PEE-PEO Block Copolymer Exchange Rate between Mixed Micelles Is Detergent and Temperature Activated.

We examine the kinetics of polymer chain exchange between mixed block copolymer-detergent micelles, a system relevant to the synthesis of protein-containing biomimetic membranes. While chain exchange between block copolymer (BCP) aggregates in water is too slow to observe, and detergent molecules exchange between micelles on a time scale of nanoseconds to microseconds, BCP chains exchange between mixed detergent-polymer micelles on intermediate time scales of many minutes to a few days. We examine a membrane-protein-relevant, vesicle-forming, ultrashort polymer, poly(ethyl ethylene)20-poly(ethylene oxide)18 (PEE20-PEO18). PEE20-PEO18 was solubilized in mixed micelles with the membrane protein compatible nonionic detergent octyl-β-D-glucoside (OG). Using cryo-TEM and small-angle neutron scattering (SANS), we demonstrate complete solubilization of the polymer into micelles. Using time-resolved SANS (TR-SANS), we provide the first direct evidence that detergents activate BCP chain exchange and determine kinetic parameters at two detergent concentrations slightly above the critical micellar concentration (CMC) of OG. We find that chain exchange increases 2 orders of magnitude when temperature increases from 35 to 55 °C and that even a 1 mg/mL increase in OG concentration leads to a noticeable increase in exchange rate. Our kinetic data are consistent with a single rate-limiting process rather than the distribution of exchange rates known to exist for BCPs in the absence of detergent, indicating a different exchange mechanism than the simple chain escape dominant in single-component micelles. Using the Arrhenius equation, we determine that at the detergent concentrations examined the activation energy for polymer chain exchange is only 2-3 times higher for PEE20PEO18 than for short-chained lipids and that the activation barrier decreases with increasing OG concentration. On the basis of these results, we postulate that mixed micelles exchange BCPs through a detergent-mediated process such as the fusion and fragmentation mechanisms also known to occur in micellar systems. These findings explain the need for high detergent concentration and/or temperature to synthesize polymer/protein membranes. Further, we postulate this is a more general phenomenon applicable to mixed micelle systems containing amphiphiles with vastly different solubilities and CMCs differing by many orders of magnitude. [ABSTRACT FROM AUTHOR]