Structure, aggregation dynamics and crystallization of superfolder green fluorescent protein: Effect of long alkyl...

Green fluorescent protein (GFP) and its variants are widely used in medical and biological research, especially acting as indicators of protein structural integrity, protein-protein interactions and as biosensors. This study employs superfolder GFP (sfGFP) to investigate the impact of varying alkyl chain length of 1-C_n-3-methylimidazolium chloride ionic liquid (IL) series ([C_nmim]Cl, n = 2, 4, 6, 8, 10, 12) on the protein fluorescence, structure, hydration, aggregation dynamics and crystallization behaviour. The results revealed a concentration-dependent decrease in the sfGFP chromophore fluorescence, particularly in long alkyl chain ILs ([C₁₀mim]Cl and [C₁₂mim]Cl). Tryptophan (Trp) fluorescence showed the quenching rate increased with longer alkyl chains indicating a nonpolar interaction between Trp57 and the alkyl chain. Secondary structural changes were observed at the high IL concentration of 1.5 M in [C₁₀mim]Cl and [C₁₂mim]Cl. Small-angle X-ray scattering (SAXS) indicated relatively stable protein sizes, but with IL aggregates present in [C₁₀mim]Cl and [C₁₂mim]Cl solutions. Dynamic light scattering (DLS) data showed increased protein size and aggregation with longer alkyl chain ILs. Notably, ILs and salts, excluding [C₂mim]Cl, promoted sfGFP crystallization. This study emphasizes the influence of the cation alkyl chain length and concentration on protein stability and aggregation, providing insights into utilizing IL solvents for protein stabilization and crystallization purposes.