Aqueous two-phase system formation kinetics for elastin-like polypeptides of varying chain length

The kinetics of aqueous two-phase system (ATPS) formation for elastin-like polypeptides (ELP) with defined chemical composition and chain length was investigated by dark field microscopy in an on-chip format with a linear temperature gradient. Scattering intensities from peptide solutions in the presence and absence of sodium dodecyl sulfate (SDS) were recorded as a function of temperature and time, simultaneously. It was found that the formation of the ATPS for three ELPs of different molecular weights (36 075, 59 422, and 129 856 Da) in the absence of SDS followed a coalescence mechanism, and the rate constant and activation energy were independent of chain length. With the introduction of SDS into the ELP solutions, the rate constants were attenuated more strongly with increasing chain length. Moreover, the coalescence process in the presence of SDS showed non-Arrhenius kinetics as a function of temperature. For the two shorter ELPs, ATPS formation occurred via coalescence at all SDS concentrations and temperatures investigated. On the other hand, the coalescence process was greatly suppressed for the longest ELP at elevated temperatures and higher SDS concentrations. Under these circumstances, ATPS formation was forced to proceed via a mixed Ostwald ripening and coalescence mechanism.