Adsorption of Polyadenylic acid on graphene oxide: experiments and computer modeling.

In this work, we study the adsorption of poly(rA) on graphene oxide (GO) using AFM and UV absorption spectroscopies. A transformation of the homopolynucleotide structure on the GO surface is observed. It is found that an energetically favorable conformation of poly(rA) on GO is achieved after a considerable amount of time (days). It is revealed that GO can induce formation of self-structures of single-stranded poly(rA) including a duplex at pH 7. The phenomenon is analyzed by polymer melting measurements and observed by AFM. Details of the noncovalent interaction of poly(rA) with graphene are also investigated using molecular dynamics simulations. The adsorption of (rA)10 oligonucleotide on graphene is compared with the graphene adsorption of (rC)10. DFT calculations are used to determine equilibrium structures and the corresponding interaction energies of the adenine-GO complexes with different numbers of the oxygen-containing groups. The IR intensities and vibrational frequencies of free and adsorbed adenines on the GO surface are calculated. The obtained spectral transformations are caused by the interaction of adenine with GO.

Adsorption of poly(rA) on the carbon nanotube surface and its hybridization with poly(rU).

Adsorption of poly(rA) on a single-walled carbon nanotube surface in aqueous suspension and the subsequent hybridization of this polymer with free poly(rU) is studied. A comparison of the temperature dependence of the absorbance of free poly(rA) and poly(rA) adsorbed on the nanotube surface [poly(rA)(NT)] at nu(max)= 38,500 cm(-1) shows that the thermostability of the adsorbed polymer is higher. Molecular dynamics simulations demonstrate that more than half of the adenines are not stacked on the tube surface and some of them undergo self-stacking. After addition of a complementary poly(rU) to the poly(rA)(NT) suspension, a double-stranded polymer is formed as confirmed by the characteristic S-like form of its melting curve. However, the melting temperature of this polymer is lower than that of the free poly(rA)poly(rU) duplex. This result indicates that poly(rU) hybridization with poly(rA)(NT) occurs with defects along the whole length of the polymer because of pi-pi stacking between nitrogen bases and the nanotube surface, which hinders the usual hybridization process. Computer modeling demonstrates different possible structures of hybridized polymers on the nanotube surface.