Inelastic electron tunneling study of UV radiation damage in surface adsorbed nucleotides

Abstract

Ultraviolet radiation damage to nucleotides has been studied using inelastic electron tunneling (IETS) to monitor the direct bond damage as reflected in the vibrational mode intensity. The damage rate is found to be directly correlated to the resonance energy per π electron of the purine or pyrimidine base residue. Low lying ring modes are damaged most rapidly while modes associated with CH components are less rapidly damaged. The mononucleotides are damaged more rapidly than the corresponding bases, but no cleaving of the glycodidic bond between the base and sugar is observed. In the case of the purine base nucleotides double bond structure develops during UV exposure and is attributed to the more complex damage sequence of the base ring. The nucleotides are adsorbed on the alumina barrier of an Al–AlO_x–Pb tunnel junction and the PO⁻₂ group interacts with the surface. The majority of the other vibrational modes are unperturbed and indicate that the sugar and base rings form a complex on the surface of relatively fixed orientation. The IETS spectra of all nucleotides indicate a similar surface adsorption and this is not influenced by variation of the substrate temperature as is the case for molecules with a less constrained surface orientation. This unique surface configuration is discussed along with comparison to other molecules where temperature effects on the IETS spectrum are strong. Preliminary results of UV damage to polynucleotides and polypeptides is also reported. The role of the alumina surface structure and interaction with respect to the different types of molecules is also discussed.