Bonding, organization, and dynamical growth behavior of tripeptides on a defined metal surface: Tri-L-alanine and tri-L-leucine on Cu{100}

The behavior of the tripeptides tri-L-alanine and tri-L-leucine, deposited under ultrahigh vacuum conditions onto a clean Cu{110} surface, has been studied using reflection-absorption infrared spectroscopy and low-energy electron diffraction. Both peptides remain intact upon landing on the Cu{110}surface and are present in their anionic form. Bonding to the surface is through the terminal carboxylate ions (COO-) and amino groups (NH2) with the C=O functionalities of the amide groups (CONH) also involved in the process. Tri-L-alanine shows a complex range of adsorption phases which are sensitive to growth conditions. At high flux and with the substrate held at room temperature (300 K), three phases are identified. Phase I occurs at low coverage with tri-L-alanine molecules randomly adsorbed and isolated from each other. As coverage increases, phase II is formed which represents a monolayer with intermolecular hydrogen bonding occuring across the surface. At higher coverages, a saturated bilayer, phase III, is created with the perpendicularly oriented C=O functionalities of the amide groups being involved in strong interlayer II-bonding. There is evidence that, locally, phase III has strong similarities to the antiparallel P-sheet form of the solid crystal although no long range ordered surface structures are seen. Multilayers are formed under high flux conditions when the Cu{110} surface is cooled to 83 K. Tri-L-leucine bonds to the surface with its longer, bulkier side chains aligned along the surface normal which sterically inhibit phase III bilayer growth. Under low flux conditions, both molecules reorient after initial adsorption so that their amide C=O functionalities are more flat lying, possibly chelating to the surface, making it difficult to grow higher coverage phases.