Determination of Kinetic Characteristics of Dissociation of Complexes of Meta-Tetra(Hydroxyphenyl)Chlorin and Dextran-Poly-N-Isopropylacrylamide Copolymer by Fluorescence Spectroscopy

The ability of the thermosensitive copolymer dextran-poly-N-isopropylacrylamide (D-PNIPAM) to bind and release the photosensitizer (PS) meta-tetra(hydroxyphenyl)chlorin (mTHPC) was investigated using fluorescence spectroscopy techniques. It was shown that the efficiency of the copolymer to absorb mTHPC molecules depended on its phase state: mTHPC was efficiently adsorbed by D-PNIPAM (copolymer in globular conformation) at temperatures above 34–35°C, but the PS was practically not adsorbed on D-PNIPAM (copolymer in coil conformation) at temperatures below 34–35°C. The presence of additional structural rearrangements in the globule of D-PNIPAM under the action of high temperatures (>45°C) was shown using dynamic light scattering. The mTHPC dissociation rates from complexes with D-PNIPAM formed at 37–60°C were compared. The mTHPC release rate constants from the polymer globule were calculated. It was shown that complexation of mTHPC with D-PNIPAM at high temperatures (45–60°C) almost doubled the release rate of PS molecules from the copolymer globule at physiological temperatures as compared to similar complexes formed at 37–40°C. The time required for thermally induced structural rearrangements in D-PNIPAM was estimated. The globule-coil transition in the D-PNIPAM molecule caused by a temperature decrease below 34–35°C took less than a minute, while the reversibility of conformational changes in the copolymer globule when the medium temperature rose above 45°C required ~150 min. The results suggested that thermally dependent conformational rearrangements in the D-PNIPAM copolymer could be used for efficient regulation of the PS release rate in cellular and tissue systems.