Interaction of Amino Acids with Single-Walled Carbon Nanotubes: Insights from Density Functional Theory Calculations

Using the BLYP/DND level of density functional theory, we studied the non-covalent interaction of 20 proteinogenic L-amino acids with the closed-ended armchair (5,5) and zigzag (10,0) single-walled carbon nanotubes (ANT and ZNT, respectively). The calculated adsorption energies ranged from −0.16 kcal mol⁻¹ for alanine adsorbed on ANT, to −2.45 kcal mol⁻¹ for tryptophan adsorbed on ZNT. Aliphatic amino acids with no heteroatoms in the side chain showed no detectable changes in the distribution of molecular electrostatic potential upon adsorption. For amino acids containing heteroatoms in the side chain, an evident decrease in the negative lobes of electrostatic potential was detected, which was more evident for the adsorption on ZNT. HOMO and LUMO orbitals of the adsorption complexes remained essentially the same as in the isolated ANT and ZNT. Plotting the calculated binding energies versus sixteen series of hydrophobicity indexes systematically produced a random dispersion of data points, though in some cases it was possible to conclude that stronger adsorption generally correlates with higher hydrophobicity. Apparently, despite of the nanotube sidewalls are typical hydrophobic surfaces, the adsorption of amino acids takes place according to a mechanism different from classical hydrophobic interactions, or due to an interplay of several types of interactions with a limited contribution of hydrophobic mechanism.