift of the π band to polaron band transition. The POMA was doped by HCl to the level of ∼52% during the preparation of POMA (ES). So the slowness of the red shift of the π band to polaron band is not due to any primary doping process. The replacement of Cl- ions by DNA anions is fast as ionic reactions are usually very fast. So it may be argued that after POMA-DNA complex formation some physical process occurs causing the slow red shift of the π band to polaron band transition of the hybrid in the solution state. It may be argued that the nature of the physical process is such that it increases the conjugation length of POMA, causing a red shift. Obviously, the conformational change is the only process that may increase the conjugation length. The conformational change may occur through the repulsive interactions of radical cations (Scheme 1) bound on the DNA surface. Such a repulsive interaction might exist in HCl or H3PO4 doped POMA, but due to the small size of Cl- and PO43- ions, thermal fluctuation destabilizes any extension of chain, making the process an oscillating process. As a result, the UV-vis spectra of the POMA/H3PO4 system do not show any red shift with time.15 To support the above assertion, we have made a temperature dependent study of the aging of mixtures around room temperature such that any possibility of denaturation of DNA is minimized. A representative figure of UV-vis spectra for different aging temperatures at the same time of aging (e.g., 6 h) is presented in Figure 6. The figure clearly illustrates that there is a large shift of π band to polaron band transition peak from 666 nm at 20 °C to 729 nm at 40 °C for the blend composition WDNA ) 0.5. The other two hybrid compositions also exhibit similar behavior (Figure 2a,b in the Supporting Information). This red shift of the π band with an increase in temperature might be considered as thermochromism, and this phenomenon is already reported for P3HT.32 The red shift with increase in aging temperature (for same time of aging) certainly arises from the increased uncoiling of the POMA chain due to its conformational transition on DNA template. In Figure 7a-c, the wavelength of the π band to polaron band transition is plotted with the logarithm of aging time for different isothermal temperatures for all three hybrid compositions. In each case, at first there is a slow increase followed by a sudden rise, and finally, there is a leveling of the absorption peak with log time. This sigmoidal behavior is really interesting and signifies it as an autocatalytic process. Figure 7 also corroborates that with an increase in aging temperature the (32) Rughooputh S. D. D. V.; Hotta, S.; Heeger, A. J.; Wudl, F. J. Polym. Sci. Part B: Polym. Phys. 1987, 25, 1071.
Figure 6. UV-vis spectra of POMA-DNA hybrid (WDNA ) 0.50) solution at indicated temperatures for 6 h of aging.
uncoiling rate is faster; that is, at higher aging temperature, they shift in the backward direction. In other words, it may be called as the rate has a positive temperature coefficient. To analyze the temperature coefficient of the above thermochromism, the Arrhenius equation of rate constant (k) may be used
k ) A-E/RT
(ii)
where A is the frequency factor, E is the activation energy of the conformational transition responsible for uncoiling process, R is the gas constant, and T is the aging temperature. The rate constant k of the conformational change may be approximated as 1/τ70024,33 where τ700 is the time required to reach the absorption maxima at 700 nm in the UV-vis spectra of the samples. If the process obeys the Arrhenius equation, the ln(1/τ700) vs 1/T plot would be a straight line with negative slope. In Figure 8, the plots are shown and straight lines with negative slopes are obtained in each case. The correlation coefficient, least-squares slope, and intercept values are shown in Table 2. The correlation coefficient values suggest that the straight line fittings of the data are good, indicating that the Arrhenius equation is applicable in the physical (33) Mal, S.; Nandi, A. K. Polymer 1998, 39, 6301.
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Dawn and Nandi Table 2. Activation Energy, Intercept, and Correlation Coefficient Values of the Arrhenius Plots of POMA(ES)-DNA Hybrid Solutions at Different Blend Compositions composition (WDNA)
E (kcal/mol)
intercept
correlation coefficient
0.25 0.50 0.75
13.1 14.5 14.9
19.4 22.0 22.1
0.983 0.975 0.974
Table 3. Conductivity(σ) of POMA(ES)-DNA Hybrids at 30 °C
Figure 7. Wavelength of the π band to polaron band transition at 20 (9), 25 (O), 30 (2), and 40 °C (3) plotted against log t for POMA-DNA hybrid solutions (a) WDNA ) 0.25, (b) WDNA ) 0.5, and (c) WDNA ) 0.75.
Figure 8. Arrhenius plot of ln(1/τ700) vs 1/T for POMA-DNA hybrids (a) WDNA ) 0.25, (b) WDNA ) 0.5, and (c) WDNA ) 0.75.
process presented here. The frequency factor values are nearly same and a small tendency of its increase with increasing DNA concentration is observed. The least-squares slope value also increases with increasing DNA concentration in the blend. Thus the activation energy values are 13.1, 14.5, and 14.9 kcal/mol for the blends WDNA ) 0.25, 0.5, and 0.75, respectively. This activation energy value
WDNA
σ (S/cm)
WDNA
σ (S/cm)
0 0.25 0.50
3.2 × 10-5 8.9 × 10-6 5.3 × 10-6
0.75 1.00
9.8 × 10-7 1.3 × 10-10
is similar to the activation energy values for the conformational transition of common polymers. As for example, the activation energy for the conformational change of R-polymorph poly(vinylidene fluoride) (PVF2) is 9 kcal/mol, obtained the from potential energy calculation of Farmer et al.34 However the same measured from gelation kinetics of PVF2 in diethyl adipate is ∼15 kcal/mol.23 Shibaev et al. calculated the maximum energy change for the rotation of poly(3-hexyl thiophene) (P3HT) in the dihedral angle 0°-90° to be ∼15 kcal/mol,35 and the measured value of the conformational activation energy of P3HT from gelation kinetics in xylene is 23.7 kcal/mol.24 So the present activation energy values are on the same order with those of the other polymers. These results support that the above red shift of π band to polaron band transition occurs due to conformational transition, which promotes the uncoiling of POMA chain producing the slow red shift. The activation energy values of the conformational changes of POMA increase with DNA concentration indicating that the rate of such a transition decreases with increasing DNA concentration. Due to lesser concentration of radical cations the repulsive force required for conformational transition is lower in DNA rich blend causing an increase of activation energy. Thus it may be concluded that DNA surface is catalyzing the uncoiling process of adsorbed POMA arising from the repulsive interaction of radical cations. E. Conductivity. It is now interesting to discuss the conductivity values (Table 3). The POMA-ES has a 52% doping level and showed a conductivity of 3.2 × 10-5 S/cm. The reported conductivity of this polymer with HCl doping is usually
