Investigation of protein conformation and interactions with salts via X-ray absorption spectroscopy

• Published in: Proceedings of the National Academy of Sciences - PNAS 2010-08, Vol.107 (32), p.14008-14013
• Main Authors: Schwartz, Craig P., Uejio, Janel S., Duffin, Andrew M., England, Alice H., Kelly, Daniel N., Prendergast, David, Saykally, Richard J., Klein, Michael L.
• Format: Article
• Language: English
• Subjects: Amino acids; Anions; Atoms; Bromides; Ions; Liquids; Molecular chemistry; Molecules; Nitrogen; Oligopeptides - chemistry; Peptides; Peptides - chemistry; Physical Sciences; Protein Binding; Protein Conformation; Protein Structure, Secondary; Proteins; Proteins - chemistry; Salts; Salts - chemistry; Sodium; Sodium sulfites; Solubility; Solutions; Spectrum analysis; Temperature; X-Ray Absorption Spectroscopy - methods
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1006435107

Nitrogen K-edge spectra of aqueous triglycine were measured using liquid microjets, and the effects of Hofmeister-active salts on the spectra were observed. Spectra simulated using density functional theory, sampled from room temperature classical molecular dynamics trajectories, capture all major features in the measured spectra. The spectrum of triglycine in water is quite similar to that in the presence of chaotropic sodium bromide (and other halides), which raises the solubility of proteins. However, a new feature is found when kosmotropic Na₂SO₃, which lowers solubility, is present; this feature results from excitations of the nitrogen atom in the terminal amino group of triglycine. Both direct interactions between this salt and the protonated amino terminus, as well as corresponding changes in the conformational dynamics of the system, contribute to this new feature. These molecular measurements support a different mechanism for the Hofmeister effect than has previously been suggested based on thermodynamic measurements. It is also shown that near edge X-ray absorption fine structure (NEXAFS) is sensitive to strong direct interaction between certain salts and charged peptides. However, by investigating the sensitivity of NEXAFS to the extreme structural differences between model β-sheets and α-helices, we conclude that this technique is relatively insensitive to secondary structure of peptides and proteins.