Polymer principles of protein calorimetric two-state cooperativity

The experimental calorimetric two state criterion requires the van't Hoff e nthalpy Delta H-vH around the folding/unfolding transition midpoint to be e qual or very close to the calorimetric enthalpy Delta H-cal of the entire t ransition. We use an analytical model with experimental parameters from chy motrypsin inhibitor 2 to elucidate the relationship among several different van't Hoff enthalpies used in calorimetric analyses. Under reasonable assu mptions, the implications of these Delta H-vH's being approximately equal t o Delta H-cal are equivalent: Enthalpic variations among denatured conforma tions in real proteins are much narrower than some previous lattice-model e stimates, suggesting that the en -ergy landscape theory "folding to glass t ransition temperature ratio" T-f/T-g may exceed 6.0 for real calorimetrical ly two-state proteins. Several popular three-dimensional lattice protein mo dels, with different numbers of residue types in their alphabets, are found to fall short of the high experimental standard for being calorimetrically two-state. Some models postulate a multiple-conformation native state with substantial pre-denaturational energetic fluctuations well below the unfol ding transition temperature, or predict a significant post-denaturational c ontinuous conformational expansion of the denatured ensemble at temperature s well above the transition point, or both. These scenarios either disagree with experiments on protein size and dynamics, or are inconsistent with co nventional interpretation of calorimetric data. However, when empirical lin ear baseline subtractions are employed, the resulting Delta H-vH/Delta H-ca l's for some models can be increased to values closer to unity, and baselin e subtractions are found to correspond roughly to an operational definition of native-state conformational diversity. These results necessitate a reas sessment of theoretical models and experimental interpretations. (C) 2000 W iley-Liss, Inc.