Aromatic SOFAST-HMBC for proteins at natural C abundance.

We propose here SOFAST-HMBC as a new complementary NMR tool for aromatic side chain assignment of protein samples at natural C abundance. The characteristic peak patterns detected in SOFAST-HMBC for each aromatic side chain allow straightforward assignment of all protons and carbons (including quaternary ones) of the aromatic ring, and for tyrosine and phenylalanine, connection to the CB of the aliphatic chain. The performance of SOFAST-HMBC is demonstrated for three small proteins (7-14 kDa) at millimolar sample concentration using modern high-field NMR instruments equipped with cryogenically cooled probes. Despite the low amount of NMR-active C nuclei in these samples, H-C multiple-bond correlation spectra of good quality were obtained in reasonable experimental times of typically less than 24 h.