Attachment of alanine and arginine to the Ge(100)-2x1 surface

The reactions of the amino acids alanine and arginine on the Ge(100)-2x1 surface are investigated using density functional theory. Arginine presents a unique reactivity not previously explored on semiconductor surfaces. Arginine dative bonds to the surface dimer through its imine lone pair, resulting in two adsorbed conformations with adsorption energies of 30.7 and 33.8 kcal/mol, significantly stronger than amine dative bonds on Ge(100)-2x1. We predict a N-H dissociation and an ene reaction pathway from each of these two conformers with the ene reactions kinetically favored over N-H dissociation of the imine N-H bond and the ene reaction from the cis state favored both kinetically and thermodynamically over the trans ene reaction. Our results for the reactions of the hydroxyl, carbonyl, and amine functional groups are similar to previous observations for analogous reactions of small organic molecules on Ge(100)-2x1. We find that O-H dissociation from the carbonyl dative bonded state is the most favorable pathway. The amine dative bonded state is slightly more stable than the carbonyl dative bonded state. We find that [2+2] cycloaddition through the CN double bond is exothermic by only 3.3 kcal/mol.