Journal
ACS APPLIED MATERIALS & INTERFACES
Volume 6, Issue 15, Pages 13242-13256Publisher
AMER CHEMICAL SOC
DOI: 10.1021/am5031177
Keywords
protein nanopore; single-molecule electrophysiology; peptide folding; histidine
Funding
- National Research Foundation of Korea (NRF) - Korea government (MEST) [2011-0017532]
- [PN-II-ID-PCCE-2011-2-0027]
- [PN-II-PT-PCCA-2011-3.1-0595]
- [PN-II-PT-PCCA-2011-3.1-0402]
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Inter-amino acid residues electrostatic interactions contribute to the conformational stability of peptides and proteins, influence their folding pathways, and are critically important to a multitude of problems in biology including the onset of misfolding diseases. By varying the pH and ionic strength, the inter-amino acid residues electrostatic interactions of histidine-containing, beta-hairpin-like peptides alter their folding behavior, and we studied this through quantifying, at the unimolecular level, the frequency, dwell-times of translocation events, and amplitude of blockades associated with interactions between such peptides and the alpha-hemolysin (alpha-HL) protein. Acidic buffers were shown to dramatically decrease the rate of peptide capture by the alpha-HL protein, through the interplay of enthalpic and entropic contributions brought about on the free energy barrier, which controls the peptides-alpha-HL association rate. We found that in acidic buffers, the amplitude of the blockage induced by an alpha-HL, beta-barrel-residing peptide is smaller than the value seen at neutral pH, and this supports our interpretation of the pH-induced change in the conformation of the peptide, which behaves as a less-stable hairpin at acidic pH values that obstructs, to a lesser extent, the protein pore. This is also confirmed by the fact that the dissociation rate of such model peptide from the alpha-HL's beta-barrel is higher at acidic, as compared to neutral, pH values. Experiments performed in low-salt buffers revealed the dramatic decrease of the peptide capture rate by the alpha-HL protein, most likely caused by the increase in the radius of counterions cloud around the peptide that hinders peptide partition into the alpha-barrel, and histidines protonation at low pH bolsters this effect. Reduced electrostatic screening in low-salt buffers, at neutral pH, leads to a decrease in peptides effective cross-sectional areas and an increase of their mobility inside the alpha-HL pore, due most likely to the chain stretching augmentation, via increased inter-residues electrostatic interactions.
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