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Langmuir 1999, 15, 2898-2900
Penetration of DNA into Mixed Monolayers of 1,3-Bis(salicylideneamino)propanechromium(III) Perchlorate and Octadecylamine at an Air/Water Interface R. Vijayalakshmi, A. Dhathathreyan, M. Kanthimathi, V. Subramanian, Balachandran Unni Nair, and T. Ramasami* Chemical Laboratory, Central Leather Research Institute, Chennai (Madras) 600 020, India Received June 23, 1998. In Final Form: January 21, 1999 The dynamics of surface pressure (π) of DNA adsorbed onto mixed monolayers of octadecylamine (ODA) with trans-diaquo-1,3-bis(salicylideneamino)propanechromium(III) perchlorate, [Cr(salprn)(H2O)2]ClO4, has been measured. The adsorption of DNA takes place only when a lower concentration limit of [Cr(salprn)(H2O)2]ClO4 is present in the spread film, indicating synergism due to the possible interaction of DNA with [Cr(salprn)(H2O)2]ClO4 after interacting with loosely or moderately packed ODA molecules through hydrogen bonds. The results point to an enhanced DNA adsorption by relatively dilute ODA monolayer densities and a decline of adsorption with dense lipid monolayer.
Introduction The interaction of metal complexes in terms of binding/ cleavage with DNA is of great interest.1-4 In this regard, a number of metal complexes have been used as structural probes of DNA in solution,5 as agents for strand scission6 of duplex DNA, and as chemotherapeutic agents.7 Binding studies of small molecules with DNA are very important in the development of new reagents which bind strongly to DNA. The effect of size, shape, hydrophobicity, and the charge on the binding of the complex to DNA has been studied by changing the type of heteroatomic ligand8,9 or metal center.10,11 In this context, two-dimensional organized monolayers of DNA mimics at air/water interface have proved to be ideal model systems for studying the above interactions.12 Weak intermolecular interactions (e.g., hydrogen bonds, van der Waals forces, and hydrophobic interactions) are indispensable architectural tools for assembling molecular organization.13,14 Double-helical DNA, one of the natural products of molecular organizations based on the specific intermolecular interaction, comprises one-dimensionally stacked base pairs, adeninethymine and guanine-cytosine, formed by complementary multiple hydrogen bonds. A hydrophobic interface of * E-mail:
[email protected] or
[email protected]. Fax: 91(44)4911589. (1) Farinas, E.; Tan, J. D.; Baidza; Mascharak, P. K . J. Am. Chem. Soc. 1993, 115, 2996. (2) Bailly, C.; Kenani, A.; Helbecque, N.; Bernier, J. L.; Houssin, R.; Henichart, J. P. Biochem. Biophys. Res. Commun. 1988, 152, 695. (3) Stubbe, J.; Kozarich, J. W. Chem. Rev. 1987, 87, 1107. (4) Yashimoto, Y.; Iijma, H.; Nozaki, Y.; Shudo, K. Biochemistry 1986, 25, 5103. (5) Barton, J. K. J. Biomol. Struct. Dyn. 1983, 1, 621. (6) Dervan, P. B. Science 1986, 232, 464. (7) Roberts, J. J.; Thomson, A. J. Prog. Nucleic Acid Res. Mol. Biol. 1979, 22, 71. (8) Sathyanarayana, S.; Dabrowiak, J. C.; Chaires, J. B. Biochemistry 1993, 32, 2573. (9) Pyle, A. M.; Rehmann, J. P.; Meshoyrer, R.; Kumar, C. V.; Turro, N. J.; Barton, J. K. J. Am. Chem. Soc. 1989, 111, 3051. (10) Barton, J. K. Science 1986, 233, 727. (11) Carter, M.; Rodriguiz, M.; Bard, A. J. J. Am. Chem. Soc. 1989, 111, 8901. (12) Shimomura, M.; Nakamura, F.; Ijiro, K.; Taketsuna, H.; Tanaka, M. Nakamura, H.; Hasebe, K. J. Am. Chem. Soc. 1997, 119, 2341. (13) Rebek, J., Jr. Acc. Chem. Res. 1990, 23, 399. (14) Zerkowski, J.; MacDonald, J.; Seto, C.; Wierda, D.; Whitesides, G. J. Am. Chem. Soc. 1994, 116, 2382.
a monolayer assembly formed on a water surface has been reported to be a sufficient environment for hydrogen bonding, even though the chemical substances are surrounded by a large number of water molecules.15,16 By systematic variations of the composition of mixed monolayers of different lipids in combination with suitable metal ligand, the relative contribution of the different intermolecular interaction may be characterized.17 In the present work, the adsorption of DNA at the solution/air interface in the presence of [Cr(Schiff- base)(H2O)2]X, where Schiff base ) trans-diaquo-1,3-bis(salicyldeneamino)propane, i.e., [Cr(salprn)(H2O)2]+ (shown below), in mixed monolayers of octadecylamine (ODA) has been monitored using surface pressure-molecular area (π-A) isotherms, surface pressure-time (π-t), and surface tension-time at constant areas.
