The influence of protic non-solvents present in the environment on structure formation of poly(γ-benzyl-L-glutamate) in organic solvents

We present an experimental study of structure formation in a polypeptide hetero-arm star block copolymer solution, obtained by swelling thin films in chloroform solvent vapor to variable poly(gamma-benzyl-L-glutamate) (PBLGlu) concentrations (c(p)). Direct observation by optical microscopy allowed us to follow in real time nucleation and growth of ordered three-dimensional structures of ellipsoidal shape. At low c(p), growth stopped when c(p) decreased below the solubility limit (c(critical)) but additional structures were formed when c(p) was rapidly increased to a higher value. Although water is not a solvent for this polymer, we demonstrate that water, even in trace amounts, is nonetheless considerably affecting solubility and consequently the process of structure formation. We have varied systematically the amount of water present in the environment. c(critical) changed from about 0.53 (dry, i.e. desiccated surrounding vapour phase) via c(critical) approximate to 0.16-0.25 for 30-50% humidity of the vapour phase to c(critical) approximate to 0.03 for a vapour phase at 100% humidity. We attribute this change in solubility to complexation of water molecules with PBLGlu alpha-helix, which increases the interfacial tension between the polymer and the solvent. We have tested our hypothesis by replacing water with other non-solvents for the polymer. Only protic non-solvents changed the solubility of PBLGlu in chloroform.