SAIL-labelled protein is prepared using cell-free methods. The amino acids used for the protein production are prepared using chemical and enzymatic syntheses. Their labelling is guided by the following principles:
– in each methylene group, one of the 1H atoms is stereo-selectively replaced by a 2H atom.
– in each methyl group, two of the 1H atoms are replaced by 2H atoms
– the prochiral methyl groups of Leu and Val are stereo-selectively 12C2H3 and 13C1H2H2 labelled
– six-membered aromatic rings have alternating 12C2H and 13C1H moieties
The SAIL Wiki site has a figure illustrating the labelling scheme.
The SAIL labeling pattern is particularly useful for the structure calculation of large proteins with advantages seen in the NOESY spectra. Non-exchangeable side-chain protons are essential for defining side-chain conformations, but are often prone to overlap. By reducing the number of these protons in SAIL-labelled samples, the overlap is decreased and the quality of NOESY spectra is improved. Indeed, although there are fewer NOE peaks for a SAIL-labelled protein than for a uniformly 15N,13C-labelled one, it is likely that there are more interpretable NOE peaks. In any case, many of the peaks which are removed, have a low information content because they involve fixed (geminal) distances or duplicate information already present from other NOEs. In addition, the information obtained is immediately stereo-specific. A further advantage is that the most serious sources of spin diffusion are removed which improves the accuracy of inter-proton distance measurements.
More general advantages of SAIL-labelled samples include the fact that lines become sharper due to a decrease in long-range couplings and by eliminating dipolar relaxation pathways. The aromatic ring labeling strategy removes one-bond 13C-13C couplings, which often complicate spectra or require the use of constant time data collection methods to reduce spectral complexity. And the measurements of couplings is made easier because CH2 and CH3 groups are converted to CHD and CHD2 groups.
The labelleding of methyl and methylene groups in SAIL-labelled samples also makes them amenable to straight forward relaxation experiments for the investigation of side-chain motions.
Despite the clear advantages to using SAIL-labelled protein for structure determination, this labelling method is not very common – probably because it is very expensive and requires cell-free technologoy which is not available in many labs.
M. Kainosho, T. Torizawa, Y. Iwashita, T. Terauchi, A.M. Ono and P. Güntert (2006) Nature 440 52-57. (Link to Article)
SAIL Wiki run by the Kainosho and Güntert labs