Was some effort made to allow amide exchange to occur, ie unfolding and refolding in H2O, before you ran your spectra? If not you may not have enough amide protons there for HN experiments to work. Do you get a good N15 HSQC with the same number of transients that worked for the protonated sample?
You might also try to compare a TROSY-HSQC with a regular HSQC to see if that helps the sensitivity although for a globular 20kDa protein in a spectrometer field lower than 700 MHz the sensitivity gain will probably be marginal.
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