Resonance Assignment/Principles and concepts

Introduction

This section is intended to introduce a few definitions and concepts in resonance assignment protocols. For in depth description of the process see e.g. Kurt Wütrich's book NMR of Proteins and Nucleic Acids (Wiley, 1986) and John Cavangh's et al. textbook Protein NMR Spectroscopy. Principles and Practice (2^nd edition, Elsevier, 2007). 

Stable isotope labeling schemes

Through the NESG consortium, the most prevalent isotope labeling schemes are as follows:

• 100% ¹⁵N, 100%¹³C-labeled (or doubly-labeled) samples are the main category, to which the majority of the information on this site applies. They are used for complete resonance assignments and structure calculation.
  
• 100% ¹⁵N-labeled samples are used for screening with 15N-HSQC. They can also find limited use in collecting RDC-type data.
  
• 100% ¹⁵N, 5-7% ¹³C-labeled samples are used to obtain stereospecific assignment of Val and Leu side chain methyl groups, usually important for proper packing of hydrophobic core.
• 100% 14N, 100% 12C (or unlabeled) or alternatively natural abundance samples can be used in 50%-50% mixtures for homodimer structure determination.
  

In addition to these labeling schemes, one can find it useful, especially larger proteins to have selectively labeled samples, such as SAIL NMR (http://www.sailnmr.org/). To reduce signal broadening due to spin-spin relaxation, it may be advantageous to deuterate the protein to a certain level.