title: Coulomb repulsion in short polypeptides.
abstract: Coulomb repulsion between like-charged side chains is presently viewed as a major force that impacts the biological activity of intrinsically disordered polypeptides (IDPs) by determining their spatial dimensions. We investigated short synthetic models of IDPs, purely composed of ionizable amino acid residues and therefore expected to display an extreme structural and dynamic response to pH variation. Two synergistic, custom-made, time-resolved fluorescence methods were applied in tandem to study the structure and dynamics of the acidic and basic hexapeptides Asp6, Glu6, Arg6, Lys6, and His6 between pH 1 and 12. (i) End-to-end distances were obtained from the short-distance Förster resonance energy transfer (sdFRET) from N-terminal 5-fluoro-l-tryptophan (FTrp) to C-terminal Dbo. (ii) End-to-end collision rates were obtained for the same peptides from the collision-induced fluorescence quenching (CIFQ) of Dbo by FTrp. Unexpectedly, the very high increase of charge density at elevated pH had no dynamical or conformational consequence in the anionic chains, neither in the absence nor in the presence of salt, in conflict with the common view and in partial conflict with accompanying molecular dynamics simulations. In contrast, the cationic peptides responded to ionization but with surprising patterns that mirrored the rich individual characteristics of each side chain type. The contrasting results had to be interpreted, by considering salt screening experiments, N-terminal acetylation, and simulations, in terms of an interplay of local dielectric constant and peptide-length dependent side chain charge-charge repulsion, side chain functional group solvation, N-terminal and side chain charge-charge repulsion, and side chain-side chain as well as side chain-backbone interactions. The common picture that emerged is that Coulomb repulsion between water-solvated side chains is efficiently quenched in short peptides as long as side chains are not in direct contact with each other or the main chain.
authors.affiliation: Department of Life Sciences and Chemistry, Jacobs University Bremen , Campus Ring 1, D-28759 Bremen, Germany.
authors.forename: Amir
authors.initials: A
authors.lastname: Norouzy
authors.forename: Khaleel I
authors.initials: KI
authors.lastname: Assaf
authors.forename: Shuai
authors.initials: S
authors.lastname: Zhang
authors.forename: Maik H
authors.initials: MH
authors.lastname: Jacob
authors.forename: Werner M
authors.initials: WM
authors.lastname: Nau
mesh_terms: D031541:Fluorescence Resonance Energy Transfer; D056004:Molecular Dynamics Simulation; D015394:Molecular Structure; D010455:Peptides / Q000138:chemical synthesis / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D010455:Peptides