Effect of Additives on Swelling of Covalent DNA Gels ² Diana Costa,* ,‡,§ M. Grac ¸ a Miguel, ‡ and Bjo 1 rn Lindman ‡,§ Physical Chemistry 1, Centre for Chemistry and Chemical Engineering, Lund UniVersity, Box 124, S-22100 Lund, Sweden, and Department of Chemistry, UniVersity of Coimbra, Coimbra, Portugal ReceiVed: NoVember 29, 2006; In Final Form: March 6, 2007 The volumetric response of polymer gels on cosolute addition depends on the interaction of the polymer with the cosolute and can be used as a simple and sensitive way of elucidating these interactions. Here we report on DNA networks, prepared by crosslinking double-stranded DNA with ethylene glycol diglycidyl ether (EGDE); these have been investigated with respect to their swelling in aqueous solution containing different additives, such as metal ions, polyamines, charged proteins, and surfactants. The deswelling on addition of metal ions occurs at lower concentrations with increasing valency of the counterion. The collapse of the gels in the presence of trivalent ions seems to follow the same kind of mechanism as the interaction in solution, but addition of these ions leads to DNA denaturation and formation of single-stranded DNA. Striking features were found in the deswelling of DNA gels by chitosan, spermine, spermidine, lysozyme, poly-L-lysine and poly-L-arginine. Chitosan is the most efficient cosolute of those investigated with respect to DNA gel collapse. The effect of the cationic surfactant tail length on the volume phase transition of DNA gels was studied as a function of surfactant concentration. Cationic surfactants effectively collapsed the gel from the critical aggregation concentration (cac), decreasing with increasing length of the hydrophobic tail. In several cases, the deswelling as a function of cosolute concentration shows a pronounced two-step behavior, which is interpreted in terms of a combination of DNA chain condensation and general osmotic deswelling. The studies included investigations on the state of the DNA chain after deswelling, on the reversibility of the deswelling as well as on the kinetics. With the exception for the trivalent lanthanide ions, it appears that the DNA chain always retains a double-helix conformation; with these metal ions, single-stranded DNA is found. The deswelling appears to be reversible as exemplified by addition of anionic surfactant subsequent to gel collapsed by cationic surfactant and addition of sodium bromide to gels collapsed by a polycation. An investigation of the kinetics shows that an increase in the surfactant tail length gives a pronouncedly slower deswelling kinetics. Introduction Polymer gels that respond to changes in the surrounding environment with a volume transition, often referred as respon- sive gels, have attracted much interest in the past few years. 1-8 In this group are the polyelectrolyte gels, which consist of charged polymer networks, counterions, and solvent and are usually synthesized by chemically cross-linking charged or titrating polymers. The environmental conditions include changes in different parameters such as pH, 6,9-11 solvent composition, 12 ionic strength, 13 temperature, 14,15 pressure, 16 buffer composi- tion, 17 chemicals, 18 surfactants, 19-28 and photoelectric stimuli. 29 Because of their significant swelling and syneresis in response to external stimuli, these polymeric networks are used for a variety of applications such as contact lenses, 30 diapers, wround dressing, membrane materials, pharmaceutical products, 31 mono- lithic drug delivery systems, 32 chromatography packing materi- als, and agriculture. 33 Moreover, cross-linked gels have been investigated for many biomedical uses, including tissue culture, 34 enzyme activity controlling systems, 35 and materials for im- proved biocompatibility, 36 in the design and analysis of artificial muscles and biosensors and in the design of intelligent controlled drug release devices for site-specific drug delivery. The scientific interest is due to the complex interplay of counteracting forces, which are of equal order of magnitude and which determine the swelling of the gels. 36 The main contributions are the osmotic pressure originating from the electrostatically confined small ions inside the gel, 37 the effective electrostatic interactions between the charged groups 38-41 and hydrophobic interactions. Factors such as the nature of the polymer, the polymer-solvent compatibility, the degree of cross-linking, the nature of the buffer composition, and the nature of the counterion as well play an important role in determining the swelling behavior. 42-44 Complexes of polyelectrolyte gels with oppositely charged surfactants were intensively studied in the past decade 19-28 due to their interesting practical applications. Phase coexistence has been observed during volume transitions of polyelectrolyte gels following the uptake of surfactant ions from the solution. 45-47 Here the transition to the collapsed state is promoted by the favorable electrostatic interaction between the polyion and surfactant micelles. No important differences between the ordered micelle structures formed with cross-linked and linear polyions have been reported so far. However, the structure and stability of the phases have important consequences for swelling and volume transition dynamics in gels. Surfactants are not the only class of molecules interacting strongly with DNA. For instance, the binding of charged proteins, as well as poly-L-lysine and poly-L-arginine, multi- ² Part of the special issue “International Symposium on Polyelectrolytes (2006)”. * Corresponding author. Address: Departamento de Quı ´mica, Univer- sidade de Coimbra, 3004-535 Coimbra, Portugal. Fax: +351- 239 82 7703. E-mail: diana.costa@fkem1.lu.se. ‡ Lund University. § University of Coimbra. 8444 J. Phys. Chem. B 2007, 111, 8444-8452 10.1021/jp067917q CCC: $37.00 © 2007 American Chemical Society Published on Web 05/10/2007 Downloaded by PORTUGAL CONSORTIA MASTER on June 29, 2009 Published on May 10, 2007 on http://pubs.acs.org | doi: 10.1021/jp067917q