CoulombRepulsion in Short Polypeptides.

Coulomb repulsion between like-chargedside chains is presentlyviewed as a major force that impacts the biological activity of intrinsicallydisordered polypeptides (IDPs) by determining their spatial dimensions.We investigated short synthetic models of IDPs, purely composed ofionizable amino acid residues and therefore expected to display anextreme structural and dynamic response to pH variation. Two synergistic,custom-made, time-resolved fluorescence methods were applied in tandemto study the structure and dynamics of the acidic and basic hexapeptidesAsp6, Glu6, Arg6, Lys6, and His6between pH 1 and 12. (i) End-to-end distanceswere obtained from the short-distance Förster resonance energytransfer (sdFRET) from N-terminal 5-fluoro-l-tryptophan (FTrp)to C-terminal Dbo. (ii) End-to-end collision rates were obtained forthe same peptides from the collision-induced fluorescence quenching(CIFQ) of Dbo by FTrp. Unexpectedly, the very high increase of chargedensity at elevated pH had no dynamical or conformational consequencein the anionic chains, neither in the absence nor in the presenceof salt, in conflict with the common view and in partial conflictwith accompanying molecular dynamics simulations. In contrast, thecationic peptides responded to ionization but with surprising patternsthat mirrored the rich individual characteristics of each side chaintype. The contrasting results had to be interpreted, by consideringsalt screening experiments, N-terminal acetylation, and simulations,in terms of an interplay of local dielectric constant and peptide-lengthdependent side chain charge–charge repulsion, side chain functionalgroup solvation, N-terminal and side chain charge–charge repulsion,and side chain–side chain as well as side chain–backboneinteractions. The common picture that emerged is that Coulomb repulsionbetween water-solvated side chains is efficiently quenched in shortpeptides as long as side chains are not in direct contact with eachother or the main chain. [ABSTRACT FROM AUTHOR]