A theoretical study of beta-sheet models: is the formation of hydrogen-bond networks cooperative?

The cooperativity in terms of enthalpy contribution for beta-sheet formation of polyglycine models in a vacuum has been studied theoretically by using a repeating unit approach. No cooperativity is found in the parallel direction for both the parallel and antiparallel beta-sheets. Cooperativity in the perpendicular direction is dependent upon the residue number (m) in each beta-strand. While there is large cooperativity in the acetamide hydrogen-bond chain (m = 0), the cooperativity is not large in beta-sheet networks (m > 0). SCIPCM solvent model calculations also significantly reduce the cooperativity in hydrogen-bond chains. It is concluded that cooperativity is mainly due to long-range electrostatic interactions and not due to the resonance effect.