Femtosecond to Second Studies of a Water-Soluble Porphyrin Derivative in Chemical and Biological Nanocavities

The interactions of 5,10,15,20-tetrakis­(4-sulfonatophenyl)-porphyrin (TSPP) with a quaternary ammonium modified β-cyclodextrin (QA-β-CD) and human serum albumin (HSA) protein in aqueous solutions at pH 7 were studied using steady-state, stopped-flow, and femtosecond to millisecond spectroscopy. TSPP forms 1:1 and 1:2 complexes with QA-β-CD (<i>K</i>₁ = 1.9 × 10⁵ M^–1 and <i>K</i>₂ = 7 × 10³ M^–1) at 293 K, whereas with the HSA protein only 1:1 complex (<i>K</i>₁ = 1.7 × 10⁶ M^–1) has been found. The chemical and biological nanocavities have notable effects on the fluorescence lifetimes of the <i>Q</i>_<i>x</i> state (from 9.3 to 11.1 ns in QA-β-CD and 11.6 ns in HSA). Furthermore, the rotational times (400 ps for the free TSPP, 1.6 and 19 ns for QA-β-CD and HSA protein complexes, respectively) clearly indicate the robustness of the formed entities. The confined environment does not affect much the fs dynamics (0.1–0.2 ps) of the encapsulated molecule. However, it clearly affect the ps one (1–2 ps (H₂O) and 5–10 ps (QA-β-CD and HSA)). The effect of O₂ on the relaxation of the triplet state of the free and encapsulated TSPP is also studied and the obtained results are discussed in light of the shielding effect provided by the chemical and biological cavities. The observed difference, longer triplet lifetime upon encapsulation, might be relevant to the efficiency of this porphyrin in photodynamic therapy. The presteady-state kinetics of the TSPP:HSA has been studied by the stopped-flow spectrometer, and a two-step model was proposed for the complexation processes. The results show the importance of the initial association step for the overall ligand recognition process. This first step occurs with rate constant of ∼4 × 10⁵ M^–1 s^–1, which is about 5 orders of magnitude larger than the rate constant of the consecutive relaxation processes. We believe that our observations of molecular interaction between TSPP, QA-β-CD, and HSA protein from femtosecond to second at both ground and electronically first excited state give detailed information to improve our understanding of this kind of system and thus for a better design of drug delivery nanocarriers

Nucleophilic Substitutions of 6^I‑<i>O</i>‑Monotosyl-β-cyclodextrin in a Planetary Ball Mill

The tosyl group (Ts) on 6^I-<i>O</i>-(<i>p</i>-toluenesulfonyl)-β-cyclodextrin has been substituted with halogenides and nitrogen and sulfur nucleophiles under mechanochemical conditions, and the reaction has been investigated in this work. The preparation of monosubstituted cyclodextrin (CD) derivatives, such as azido-, thioureido-, iodo-, and thioethers, is shown to be more advantageous in a planetary ball mill (BM) than classic solution methods. All BM reactions displayed poor salt cation dependency, but a considerably stronger anion nucleophilicity effect has been observed. Although CD monoderivative syntheses were performed on a 100 milligram scale, the scalability of the method has been investigated and supported by the preparation of 6^I-monoazido-6^I-monodeoxy-β-CD

Structural Equilibrium in New Nitroxide-Capped Cyclodextrins: CW and Pulse EPR Study

Design of the new spin-labeled cyclodextrins can significantly extend the functionality of nitroxides. A series of new complexes based on fully methylated cyclodextrin (TRIMEB) covalently bound to the piperidine, pyrroline, pyrrolidine, and pH-sensitive imidazoline type nitroxides has been synthesized and studied using pulse and continuous wave electron paramagnetic resonance (EPR). The influence of the radical and linker properties on the structure of complexes formed has been investigated. Using the electron spin echo envelope modulation technique, we have analyzed quantitatively the accessibility of radicals to solvent molecules in studied complexes depending on the structure and length of the linkers. In all studied systems we observed different types of equilibria between conformations with radical fragment being outside the TRIMEB cavity and radical fragment capping the cavity of TRIMEB. The observed guest-induced shift of equilibrium toward the complex with radical capping TRIMEB cavity was explained by a change of macrocyclic configuration of TRIMEB. Complex with the −NH–CO– linker has been found most perspective for the applications requiring close location of nitroxide to the inclusion complex of TRIMEB. Using continuous wave EPR, we have shown that the pH-sensitive radical covalently bound to TRIMEB maintains its pH-sensitivity, but this complexation does not reduce radical reduction rate in the reaction with ascorbic acid