Finite size effects on calorimetric cooperativity of two-state proteins

Finite size effects on the calorimetric cooperatity of the folding-unfolding transition in two-state proteins are considered using the Go lattice models with and without side chains. We show that for models without side chains a dimensionless measure of calorimetric cooperativity κ2 defined as the ratio of the van't Hoff to calorimetric enthalpy does not depend on the number of amino acids N. The average value κ2¯≈34 is lower than the experimental value κ2≈1. For models with side chains κ2 approaches unity as κ2∼Nμ, where μ≈0.17. Above the critical chain length Nc≈135 these models can mimic the truly all-or-non folding-unfolding transition.