Heat-induced coacervation of elastin and its possible thermoreversibility

Elastin (E) coacervation was studied close to isoelectric pH (pI = 4.7 ± 0.5), and this thermally activated self-assembly leading to phase separation involved the following three distinguishable temperatures: onset of interpolymer interaction at Tos, coacervation at Tc, and mesophase separation at Tms. Such behavior was absent in elastin solutions made at other pHs. The pH-dependent particle size histogram revealed bimodal distribution between pH 2 to 4 and 5.5 to 6.2, whereas the region pH = pI ± 0.5, a coacervate-rich domain with trimodal size distribution. The coacervation temperature TC decreased from 38 to 33 °C with increase in solution ionic strength (0–40 mM, NaCl) implying the importance of hydrophobic forces that governed the molecular self-assembly. The thermoreversibility of the turbidity profile implied that the elastin complexes did not fully dissociate into their monomeric state upon temperature reversal. Two observations were made here: (i) the hysteresis loop formed by the heating-cooling cycle had an area that decreased with ionic strength of the solution and (ii) the remnant aggregated E content increased with solution ionic strength. The present study highlights some of the key issues related to the reversibility of self-assembled soft matter systems which remains somewhat poorly explored until now.