The effects of terminal ion pairs on the stability of a beta-hairpin peptide corresponding to the C-terminal residues of the B1 domain of protein G were determined using thermal unfolding as monitored by nuclear magnetic resonance and circular dichroism spectroscopy. Molecular dynamics (MD) simulations were also performed to examine the effect of ion pairs on the structures. Eight peptides were studied including the wild type (G41) and the N-terminal modified sequences that had the first residue deleted (E42), replaced with a Lys (K41), or extended by an additional Gly (G40). Acetylated variants were made to examine the effect of removing the positive N-terminal charge on beta-hairpin stability. The rank in stability determined experimentally is K41 > E42 approximately G41 approximately G40 > Ac-K41 > Ac-E42 approximately Ac-G41 > Ac-G40. The Tm of the K41 peptide is 12 degrees C higher than G41, while the Tm values for the acetylated peptides are less than their unacetylated forms by more than 15 degrees C. NOE cross-peaks between side-chain methylene groups at the N- and C-termini and larger CalphaH shifts compared to random values are seen for K41. The addition of 20% methanol increases the stability in K41 and G41. The MD studies complement these results by showing that the charged N-terminus is important to stability. The type of ion pair observed varies with peptide, and when formed the simulations show that the ion pair can prevent fraying of the beta-strands through electrostatic and hydrophobic contacts. Therefore, introducing favorable electrostatic interactions at the N- and C-termini can substantially enhance beta-hairpin stability and help define the structure. Study holds ProTherm entries: 22038, 22039, 22040, 22041, 22042, 22043, 22044, 22045, 22046, 22047, 22048 Extra Details: B1 domain (N-terminal b-hairpin peptide is ADDYTWEK); Buffer used in the experiment is either 9:1 mixture of H2O and D2O or in 99.8% D2O in 5 mM sodium phosphate. beta-turn; ion pair; main-chain hydrogen bond; hydrophobic contacts; NMR; circular dichroism; molecular dynamic simulations
Submitter: Connie Wang
Submission Date: April 24, 2018, 8:53 p.m.
|Number of data points||11|
|Proteins||Immunoglobulin G-binding protein G ; Immunoglobulin G-binding protein G|
|Assays/Quantities/Protocols||Experimental Assay: Tm|
|Libraries||Mutations for sequence MTYKLILNGKTLKGETTTEAVDAATAEKVFKQYANDNGVDGEWTYDDATKTFTVTE|