Even though there are reports on the DNA binding/ cleaving properties of metalloporphyrins,18 bis(1,10phenanthroline)cobalt complexes,19 and octahedral complexes of Ru20 and Rh,21 there are no reported studies on the interaction of DNA with octahedral complexes of chromium. Hence, our focus is on the DNA binding properties with respect to the ligand environment of chromium compounds.22 Our aim in this work has been (15) Ahuja, R.; Caruso, P.; Mobius, D.; Paulus, W.; Ringsdorf, H.; Wildburg, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 352. (16) Cha, X.; Ariga, K.; Onda, M.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 11833. (17) Mobius, D.; Dhathathreyan, A.; Kozarac, Z.; Loschek, R.; Muller, A. In Biosensors: Applications in Medicine, Environment Protection and Process Control; VCH Publications; New York, 1990. (18) Pasternack, L.; Yang, R. F.; Gibbs, E. J.; Villafranca, J. J. Biochemistry 1983, 22, 2406. (19) Fin, L.; Yang, P. J. Inorg. Biochem. 1997, 68, 79. (20) Hartshorn, R. M.; Barton, J. K. J. Am. Chem. Soc. 1992, 114, 5919. (21) Hall, D. B.; Holmlin, R. E.; Barton, J. K. Nature 1996, 273, 475.
10.1021/la9807356 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/26/1999
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to study the nature of [Cr(salprn)(H2O)2]+/DNA interaction at the interface. In this work, we have investigated the incorporation of DNA molecules into monolayers containing octadecylamine and [Cr(salprn)(H2O)2]+ spread at constant area by spreading successive addition of aliquots of [Cr(salprn)(H2O)2]+ and thereby decreasing the area per molecule in steps. The interaction of DNA from the aqueous subphase into the monolayers has been measured in terms of penetration pressure (∆π) which represent an increment in initial surface pressure (πi). Materials and Methods Chemicals. Calf thymus DNA, purchased from Bangalore Genei Chemicals Ltd., India, was used as received. The concentration of DNA was determined spectrophotometrically by employing an extinction coefficient of 6600 cm-1 at 260 nm. Octadecylamine was purchased from Sigma Chemicals, USA. [Cr(salprn)(H2O)2]+ was prepared by an earlier reported procedure.23 Pure water was obtained from Milli Q Millipore system. The pH was adjusted to 7.0 using HEPES buffer, and its surface tension was 72.2 mN/m at 25 °C. Methanol and chloroform (HPLC grade) were used for spreading. Mixed Monolayers of Octadecylamine and [Cr(salprn)(H2O)2]+ at Constant Area. Both ODA and [Cr(salprn)(H2O)2]+ were spread from a solution of chloroform/methanol (3:1 v/v) by means of a micropipet (Microman Gilson; 50 µL) at the surface of water containing DNA (DNA concentration ) 5 × 10-6 M). The organic solvents were allowed to evaporate for at least 10 min. DNA was then allowed to adsorb from the unstirred subphase to the air/water interface. Two techniques were used to investigate the interaction of DNA with ODA and [Cr(salprn)(H2O)2]+. For the surface tension measurement, the Wilhelmy plate technique was used as follows: the force transducer was calibrated with the weight of the plate in the air. The weight of the meniscus was then recorded assuming zero contact angle. This weight was then related to the surface tension. The Wilhelmy plate (filter paper; 20 × 10 mm2) was configured so that it hung directly above a Teflon beaker. The aqueous solution of DNA of known concentration and volume was poured into the beaker. The liquid level was raised so that the solution came into contact with the Wilhelmy plate edge and a meniscus was formed. The surface tension was recorded continuously with the force transducer configured with a PC. Prior to the measurement, the filter paper was prewetted with the subphase buffer for at least 10 min. Dynamic surface pressure-molecular area (π-A) isotherms were measured using a trough from NIMA (model 611) with a Wilhelmy balance. The measurements were performed in a thermostat enclosure at T) 25 °C. The adsorption experiment, π versus time (π-t), at different [Cr(salprn)(H2O)2]+ concentrations were performed according to the procedure of Bos and Nylander.24 Here π denotes difference between γH2O+monolayer and γH2O. After the DNA solution was poured into the trough, the surface was cleaned by sweeping the interface with Teflon barrier and any surface active impurities were removed by suction of the surface film. ODA and [Cr(salprn)(H2O)2]+ were then spread directly onto the surface of DNA. The atmosphere over the trough was maintained in equilibrium with the vapor pressure of the solution. Experiments have been run with only the DNA in the subphase, with ODA and [Cr(salprn)(H2O)2]+ on water, and with only octadecylamine spread on DNA. The last experiment showed no change in the π value of the subphase with time indicating that there was no appreciable adsorption of DNA when only the amine was present in the subphase.
Results and Discussion Representative π-A isotherms recorded with the dynamic compression of ODA and [Cr(salprn)(H2O)2]+ are (22) Vijayalakshmi, R.; Kanthimathi, M.; Subramanian, V.; Nair. B. U.; Ramasami, T. Unpublished work. (23) Kanthimathi, M.; Nair, B. U.; Ramasami, T.; Shibahara, T.; Tada, T. Proc. Indian Acad. Sci. (Chem. Sci) 1997, 109, 235. (24) Bos, M. A.; Nylander, T. Langmuir 1996, 12, 2791.
