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 st udied using reflection-absorption infrared spectroscopy and low-energy elec tron diffraction. Both peptides remain intact upon landing on the Cu{110}su rface and are present in their anionic form. Bonding to the surface is thro ugh 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 sensitiv e to growth conditions. At high flux and with the substrate held at room te mperature (300 K), three phases are identified. Phase I occurs at low cover age with tri-L-alanine molecules randomly adsorbed and isolated from each o ther. As coverage increases, phase II is formed which represents a monolaye r with intermolecular hydrogen bonding occuring across the surface. At high er coverages, a saturated bilayer, phase III, is created with the perpendic ularly oriented C=O functionalities of the amide groups being involved in s trong 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 bilaye r growth. Under low flux conditions, both molecules reorient after initial adsorption so that their amide C=O functionalities are more flat lying, pos sibly chelating to the surface, making it difficult to grow higher coverage phases.