Prediction of the charge states of folded proteins in electrospray ionization

Earlier work from this laboratory dealt with the observation that the charge states of non-denatured proteins can be decreased by use of buffer salts in which the gas-phase basicity of conjugate base B, GB(B), of the buffer cations is high. A theoretical model was developed and applied to several small proteins. The predictions of the charge states were found to be in good agreement with those observed experimentally. Because the computational model is based on the charge residue model (CRM), the observed agreement lends support for the CRM. In the present work, the same model is applied to recent data by Catalina et al. who showed that very large charge reductions are achieved with very high GB(B) proton sponges. Their data included lysozyme but also the very much larger proteins, p-hydroxybenzoate hydroxylase (PHBH), 90 kDa and glutamate synthase (GLTS), 166 kDA. The present work examines the performance of the model for the much stronger bases and the very much larger proteins. It is found that the predictions of the charge states agree well for the small protein lysozyme but somewhat less well with the experimental results for PHBH and GLTS. The causes for the lack of good agreement with the large proteins are examined.