Solvent-Driven Control of Poly-l-Lysine and Poly-l-Glutamic Acid Properties and Multilayer Growth

Despite the widespread use of cosolvents to modulate polypeptide conformation, their solvent-specific role in coupling polypeptide charge regulation in solution along with interfacial multilayer growth and mechanics remains poorly understood. Here, we systematically investigate how three common cosolvents, namely ethanol, urea, and dimethyl sulfoxide (DMSO), alter the solution behavior of poly-l-lysine (PLL) and poly-l-glutamic acid (PGA) across a broad pH range (3.0-11.0) at controlled ionic strength. We also examine how the cosolvent and the pH effects govern PLL and PGA layer-by-layer assembly on silica surfaces. By combining electrokinetic measurements, circular dichroism spectroscopy, molecular dynamics simulations, and in situ mechanical characterization, we identify distinct, solvent-dependent mechanisms of charge compensation and structural disruption. Ethanol and DMSO primarily promote counterion condensation and partial chain dehydration, whereas urea compensates charge through direct hydrogen bonding to the polypeptide backbone, leading to a more pronounced loss of secondary structure. These molecular-level differences directly translate into multilayer architecture and mechanics and affect multilayer build-up. Ethanol yields thin, compact, and mechanically stiff layers, while urea and DMSO produce thicker, softer, and more dissipative layers. Overall, this work establishes a direct, solvent-specific link between polypeptide charge regulation in solution and the growth and mechanical properties of polypeptide multilayers, providing a versatile strategy to tailor multilayer architectures through solvent control alone.