COVALENT IMMOBILIZATION OF DNA ONTO GLASSY-CARBON ELECTRODES

Denatured calf thymus deoxyribonucleic acid (DNA) and synthetic poly(dG)poly(dC) have been covalently immobilized onto oxidized glassy carbon electrode surfaces using the water-soluble carbodiimide, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide. Immobilized DNA has been detected using the complex Co(bpy)3(3+) (bpy = 2,2'-bipyridine). This complex has a reversible one-electron oxidation (E-degrees' = 0.33 V versus SHE), binds electrostatically in the minor groove of the DNA double helix, and preconcentrates at the surface of the DNA-modified electrode prior to a voltammetric scan. Our cyclic voltammetric data indicate that larger Co(bpy)3(3+/2+) signals are observed at low ionic strength; this is consistent with the larger association constants obtained for the binding of Co(bpy)3(3+) with calf thymus DNA at low ionic strength. While immobilizations were attempted for four species, poly(dA)poly(dT), native calf thymus DNA, denatured calf thymus DNA, and poly(dG)poly(dC), only the last two species could be successfully immobilized with the carbodiimide procedure; these results show that guanosine and/or cytosine bases will bind covalently to the o-acylisourea on the glassy carbon surface.