Temperature and chemically induced denaturation comprise two of the most characteristic mechanisms to achieve the passage from the native state to any of the unstructured states in the denatured ensemble in proteins and peptides. In this work we present a full analytical solution for the configurational partition function of a homopolymer chain poly-X in the extended Zwanzig model (EZM) for a quasisigmoidal denaturation profile. This solution is built up from an EZM exact solution in the case where the fraction of native contacts follows exact linear dependence on denaturant’s concentration ; thus an analytical solution for in the case of an exact linear denaturation profile is also provided. A recently established connection between the number of potential nonnative conformations per residue and temperature-independent helical propensity complements the model in order to identify specific proteinogenic poly-X chains, where X represents any of the twenty naturally occurring aminoacid residues. From , equilibrium thermodynamic potentials like entropy and average internal energy and thermodynamic susceptibilities like specific heat are calculated for poly-valine (poly-V) and poly-alanine (poly-A) chains. The influence of the rate at which native contacts denature as function of on thermodynamic stability is also discussed. 1. Introduction The early recognition that ordered three-dimensional macromolecular structures, later known as the native state, play a fundamental role on biological activity of proteins and peptides led to a large interest in the fundamental mechanisms that drive formation and stabilization of such structures [1–4]. According to the classical view of protein folding, the native state is a unique three-dimensional structure in which the protein displays biological activity [1, 5]. More recent studies, however, have shown that the structure-function paradigm may have remarkable exceptions. A large class of proteins known as intrinsically disordered proteins (IDPs) is one of the most outstanding examples of this category. IDPs can show a high level of biological activity even if the protein is in one of the states associated with the disordered ensemble [6–9]. The transition from has been, accordingly, a subject of intense research. Temperature and chemically induced denaturation are two of the most common factors to induce such transitions being, the latter, probably the second most studied denaturation mechanism [10]. In order to chemically induce the transition, urea and guanidine hydrochloride (GdnHCl) are the most usual organic
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