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

Abstract

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(γ-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) viac_critical ≈ 0.16–0.25 for 30–50% humidity of the vapour phase to c_critical ≈ 0.03 for a vapour phase at 100% humidity. We attribute this change in solubility to complexation of water molecules with PBLGlu α-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.