Adsorption Free Energy of Single Amino Acids at the Rutile (110)/Water Interface Studied by Well-Tempered Metadynamics

Single amino acids are present in blood plasma and are the building blocks of larger organic residues. Their interaction with surfaces is therefore crucial for biomedical applications in contact with blood. In this work, we use well-tempered metadynamics to study the adsorption of six amino acids, with nonpolar (Ala and Leu), polar (Ser), positively charged (Arg and Lys), and negatively charged (Asp) side groups, on a negatively charged rutile (110) surface. The free energy of adsorption and the desorption barriers were determined for all the amino acids under different adsorption conformations. When using the center of mass as the collective variable in well-tempered metadynamics, results for different amino acids were difficult to interpret because of different adsorption conformations on the surface overlapping in collective variable space. After projecting onto separate collective variables for the backbone and the side group, much clearer trends were observable. We show that, on the negatively charged surface of rutile, adsorption via the backbone occurs for all the amino acids irrespective of their side group. Adsorption, driven via the side group, only occurs for the polar and charged side groups as opposed to the nonpolar side groups. This points to the importance of interactions of the side group with the strongly structured water layer rather than direct side group-surface interactions in determining the adsorption behavior.