Dynamics of Oligonucleotides Adsorbed on Thermosensitive Core−Shell Latex Particles

The dynamics of a rhodamine X-labeled oligonucleotide composed of 25 mers of thymine (dT 25 −ROX) in solution or adsorbed on the thermosensitive shell of poly(methyl methacrylate/ N -isopropylacrylamide), poly(MMA/NIPAM), core−shell nanoparticles, with a characteristic volume phase transition temperature ( T VPT ) at ∼32 °C, were studied as a function of temperature by fluorescence anisotropy. The time-resolved anisotropy decay of the dT 25 −ROX oligonucleotides in buffered solution (pH 5.5) is biexponential. The results are interpreted in terms of the “two-step” model, where the fast correlation time ( θ w ) is attributed to the restricted “wobbling” motion of the bound ROX to the oligonucleotide (ODN) strand in a cone, while the slow correlation time ( θ oligo ) is related to the rotation of the oligonucleotide as a whole. Both correlation times decrease with temperature due to the variation of the viscosity of the medium. The anisotropy decays of dT 25 −ROX adsorbed on the PNIPAM thermosensitive shell were interpreted in terms of two models: the “ wobbling-in-two-cones ” and the heterogeneous population model. In both models, the fast correlation time is due to the “wobbling” motion of ROX ( θ w ). The half-angle of the cone of the ROX “wobbling” motion ( φ ) decreases from ∼34° below the VPT to ∼11° above it, due to the increasing constraints imposed by the PNIPAM chains. The slow correlation time describes the overall rotational motion of the ODNs (θ oligo ) without restrictions (heterogeneous population model) or restricted to the “wobbling” in a cone (“wobbling-in-two-cones” model). The θ oligo values of the adsorbed ODNs are higher than in solution and increase with temperature from ∼2.6 ns at 15 °C to reach a constant value of ∼6.6 ns after the transition due to the increasing friction by the PNIPAM chains. The anisotropy decay shows a residual component at long times ( r ∞ ) that increases with temperature along the VPT, which is essentially due to the increasing percentage of immobilized ODNs from ∼14% at 15 °C to ∼70% above the transition (heterogeneous population model) or to the increase of the order parameter, S oligo, with temperature from 0.38 (φ oligo ∼ 60°) at 15 °C to 0.85 (φ oligo ∼ 27°) at 45 °C (“wobbling-in-two-cones” model). Both models show that the whole motion of the ODNs and the randomization of their orientations are strongly restricted along the volume phase transition of the PNIPAM shell.