The immobilisation of medicinal substances, in particular antibiotics of the anthracycline series, on the surface of nanosized carriers for the targeted delivery of drugs to target organs or target tissues allows the creation of an optimal concentration of the drug in the area of therapeutic effect. Doxorubicin is a drug that interacts with DNA and is a common component of chemotherapy regimens. The toxic effect of doxorubicin represents a significant challenge to the implementation of highly effective cytostatic chemotherapy, providing a compelling rationale for treatment cessation even before the attainment of a clear antitumour effect. In particular, nanoscale carbon materials, such as carbon nanotubes (CNTs), are emerging as promising auxiliary substances. Nevertheless, the particulars of the interaction between doxorubicin and CNTs at the atomic level remain insufficiently understood. It is therefore important to investigate the energy parameters of the interaction between single-walled CNTs and doxorubicin in its various protolytic forms, which exist at different pH values in aqueous media, using quantum chemistry methods. Furthermore, it is also important to investigate how the diameter of CNTs affects the adsorption properties of doxorubicin in different protolytic forms. The results of the quantum chemical calculations indicate that all values of ΔH298 for intermolecular interactions are negative, which suggests that the adsorption process for all considered protonated forms of doxorubicin on the outer surface of the nanotube is thermodynamically self-activating, irrespective of the nanotube diameter. At pH values below 7, the protonated form of doxorubicin exhibits the greatest enthalpy of adsorption on CNTs, irrespective of the diameter of the carbon nanotube fragment. As the diameter of the carbon nanotube increases, the intermolecular interaction energy rises for both the molecular and protonated forms of doxorubicin. The lowest value of the enthalpy of interaction was observed for the molecular form of doxorubicin and the smallest CNT (diameter 10 Å). Conversely, the highest value of the interaction enthalpy was recorded for the protonated form of doxorubicin and the maximum size CNT (diameter 20 Å).
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