Theoretical evidence for the stronger ability of thymine to disperse SWCNT than cytosine and adenine: Self-stacking of DNA bases vs their cross-stacking with SWCNT

Self-stacking of four DNA bases, adenine (A), cytosine (C), guanine (G) and thymine (T), and their cross-stacking with (5,5) as well as (10,0) single walled carbon nanotubes (SWCNTs) were extensively investigated with a novel hybrid DFT method, MPWB1 K/cc-pVDZ. The binding energies were further corrected with MP2/6-311++G(d,p) method in both gas phase and aqueous solution, where the solvent effects were included with conductor-like polarized continuum model (CPCM) model and UAHF radii. The strongest self-stacking of G and A takes displaced antiparallel configuration, but undisplaced or eclipsed antiparallel configuration is the most stable for C and T. In gas phase the self-stacking of nucleobases decreases in the sequence G > A > C > T. while because of quite different solvent effects their self-stacking in aqueous solution exhibits a distinct sequence A > G > T > C. For a given base, cross-stacking is stronger than self-stacking in both gas phase and aqueous solution. Binding energy for cross-stacking in gas phase varies as G > A > T > C for both (10,0) and (5,5) SWCNTs, and the binding of four nucleobases to (10,0) is slightly stronger than to (5,5) SWCNT by a range of 0.1-0.5 kcal/mol. The cross-stacking in aqueous solution varies differently from that gas phase: A > G > T > C for (10,0) SWCNT and G > A > T > C for (5,5) SWCNT. It is suggested that the ability of nucleobases to disperse SWCNT depends on relative strength (Delta Delta E-bin(sol)) of self-stacking and cross-stacking with SWCNT in aqueous solution. Of the four investigated nucleobases thymine (T) exhibits the highest Delta Delta E-bin(sol), which can well explain the experimental finding that T more efficiently functionalizes SWCNT than C and A.