Homonuclear J-couplings and heteronuclear structural constraints.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Structural Biology
• Biophysics

Background:

• Magic Angle Spinning (MAS) NMR experiments utilize ¹³C-¹³C and ¹³C-¹⁵N dipolar recoupling to measure internuclear distances for molecular structure determination.
• For distances >4 Å, both dipolar and scalar (J) couplings are significant, requiring their inclusion for precise measurements.
• Existing frequency-selective dipolar recoupling methods struggle with spectral crowding in larger molecules, leading to sensitivity loss or excitation issues.

Purpose of the Study:

• To develop novel approaches for high-precision internuclear distance measurements in NMR experiments, particularly for systems with spectral crowding.
• To overcome limitations of current methods that ignore or struggle to account for J-couplings in complex molecular structures.
• To facilitate more accurate structural constraints for challenging biological macromolecules.

Main Methods:

• Integration of the in-phase anti-phase (IPAP) sequence into a non-selective dipolar recoupling technique for high-precision J-coupling measurements.
• Implementation of a double quantum filter experiment utilizing measured J-couplings to enhance spectral resolution.
• Application of these methods to a U-¹³C/¹⁵N labeled peptide segment (GNNQQNY) from the amyloidogenic Sup-35p protein.

Main Results:

• Demonstrated a method for precise measurement of specific J-couplings within a non-selective dipolar recoupling framework.
• Developed a double quantum filter approach that provides necessary resolution for frequency-selective dipolar recoupling without multidimensional NMR.
• Successfully measured 25 out of 27 possible one-bond ¹³C-¹³C J-couplings in the GNNQQNY peptide segment.

Conclusions:

• The presented methods enable higher precision internuclear distance measurements in challenging, spectrally crowded systems.
• These techniques effectively address the limitations of ignoring J-couplings or relying on frequency-selective methods in complex NMR spectra.
• The successful application to the Sup-35p peptide segment validates the utility of these approaches for structural biology research.