Thermodynamics of ideal proteinogenic homopolymer chains as a function of the energy spectrum E, helical propensity ω and enthalpic energy barrier

Jump transition observed in translocation time for ideal poly-X proteinogenic chains as a result of competing folding and anchoraging contributions

In this work we analyze the translocation of homopolymer chains poly-X, where X represents any of the 20 naturally occurring amino acid residues, in terms of size N and single-helical propensity ω. We provide an analytical framework to calculate both the free energy F of translocation and the translocation time τ as a function of chain size N, energies U and ε of the unfolded and folded states, respectively. Our results show that free energy F has a characteristic bell-shaped barrier as function of the percentage of monomers translocated. Inclusion of single-helical propensity ω associated to monomer X and chain's native energy ε in the translocation model increases the energy barrier ΔF up to one order of magnitude as compared with the well-known Gaussian chain model. Computation of the mean first-passage time as function of chain size N shows that the translocation time τ exhibits a significant jump of several orders of magnitude at a critical chain size N. This jump markedly slows down translocation of chains larger than N. Existence of the transition jump of τ has been observed experimentally at least in poly(ethylene oxide) chains [R. P. Choudhury, P. Galvosas, and M. Schönhoff, J. Phys. Chem. B 112, 13245 (2008)]JPCBFK1520-610610.1021/jp804680q. Our results suggest the transition jump of τ as a function of N may be a very well spread feature throughout translocation of poly-X chains.

Role of single-point mutations and deletions on transition temperatures in ideal proteinogenic heteropolymer chains in the gas phase

A coarse-grained statistical mechanics-based model for ideal heteropolymer proteinogenic chains of non-interacting residues is presented in terms of the size K of the chain and the set of helical propensities [Formula: see text] associated with each residue j along the chain. For this model, we provide an algorithm to compute the degeneracy tensor [Formula: see text] associated with energy level [Formula: see text] where [Formula: see text] is the number of residues with a native contact in a given conformation. From these results, we calculate the equilibrium partition function [Formula: see text] and characteristic temperature [Formula: see text] at which a transition from a low to a high entropy states is observed. The formalism is applied to analyze the effect on characteristic temperatures [Formula: see text] of single-point mutations and deletions of specific amino acids [Formula: see text] along the chain. Two probe systems are considered. First, we address the case of a random heteropolymer of size K and given helical propensities [Formula: see text] on a conformational phase space. Second, we focus our attention to a particular set of neuropentapeptides, [Met-5] and [Leu-5] enkephalins whose thermodynamic stability is a key feature on their coupling to [Formula: see text] and [Formula: see text] receptors and the triggering of biochemical responses.