  We analyze the influence of the charge and the degree of oxidation of the
surface of graphene (Gr) on its interaction with cytosine and oligonucleotide
r(C)10. This is a computational study involving DFT calculations and molecular
dynamics simulations. It is shown that cytosine interacts stronger with
graphene oxide (GO) than with Gr, while the energy of the interaction of
cytosine with GO only weakly depends on the degree of the Gr oxidation. A
correlation between the shifts of vibrational frequencies of cytosine due to
complexation with GO and the degree of the Gr oxidation is found. The
adsorption of anionic oligonucleotide r(C)10 onto neutral and positively
charged surfaces has a certain conformational similarity to conformations
formed with similar van der Waals interactions. Also, for charged surfaces, the
Coulombic attraction gives a noticeable contribution to the total interaction
energy. For a negatively charged graphene surface the electrostatic repulsion
between Gr surface and negatively charged phosphate backbone of oligonucleotide
weakens the total binding energy. Competition between the Coulombic repulsion
and the van der Waals attraction results in formation of a unique
oligonucleotide conformation where all 10 cytosines are stacked onto Gr.
