Homonuclear Chemical Shift Correlation in Solids Under MAS by Fast Cross-Relaxation Driven Spin Diffusion.

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) spectroscopy
• Biophysical chemistry
• Structural biology

Background:

• Solid-state NMR is crucial for determining the structure of biomolecules that are difficult to crystallize.
• Efficient magnetization exchange between low-gamma nuclei (¹³C, ¹⁵N) is essential for multi-dimensional NMR experiments.
• Existing methods for magnetization exchange can be slow, limiting their application in complex systems.

Purpose of the Study:

• To develop and validate a novel 2D NMR technique for efficient homonuclear spin-system correlation in solids.
• To investigate the dependence of magnetization exchange efficiency on experimental parameters like Hartmann-Hahn mismatch and MAS frequency.
• To demonstrate the utility of the new technique for structural studies of proteins.

Main Methods:

• Implementation of a 2D NMR pulse sequence utilizing double spin-lock radiofrequency (RF) pulses.
• Magnetization exchange facilitated by cross-relaxation, spin diffusion, and RF field.
• Experimental validation using ¹³C-labeled Fmoc-Leucine, ¹⁵N-labeled L-histidine, and uniformly ¹⁵N-labeled aquaporin samples.

Main Results:

• The proposed double spin-lock technique enables rapid magnetization exchange between nearby low-gamma nuclei.
• Transfer rates correlate with internuclear distances, providing structural insights.
• Significantly faster cross-peak generation compared to conventional DARR mixing in a ¹⁵N-labeled protein sample.

Conclusions:

• The developed NMR method provides a highly efficient means for magnetization exchange between ¹⁵N or ¹³C nuclei in solids.
• This technique is advantageous for achieving resonance assignments in protein structural studies.
• The rapid spin-exchange process offers a valuable tool for investigating molecular structures and dynamics.