The prototypical adsorption system adenine/Cu(110) has been studied by core-level photoemission, x-ray absorption, and ab initio calculations. At low coverage, the molecule adsorbs nearly parallel to the surface and bonds by the interaction of one amino and one imino nitrogen atom.
At high coverage, the molecular plane is strongly tilted, and the bonding is markedly different. These findings resolve contradictions between calculated adsorbate geometries and published vibrational spectra and illustrate the complexity of the interaction between a relatively simple biomolecule and a metal.