FRET-Based Determination of the Exchange Dynamics of Complex Coacervate Core Micelles

Abstract

Complex coacervate core micelles (C3Ms) are nanoscopic structures formed by charge interactions between oppositely charged macroions and used to encapsulate a wide variety of charged (bio)molecules. In most cases, C3Ms are in a dynamic equilibrium with their surroundings. Understanding the dynamics of molecular exchange reactions is essential as this determines the rate at which their cargo is exposed to the environment. Here, we study the molecular exchange in C3Ms by making use of Forster resonance energy transfer (FRET) and derive an analytical model to relate the experimentally observed increase in FRET efficiency to the underlying macromolecular exchange rates. We show that equilibrated C3Ms have a broad distribution of exchange rates. The overall exchange rate can be strongly increased by increasing the salt concentration. In contrast, changing the unlabeled homopolymer length does not affect the exchange of the labeled homopolymers and an increase in the micelle concentration only affects the FRET increase rate at low micelle concentrations. Together, these results suggest that the exchange of these equilibrated C3Ms occurs mainly by expulsion and insertion, where the rate-limiting step is the breaking of ionic bonds to expel the chains from the core. These are important insights to further improve the encapsulation efficiency of C3Ms.

DNA dynamics in complex coacervate droplets and micelles

“…The molecular exchange dynamics of complex coacervate core micelles determines how often the core components are exposed to the surroundings and in this way the level of protection that the C3M offers to a cargo that it encapsulates. We have previously used Förster resonance energy transfer (FRET) to measure this molecular exchange of C3Ms and observed a broad range of exchange rates, 22 similar to what has been observed earlier for other C3Ms. 23 We hypothesised that this large difference in exchange rates is the result of chain polydispersity.…”

“…The measured DNA exchange rate is relatively fast compared to earlier measurements of the molecular exchange of C3Ms: the nt22/PEG-pLL47 micelles exchanged within 2 minutes, which is much faster than the exchange rate that we measured for C3Ms with poly(3-sulfopropyl methacrylate) (PSPMA) and poly(ethylene glycol)-poly(2-trimethylammonioethyl methacrylate) (PEG-PTMAEMA) at similar salt concentrations, 22 where after 40 h the micelles were still not completely exchanged. The measured exchange rate of the nt22/PEG-pLL47 micelles is even faster than exchange rate of C3Ms with fluorescent proteins, 47 while the protein exchange measurements were performed very close to the critical salt concentration at which the C3Ms disassemble and the nt22/PEG-pLL47 micelles are performed far away from the critical salt concentration (their critical salt concentration is B0.6 M NaCl 48 and 1Â PBS contains 0.137 M NaCl).…”

“…S3, ESI †) and were therefore excluded from the data analysis, like we also did previously. 22 2.9 Fluorescence spectroscopy measurements…”

“…To explain the shape of the FRET-based exchange experiment curve for ssDNA/PEG-pLL micelles with atto488/atto532 labels in 0.2Â PBS solution, we have fitted the exchange experiment data with the analytical FRET model that we have developed recently. 22 Briefly, we have first determined the micelle geometric constant n by fitting the FRET efficiency at different fractions of labelled chains (Fig. S5, ESI †) to the function:…”

Universal Scaling for the Exit Dynamics of Block Copolymers from Micelles at Short and Long Time Scales

“…Interestingly, a recent article 33 has further explored the consistency of literature models based on polydispersity to explain their experimental results. The authors used Förster resonance energy transfer (FRET) to study the exchange dynamics of complex coacervate core micelles and found that the literature models required energies below the one expected for their system and speculate on the need for additional factors besides polydispersity to properly describe the observed experimental dynamics.…”