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Thermodynamic analysis of the binding process of water-soluble negatively charged meso -tetrakis( p -sulfonatophenyl) (TPPS 4 ) and positively charged meso -tetrakis(4-methylpyridyl) (TMPyP) porphyrins with bovine serum albumin (BSA) at different temperatures was carried out based on the data of BSA quenching fluorescence by porphyrins. The comparison of binding constants ( K b ) shows that negatively charged TPPS 4 possesses higher affinity to BSA than positively charged TMPyP. Thermodynamic characteristics of the binding process were obtained in accordance with the van't Hoff theory by processing nonlinear dependences of ln K b on inverse absolute temperature within the framework of two models: taking into account the dependence or independence of the change in the standard heat capacity (Δ C 0 ) on temperature. A comparison of thermodynamic characteristics with the data obtained from the Förster fluorescence quenching theory and with literature data leads to the conclusion that TPPS 4 is bound to the Sudlow I site (subdomain IIA), while TMPyP is bound to the Heme site (between the subdomains IA and IB). The analysis of Δ C 0 changes with temperature demonstrates that binding of TPPS 4 promotes hydration of nonpolar groups in the protein, which increases with the increase of temperature, while binding of TMPyP decreases the hydration of polar groups of the protein, the effect increasing with rising temperature. The obtained information may be useful for elucidating the mechanisms of interaction of porphyrins with albumins and the effect of this interaction upon the effectiveness of porphyrins in photodynamic therapy and in fluorescence diagnostics of cancer., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Modern medicine has been searching for new and more efficient strategies for diagnostics and therapeutics applications. Considering this, porphyrin molecules have received great interest for applications in photodiagnostics and phototherapies, even as magnetic nanoparticles for drug-delivery systems and magnetic-hyperthermia therapy. Aiming to obtain a multifunctional system, which combines diagnostics with therapeutic functions on the same platform, the present study employed UV/vis absorption and fluorescence spectroscopies to evaluate the interaction between meso-tetrakis(p-sulfonatofenyl)porphyrin (TPPS) and maghemite nanoparticles (γ-Fe 2 O 3 ). These spectroscopic techniques allowed us to describe the dynamics of coupling porphyrins on nanoparticles and estimate the number of 21 porphyrins per nanoparticle. Also, the binding parameters, such as the association constants (K a = 8.89 × 10 5 M -1 ) and bimolecular quenching rate constant (k q = 2.54 × 10 14 M -1 s -1 ) were obtained. These results suggest a static quenching process where the electrostatic attraction plays an essential role. The work shows that spectroscopic techniques are powerful tools to evaluate the coupling of organic molecules and nanoparticles. Besides, the system studied provides a relevant background for potential applications in bionanotechnology and nanomedicine, such as (1) nanodrug delivery system, (2) photodiagnostics/theranostics, and/or (3) a combined action of photodynamic and hyperthermia therapies, working in a synergetic way.

The present work employs a set of complementary techniques to investigate the influence of outlying Ru(II) groups on the ground- and excited-state photophysical properties of free-base tetrapyridyl porphyrin (H(2)TPyP). Single pulse and pulse train Z-scan techniques used in association with laser flash photolysis, absorbance and fluorescence spectroscopy, and fluorescence decay measurements, allowed us to conclude that the presence of outlying Ru(II) groups causes significant changes on both electronic structure and vibrational properties of porphyrin. Such modifications take place mainly due to the activation of nonradiative decay channels responsible for the emission quenching, as well as by favoring some vibrational modes in the light absorption process. It is also observed that, differently from what happens when the Ru(II) is placed at the center of the macrocycle, the peripheral groups cause an increase of the intersystem crossing processes, probably due to the structural distortion of the ring that implies a worse spin-orbit coupling, responsible for the intersystem crossing mechanism.

The excited state dynamics of Zn2+, Fe3+, and Mn3+ meso-tetra(sulfonatophenyl) porphyrin complexes were investigated with a Z-scan technique at 532 nm using 70 ps and 120 fs single pulses and 200 ns pulse trains of a Q-switched and mode locked laser. We determined the characteristic interconversion and intersystem crossing times, quantum yields of the excited S1 state, and S1 --> Sn and T1 --> Tn transition cross-sections. The ground state cross-sections were obtained using UV-vis absorption spectroscopy, and a five-energy-level diagram was used to yield the photophysical parameters mentioned previously.