Dynamics of Oligonucleotides Adsorbed on Thermosensitive Core−Shell Latex Particles

Abstract

The dynamics of a rhodamine X-labeled oligonucleotide composed of 25 mers of thymine (dT 2 5 -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 V P T ) at ∼32 °C, were studied as a function of temperature by fluorescence anisotropy. The time-resolved anisotropy decay of the dT 2 5 -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 o l i g o ) 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 2 5 -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 o l i g o ) without restrictions (heterogeneous population model) or restricted to the wobbling in a cone (wobbling-in-two-cones model). The θ o l i g o 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 o l i g o , with temperature from 0.38 (Φ o l i g o ∼ 60°) at 15 °C to 0.85 (Φ o l i g o ∼ 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.