Application of Time-Resolved Fluorescence Anisotropy To Probe Quinoline-Based Foldamers Labeled with Oligo(phenylene vinylene)

Probing the Closed Association of Oligoquinoline Foldamers by Time-Resolved Fluorescence Anisotropy

The metal-mediated dimerization of oligoquinoline foldamers terminated at one end with an oligo(phenylenevinylene) and at the other with a carboxylic acid (OPV-QnA, where n = 4, 8, 17, and 33), and the complexation of OPV-Q8A and Q16A was promoted in chloroform by the addition of a concentrated 16 M aqueous sodium hydroxide solution. UV-vis absorption and time-resolved fluorescence anisotropy (TRFA) experiments were conducted to determine, respectively, the concentration and the average rotational time ⟨ϕ⟩ of the mixture of unassociated and associated foldamers across a range of foldamer concentrations spanning 5 orders of magnitude. Plots of ⟨ϕ⟩ as a function of acid group concentration revealed that ⟨ϕ⟩ increased with increasing foldamer concentration only when the foldamer solution in chloroform was vigorously mixed with the 16 M sodium hydroxide aqueous solution. Furthermore, all plots showed that ⟨ϕ⟩ reached a plateau at high foldamer concentration. The increase in ⟨ϕ⟩ reflected the association of foldamers into larger objects through metal ion coordination with the carboxylate anions generated by deprotonation of the carboxylic acid of OPV-QnA with NaOH, while the plateau obtained at high foldamer concentration indicated that these interactions led to the dimerization of the foldamers via a closed association mechanism. Analysis of the ⟨ϕ⟩ trends yielded the equilibrium constants (K) describing the foldamer dimerization, whose value equaled 1.0 (±0.2) × 106 M-1 for the three longer OPV-QnA foldamers, but was about 10 times smaller for the shortest one (n = 4). Association of OPV-Q8A and Q16A yielded a complex with a ⟨ϕ⟩ matching that of OPV-Q24A, and K for this complexation was similar to that for dimerization. These experiments illustrate the robust nature of TRFA as an experimental method to probe the size of rigid, self-assembled foldamers in solution.

Simplification in the Acquisition and Analysis of Fluorescence Decays Acquired with Polarized Emission for Time-Resolved Fluorescence Anisotropy Measurements

This study introduces a global fluorescence decay analysis that substantially simplifies the acquisition and analysis of time-resolved fluorescence decays acquired with a vertically polarized excitation and vertically (I_VV(t)) and horizontally (I_VH(t)) polarized emission for time-resolved fluorescence anisotropy (TRFA) measurements. TRFA measurements were conducted whereby the I_VV(t) and I_VH(t) fluorescence decays of a series of oligoquinolines labeled at one end with an oligo(phenylenevinylene) dye (OPV-Q_n with n = 4, 7, 17, 24, 33) were acquired according to the standard protocol that is currently accepted in the scientific literature which involves toggling the emission polarizer before fitting linear combinations of the I_VV(t) and I_VH(t) decays or acquiring the I_VV(t) and I_VH(t) decays with static polarizers before fitting them globally. The rotational time (ϕ) and initial anisotropy (r₀) retrieved from these analyses were identical within experimental error regardless of whether the decays were acquired with toggling or static polarizers and fitted according to the standard protocol or globally. These experimental results were further supported by retrieving the parameters used to generate mono-, bi-, and tri-exponential TRFAs from the global analysis of simulated I_VV(t) and I_VH(t) fluorescence decays which were found to match perfectly the values that were inputted. Together, these experiments and simulations demonstrated that the parameters describing any type of TRFA can be extracted directly from the analysis of the I_VV(t) and I_VH(t) fluorescence decays acquired with a standard time-resolved fluorometer, a substantial simplification compared to the protocols currently in place to determine the TRFA.