Free energies of proton transfer by polar amino acid sidechain analogues anchored to the outer wall of single walled carbon nanotubes

Extensive empirical valence bond (EVB) simulation studies are reported on selected exohedrally functionalized derivatives of single walled carbon nanotube (SWCNT) in water. In each model studied, an analogue of a polar amino acid sidechain (such as Asp, His and Ser) and one hydroxyl group are covalently linked to two carbon atoms in close proximity on the SWCNT wall. We have estimated the free energies of reaction and activation associated with proton transfer between the sidechain analogue (donor) and the hydroxyl group (acceptor). All the sidechain analogues are found to be spontaneous proton acceptors. We also compare these free energy values to those in model systems where the proton donor and acceptor groups are endohedrally linked to the wall and extend into the hydrophobic core of the nanotube. Our results clearly quantify the effect of hydrophobic confinement on the proton transfer reaction studied.