Journal of the Mass Spectrometry Society of Japan

Protonated molecules with different molecular weights are produced from their solutions by “nano-sonic spray ionization” (nano-SSI). Their intensities are compared with those of protonated molecules formed by “nano-electrospray ionization” (nano-ESI). In the case of nano-SSI, the dependence of the intensities of the protonated molecules on nebulizing gas flow rate shows that they have maxima at about the sonic velocity. Furthermore, the ratio of the intensity of a doubly-protonated peptide molecule to that of a triply-protonated one (substance P and neurotensin) decreases with increasing gas flow rate or velocity below the sonic velocity. The increased efficiency of the triply-protonated-molecule formation is explained in terms of ion evaporation from charged droplets deformed due to the gas flow. In the analysis of proteins of myoglobin and cytochrome c, mass spectra obtained by using nano-SSI are similar to those obtained by using nano-ESI. Multiply-protonated protein molecules with molecular weights above 3,300 are formed by a charged-residue mechanism, which appears less dependent on gas flow rate.

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Product ion spectra of bovine carbonic anhydrase 2 (CA2) were obtained, in an attempt to examine influence of charge number of precursor ions, which are produced by electrospray ionization (ESI), on collision-induced dissociation (CID). In the product ion spectra obtained from the precursor ions with various charge numbers, the observed m/z values of the product ions were common among the product ion spectra. Product ions which are comprised of complementary ion pairs (b- and y-series ions) were observed to cover the entire CA2 sequence. The sum totals of the charge number of the paired b- and y-series ions were equal to the charge number of the precursor ions. Regarding the intensity of the product ions, the charge number of the most abundant peak for b-series ions shifted in proportion as the charge number of the precursor ions, whereas that of y-series ions were fixed at 5+. The results suggest that the gas phase CA2 ions consist of the following two parts; 1) the region containing the C-terminal, which can maintain a certain folding state, and 2) the N-terminal region which was extensively altered as the result of the change in charge state. The advantageous point for product ion measurement is that the method can provide information concerning the folding state of the partial structure of proteins. Regarding the folding states of proteins, not only the dissociation position of the product ions but also the intensities of both the precursor and the product ions appear to be important.

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