The effect of protonation of cytosine and adenine on their interactions with carbon nanotubes

Placing electrical charges on nanomaterials is a means to extend their functional capabilities in nanoelectronics and sensoring applications. This paper explores the effect of charging nitrogen bases cytosine (Cyt) and adenine (Ade) via protonation on their noncovalent interaction with carbon nanotubes (CNT) using quantum chemical calculations performed at the M05-2X/6-31++G** level of theory alongside with a molecular graphics method. It is shown that the protonation of the bases causes threefold increase of the interaction energy in the CNT·Cyt·H⁺ and СNT·Ade·H⁺ complexes as compared to the CNT complexes formed with neutral bases. There is also some shortening of the base-CNT distance by ca 0.13 Ǻ. The visualization of the electrostatic potential distribution with the molecular graphics reveals that the positive potential due to the protonated bases extends to a cylindrical domain of the nanotube segment adjacent to the base binding site. Furthermore, subtraction of the electrostatic potential maps of the protonated bases from the maps of their complexes with CNTs reveals an area of negative potential on the CNT surface, which reflects the location of the adsorbed base. The positive charge transfer of ca 0.3 e from the protonated bases to the CNT strengthens the interaction in the CNT·Cyt·H⁺ and СNT·Ade·H⁺ complexes. The analysis of the frontier orbitals shows that the LUMOs of the complexes mainly reside on the CNT, while the HOMOs spread over both components of each complex. The observed effects may facilitate the design of nanomaterials involving nitrogen bases and CNTs.