Adsorption of cytosine, thymine, guanine and adenine on Cu(1 1 0) studied by infrared reflection absorption spectroscopy

The molecular internal vibration modes and orientation of the DNA base molecules (cytosine, thymine, guanine and adenine) adsorbed on copper(1 1 0) clean single crystal surface were investigated by infrared reflection adsorption spectroscopy in ultrahigh vacuum. The adsorption was performed by thermal evaporation of powder of each species onto Cu(1 1 0) at room temperature. The surface uptakes of the molecules were determined by Auger electron spectroscopy. Cytosine exhibited characteristic asymmetric and symmetric NH2 stretching signal. The asymmetric stretching signal was missing in the submonolayer region, indicating that the whole molecule was standing upright, with the NH2 group sticking out of the surface. Thymine was also proved to stand upright at surface. Guanine exhibited weakened infrared signals, indicating that the core purine ring was tilted on the surface, with an interaction of NHCO side of the molecule. Adenine at coverages below one monolayer gave no absorption signals, evidencing the molecules laid flat along the surface. The vibrational information obtained will be helpful in developing the future methods of surface DNA sequencing.