Labelling Schemes

Several labelling schemes have been incorporated into Analysis. Along with the standard 15N, 13C and 15N,13C,2H schemes, this also includes the 1,3-13C-glycerol and 2-13C-glycerol labelling schemes which are particularly useful for solid-state MAS NMR spectroscopists. You can use the labelling schemes to filter assignment options, for example. Simply select a Labelling Scheme from the list in the Assignment Panel and the assignment possibilities will be limited to those which are in agreement with the labelling scheme. For the 2Glycerol labelling scheme, for example, it will take into account that you cannot see any intra-residue Cα-CO peaks, but it will allow inter-residue Cα-CO assignments. Whenever you can select a labelling scheme for filtering you are also given the option of selecting the Min Isotope Fraction. This is the minimum fraction which must be labelled at any site for it to be classed as being labelled. This is important for the glycerol labelling schemes: some sites are fully labelled or fully unlabelled, but many are partly labelled, because there are a variety of isotopomers. The minimum degree of labelling necessary at a site for you to see peaks depends on various factors including your protein and your signal-to-noise ratio. For this reason you can always adjust the Min Isotope Fraction yourself. Note that the glycerol labelling schemes include the various isotopomers at 1:1 ratios since this ratio appears to vary somewhat from protein to protein.

Labelling schemes can also be used when creating distance restraints.

From version 2.2.1 onwards you can also associate an experiment with a particular labelling scheme so that the labelling scheme is taken into consideration automatically and you don’t have select the labelling scheme by hand each time. Go to the Molecules menu and select the Isotope Labelling popup. In the top half of the popup select New Sample (for the correct molecule). You can now add a labelling scheme for this sample in the lower half of the popup by selecting New Pattern from Scheme (making sure that the correct labelling scheme is selected in the drop-down menu on the right). You will see that the Labelling Pattern in the top half has now been updated. By double clicking in the Experiments column you can select the experiments which are associated with this sample. One of the advantages of using this feature is that you can create a sample which contains two labelling schemes, e.g. 50% U-15N and 50% U-13C labelling. Analysis will then automatically know that any 15N-13C cross peaks are intermolecular cross peaks and distance restraints will be generated accordingly.