Spectral density mapping at multiple magnetic fields suitable for (13)C NMR relaxation studies

Area of Science:

• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Biophysical Chemistry
• Computational Chemistry

Background:

• Standard spectral density mapping protocols are effective for nitrogen-15 relaxation data but introduce systematic errors for carbon-13 relaxation data, particularly for small molecules or dynamic regions of macromolecules.
• These errors arise from motions with short correlation times, complicating the accurate analysis of molecular dynamics.

Purpose of the Study:

• To investigate the improvement in accuracy for carbon-13 relaxation data analysis by utilizing cross-correlated relaxation rates and measurements at multiple magnetic fields.
• To develop and test a suite of protocols for analyzing such multi-field, cross-correlated relaxation data.
• To demonstrate the applicability of these protocols through case studies on RNA hairpin and disaccharide systems.

Main Methods:

• Development and testing of novel protocols for spectral density mapping using both auto- and cross-correlated relaxation rates.
• Acquisition of relaxation data at multiple magnetic fields (three and five fields in case studies).
• Application of these protocols to uniformly labeled RNA hairpin and selectively labeled disaccharide systems with varying degrees of motional anisotropy.

Main Results:

• Accurate values for auto- and cross-correlated spectral density functions at zero and carbon-13 frequencies can be obtained from data acquired at three magnetic fields for uniformly carbon-13 labeled molecules with moderate rotational anisotropy.
• Analysis of auto-correlated relaxation rates at five magnetic fields provides a viable alternative for molecules exhibiting highly anisotropic motions.
• The developed protocols successfully separated effects of fast motions from conformational or chemical exchange in the RNA hairpin study.

Conclusions:

• The proposed protocols offer a significant improvement in the accuracy of spectral density mapping for carbon-13 relaxation data, overcoming limitations of standard methods.
• Multi-field NMR relaxation measurements combined with cross-correlated rates are crucial for detailed molecular dynamics studies, especially for systems with complex motions.
• These findings provide enhanced tools for characterizing the dynamics of small molecules and flexible regions in macromolecules.