Proton Affinities of Some Amino Acid Side Chains in a Restricted Environment

We investigate the dependence of proton affinity values of the side chains of amino acids such as Asp, Glu, His, Ser, and Thr on confinement in a single-walled carbon nanotube. The proton affinity values, estimated using the density functional theories (PW91/dnp and BLYP/dnp), are found to be highly sensitive toward confinement. We find that for both Asp and Glu, the proton affinity, while suspended inside the carbon nanotube, becomes much less in comparison to their respective gas phase values. In the case of His, Ser, and Thr side chains, on the other hand, the proton affinity inside the carbon nanotube becomes negative. Hydrogen bonding with neighboring polar groups is found to result in a marked increase in proton affinity inside the tube in all of the cases reported in this article. The increase is most remarkable in the case of His, Ser, and Thr side chains where the presence of polar neighboring groups within a hydrogen-bonding distance is found to augment the proton affinity value by more than 100 kcal mol⁻¹.