Mass spectrometric studies on peptides and proteins: Conformations of Escherichia coli thioredoxin and its alkylated adducts studied by hydrogen/deuteriumexchange and HPLC-electrospray ionization mass spectrometry

E. coli thioredoxin (TRX) was modified by the episulfonium ion derived from S-(2-chloroethyl)glutathione (CEG) or S-(2-chloroethyl)cysteine (CEC). The alkylation site was located at Cys-32, which was confirmed by tandem mass spectrometry. Two forms of native TRX, Oxi- and Red-TRX, and two modified TRXs, GS- and Cys-TRX, were examined by hydrogen/deuterium (H/D) exchange reactions using electrospray ionization mass spectrometry (ESI-MS) for the analysis.; Conformational dynamics during thermal denaturation were probed by H/D exchange-in experiments. Under conditions in which the folded conformational state is marginally stable, H/D exchange-in experiments resulted in mass spectra differing in the number of incorporated deuteriums which indicates the presence of two distinct populations of molecules. As the exchange-in time increased, the population representing the unfolded state increased and the population for the folded state decreased. The rate of conversion was used to estimate the rate constant of unfolding. ESI mass spectra were also recorded as a function of temperature without H/D exchange, and the observed bimodal charge state distributions were analyzed in order to estimate melting temperatures. GS-TRX showed increased resistance to hydrogen isotope exchange in comparison with Red-TRX indicating that there were enhanced intramolecular interactions in the former protein.; Pepsin digestion was performed on deuterated TRXs to analyze different structural regions. The amount of deuterium incorporated was monitored with peptic peptides from deuterated TRXs with different exchange-in incubation periods. Deuterium levels of each peptide were plotted versus the exchange time and fitted with a series of first-order rate terms. The regions 59–80 and 81–108 of Oxi- and Red-TRX showed an EX1 mechanism as evidenced by two distinct mass envelopes that appeared after a short incubation time in deuterated solvent.; Tandem mass spectrometry (MS/MS) was carried out to obtain the information on individual amide linkages. MS/MS data showed generally excellent correlations with the exchange rate constants from published NMR data on Oxi- and Red-TRXs. Two residues, Ile-75 and Ala-93 in GS-TRX indicated the most probable sites responsible for induced H-bonding by the attached glutathionyl group, which was consistent with the energy minimized structure predicted by AMBER force field constants.