Probing subtle acid-induced conformational changes of ribonuclease A by electrospray mass spectrometry

The newly developed technique, electrospray mass spectrometry, was used to probe subtle conformational changes of bovine pancreatic RNase A during acid denaturation. In a dilute acid solution of pH 2.6, RNase A lost nearly all of its activity, whereas its intrinsic fluorescence intensity at 304 nm and its ellipticity at 222 nm were fairly resistant to denaturation by acetic acid. The observed maximum charged state of the enzyme in electrospray mass spectra was increased from 11+ (at pH 3.3) to 14+ (at pH 2.6). This could result from exposure of the buried basic amino acid residues R-10, K-41, H-12 and perhaps H-48.