<p>Mixtures of bovine serum albumin and nanosized systems of the fluorescent dye methyl pheophorbide <i>a</i> based on amphiphilic copolymers of N-vinylpyrrolidone in phosphate-buffered saline were studied using UV/visible spectroscopy and dynamic light scattering. The experimental data obtained indicate the possibility of forming joint structures as a result of adsorption of albumin molecules on polymer particles. The quantum-chemical modeling performed allowed us to propose a structure and estimate the energy of intermolecular bonds of the guest molecule in the resulting nanoscale systems both on the particle surface and in hydrophobic regions. The binding energy of the albumin molecule to the surface of the polymer nanoparticle was estimated and a mechanism for the transition of the methyl pheophorbide molecule from the surface of the nanoparticle to one of the four albumin molecule binding sites found using molecular docking was proposed. Such a transition to the blood protein will facilitate the effective delivery of the active substance to target cells.</p>