Site-specific resolution of anionic residues in proteins using solid-state NMR spectroscopy

Area of Science:

• Biophysical Chemistry
• Structural Biology
• Solid-State NMR Spectroscopy

Background:

• Nuclear Magnetic Resonance (NMR) spectroscopy is vital for determining site-specific acid dissociation constants (pK_a) due to chemical shift sensitivity to protonation states.
• Probing atoms closest to functional groups offers the highest sensitivity for assessing protonation states in acid/base chemistry.

Purpose of the Study:

• To develop and validate a magic-angle-spinning (MAS) solid-state NMR approach for measuring chemical shifts of anionic aspartate and glutamate residues.
• To enhance the characterization of protonation states and derivation of pK_a values in proteins using solid-state NMR.

Main Methods:

• Utilized a combination of double quantum spectroscopy in the indirect dimension and Rotating-frame Overhauser Effect (REDOR) dephasing.
• Applied the method to measure chemical shifts on the side chains of aspartate and glutamate residues.
• Demonstrated applicability on both a soluble protein (ubiquitin) and a membrane protein (EmrE) embedded in lipid bilayers.

Main Results:

• Achieved a sensitive and resolved view of aspartate and glutamate residues.
• Successfully measured chemical shifts and derived pK_a values.
• Validated the method's effectiveness across different protein types and environments.

Conclusions:

• The described MAS solid-state NMR method offers superior convenience and accuracy for characterizing protein protonation states.
• This technique is highly effective for deriving pK_a values, particularly for aspartate and glutamate residues involved in critical biological processes.
• The enhanced resolution and identification confidence make this approach a valuable tool in structural biology and biophysical chemistry.