Electrostatic and hydrophobic interactions on the ODN adsorption process

M. L. CAROT; P. A. FIORITO; R. M. TORRESI; C. D. GARCÍA; C. E. GIACOMELLI

Montevideo, Uruguay

Congreso; 6th International Conference of Biological Physics (ICBP 2007). 5th Southern Cone Biophysics Congress; 2007

Nucleic acid hybridization is widely used in the detection of the specific complementary strand in molecular biology and applied fields, such as diagnostic in medicine. An ideal system for theses applications results when solid surfaces are combined with an oligodeoxyribonucleotide (ODN), a fragment of single strand DNA, to induce surface hybridization. Clearly, the capability of hybridization in such a system is controlled by the orientation and conformation of the adsorbed ODN molecules, which mainly depend on the surface properties and the solution conditions. The purpose of the present work is to study the adsorption behavior of synthetic oligo-ODN (20 bases) onto different solid surfaces. In view of the practical application, this study is especially focused on how this process is affected by the interactions between ODN molecules and surfaces and on how these biomolecule-surface interactions determine the adsorbed state of the ODN molecules. Information about the interactions involved was obtained by changing the physico-chemical properties of the system. Hydrophobic interactions were studied by comparing the adsorption on hydrophilic (silica) and hydrophobic (silica + CNT) surfaces. The electrostatic interactions were modulated by performing the adsorption experiments at different ionic strength with and without divalent cations. The surfaces were characterized by tapping mode AFM. The kinetic aspects of the adsorption process were studied by reflectometry, an optical technique which enables a continuous monitoring of the adsorbed amount in time, as a function of the ODN concentration. For an adequate analysis of the experimental results, an appropriate optical model considering the surface roughness and the adsorption mode was developed. The adsorption-desorption process of oligo-ODN is conditioned by the electrostatic ODN-surface interactions. On hydrophilic surfaces, adsorption only takes place under favorable electrostatic interactions. However, when the electrostatic repulsion is suppressed on the hydrophobic surface, ODN molecules adsorb with the bases (apolar region of the molecule) towards the surface. This orientation of the adsorbed ODN is unfavorable for the surface hybridization, indicating that to properly biofunctionalized the surface, hydrophobic interactions need to be minimized.