In situ surface-enhanced infrared study of hydrogen bond pairing of complementary nucleic acid bases at the electrochemical interface

Surface-enhanced infrared absorption spectroscopy with a Kretschmann-type attenuated total reflection configuration has been used to study hydrogen-bonded pairing between 6-amino-8-purinethiol, a thiol-derivatized adenine, immobilized on a gold electrode surface, and thymidine, a complimentary base derivative of adenine, in 0.1 M NaCl04 aqueous solution as a function of applied potential. 6-Amino-8-purinethiol is adsorbed on a gold surface via a sulfur atom to form a S--Au bond. Nearly half of the adsorbed molecules are protonated, and the long axis of the adenine moiety is tilted from the surface normal at open circuit potential. As the potential increases, the acid--base equilibrium is shifted toward the unprotonated form and the adenine moiety is reoriented toward a nearly perpendicular configuration. The hydrogen bond interaction between the adsorbed 6-amino-8-purinethiol with thymidine in solution is greatly affected by the protonation and orientation of the adenine moiety and is controllable by the applied potential. Due to steric hindrance, an adenine-thymine-type hydrogen bond pair is formed only at potentials more positive than 0.1 V (vs SCE) where the unprotonated adenine moiety is perpendicularly oriented.