We have investigated the relationship between protein size and the structural modification in concentrated aqueous solutions of ionic liquids (ILs) 1-alkyl-3-methylimidazolium nitrate ([C(n)mim][NO3] (n = 2, 4)). Proteins with the radii of gyration (R-g) above similar to 20 A form amorphous aggregates in an aqueous [C(4)mim][NO3] solution at X (mol% IL) = 20, whereas those with R-g values below similar to 20 angstrom assume partial globular states. Remarkably, a decrease in the alkyl-chain length causes an increase in the protein size at which protein aggregation occurs. Concentrated [C(n)mim][NO3] (n = 2, 4) solutions might control a boundary between the protein aggregation and partial globular states. (C) 2014 Published by Elsevier B.V.