Structural Basis for the
Enhanced Stability of Protein
Model Compounds and Peptide Backbone Unit in Ammonium Ionic Liquids
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Abstract
Protein folding/unfolding is a fascinating study in the
presence
of cosolvents, which protect/disrupt the native structure of protein,
respectively. The structure and stability of proteins and their functional
groups may be modulated by the addition of cosolvents. Ionic liquids
(ILs) are finding a vast array of applications as novel cosolvents
for a wide variety of biochemical processes that include protein folding.
Here, the systematic and quantitative apparent transfer free energies
(Δ<i>G</i>′<sub>tr</sub>) of protein model
compounds from water to ILs through solubility measurements as a function
of IL concentration at 25 °C have been exploited to quantify
and interpret biomolecular interactions between model compounds of
glycine peptides (GPs) with ammonium based ILs. The investigated aqueous
systems consist of zwitterionic glycine peptides: glycine (Gly), diglycine
(Gly<sub>2</sub>), triglycine (Gly<sub>3</sub>), tetraglycine (Gly<sub>4</sub>), and cyclic glycylglycine (c(GG)) in the presence of six
ILs such as diethylammonium acetate (DEAA), diethylammonium hydrogen
sulfate (DEAS), triethylammonium acetate (TEAA), triethylammonium
hydrogen sulfate (TEAS), triethylammonium dihydrogen phosphate (TEAP),
and trimethylammonium acetate (TMAA). We have observed positive values
of Δ<i>G</i>′<sub>tr</sub> for GPs from water
to ILs, indicating that interactions between ILs and GPs are unfavorable,
which leads to stabilization of the structure of model protein compounds.
Moreover, our experimental data Δ<i>G</i>′<sub>tr</sub> is used to obtain transfer free energies (Δ<i>g′</i><sub>tr</sub>) of the peptide backbone unit (or
glycyl unit) (CH<sub>2</sub>CONH), which is
the most numerous group in globular proteins, from water to IL solutions.
To obtain the mechanism events of the ILs' role in enhancing the stability
of the model compounds, we have further obtained <i>m</i>-values for GPs from solubility limits. These results explicitly
elucidate that all alkyl ammonium ILs act as stabilizers for model
compounds through the exclusion of ILs from model compounds of proteins
and also reflect the effect of alkyl chain on the stability of protein
model compounds