Figure 1. π-A isotherms of (a) ODA on water (- - -), (2) ODA and [Cr(salprn)(H2O)2]+ on water (-‚-‚-), (3) ODA and [Cr(salprn)(H2O)2]+ on 5 × 10-6 M DNA (s).
shown in Figure 1. The isotherms for the pure ODA, ODA and [Cr(salprn)(H2O)2]+ on water, and ODA and [Cr(salprn)(H2O)2]+ on DNA showed a nearly liquid expanded type behavior. All of the isotherms were stable up to 45 mN/m. The concentration of [Cr(salprn)(H2O)2]+ in the mixed layer ranged from 0.005 to 0.024 mg/mL, and surface pressure was recorded as a function of time at constant area. The isotherms were recorded for the varying amounts of [Cr(salprn)(H2O)2]+ in the mixed layer. The solution/ air interface presents a model hydrophobic surface for which at low coverage the rate of adsorption is diffusion controlled, whereas at higher surface coverage the DNA is possibly able to create space in the spread film similar to proteins and enzymes,25 and penetrate and rearrange it so that the process according to Graham and Phillips26 is rate-determining. To evaluate the rate-determining steps, the control of adsorption of DNA at the solutionair interface, π-t curves, were used to construct the commonly used plots (Figure 2). The dependence of the adsorption kinetics on the rate constant k as expressed by the first-order equation follows the relaxation mechanisms analyzed traditionally.27
[(πi - π)/(πi - πc)] ) e-kT where πi is the initial pressure, πc is the final pressure, and π is the surface pressure at any time t. In Figure 2, at low concentration, there are three distinct steps, and the diffusion step is separated from the penetration step. However, as concentration increases, these two steps disappear with only penetration, and rearrangement due to interaction with [Cr(salprn)(H2O)2]+ is seen. Immediately following the spreading of ODA and [Cr(salprn)(H2O)2]+ on DNA (constant area ) 0.35 nm2/mole), there is a sudden decrease in the surface tension values as shown in Figure 3 which then decreased gradually until it reached a plateau at 40 mN/m (for concentration of [Cr(salprn)(25) Rosilio, V.; Boissonnade, M. M.; Zhang, J.; Jiang, L.; Baszkin, A. Langmuir 1997, 13, 4669. (26) Graham, D. E.; Phillips, M. C. J. Colloid Interface Sci. 1979, 70, 403. (27) Gaines, G. L. Insoluble Monolayers at Liquid/Gas Interfaces; Wiley: N. Y., 1966.
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Vijayalakshmi et al. Scheme 1
Figure 2. [(πi - π)/(πi - πc)] vs time for different concentrations of [Cr(salprn)(H2O)2]+ (a) 0.005, (b) 0.015, and (c) 0.024 mg/mL.
Figure 3. Variation of surface tension with time for different concentrations of [Cr(salprn)(H2O)2]+: (a) 0.005 (s), (b) 0.015 (- - -), and (c) 0.024 mg/mL (- ‚ -).
(H2O)2]+ 0.024 mg/mL in ca. 300 s and for 0.005 mg/mL after 3 h). The results clearly demonstrate that the rate of change is pronounced when the concentration of [Cr(salprn)(H2O)2]+ is above a threshold value.
The observed differences in the surface pressure as well as the surface tension values were attributed to the differences in the arrangement of the lipid molecules and [Cr(salprn)(H2O)2]+ induced by the compression. This can be explained as in Scheme 1. When ODA and [Cr(salprn)(H2O)2]+ are spread on DNA, ODA may form a physical complex with DNA molecules which then reacts with [Cr(salprn)(H2O)2]+. This may lead to the rearrangement of the closed, packed monolayer as seen in the increased DNA adsorption. In the present work, the presence of [Cr(salprn)(H2O)2]+ probably makes the adsorption irreversible. This irreversibility indicates the presence of a large activation barrier which hinders the desorption process. From these measurements, it is seen that DNA adsorption is time- and concentration-dependent. In conclusion, the interaction between [Cr(salprn)(H2O)2]+ and DNA causes the DNA to be preferentially adsorbed at the gas-liquid interface but only in the presence of the octadecylamine. The presence of amine headgroups near the interface possibly helps the DNA molecules to assemble in an organized manner through hydrogen bonds. Subsequent penetration and interaction with [Cr(salprn)(H2O)2]+ then takes place. The evolution of surface pressure with time is seen to have an initial lag phase which is more pronounced at lower concentration of [Cr(salprn)(H2O)2]+. The initial build up of π is due to the diffusion of DNA to the interface which is then followed by a much slower increase because of energy barrier to adsorption. Work is presently underway to understand the change in DNA structure at air/water interface using spectroscopic techniques. Acknowledgment. R.V. thanks C. S. I. R for a research fellowship. This research work was supported by No. 80(00022)/96-EMR-11. LA9807356
