Site-specific sodiation of peptides studied by pulsed nanoelectrospray ionization

Metal ions play significant roles in biological systems. Although sodium ions play a key role in biological systems, site-specific interactions between sodium ions and protonated peptides and proteins have not been thoroughly explored. In this work, the time-dependent sodiation of protonated peptides was investigated by pulsed nanoelectrospray ionization. Mass spectra for aqueous mixtures of 5x10 (-6) M angiotensin II (A), bradykinin (B), and gramicidin S (G) were measured as a function of the high voltage (4000 V) pulse width (PW) with and without the application of a bias voltage (BV) to the solution. With BV = 0 V, only [B + 2H H + Na](2+) was detected with an increase in PW from 220 mu s (threshold for the signal appearance) to 1 ms. In contrast, [A + 2H nH + nNa](2+) (n = 1-3) started to be detected with PW = 500 mu s and up to n = 4 with PW = 1 ms. By the application of BV = 900 V, [B + 2H nH + nNa](2+) with n = 0 and 1 started to be detected with PW = 40 mu s. With further increase in PW up to 1 ms, [B + 2H nH + nNa](2+) with n = 0-3 and [A + 2H nH + nNa](2+) (n = 1-5) were detected. The observation of [B + 2H 3H + 3Na](2+) and [A + 2H 5H + 5Na](2+) with BV = 900 V suggests that carboxyl ( COOH), imidazole > N-H, hydroxyl ( OH) as well as amide > N-H bonds are deprotonated to form respective sodiated forms of carboxylate -COO- center dot center dot center dot center dot Na+, imidazole > N- center dot center dot center dot center dot Na+, hydroxyl -O- center dot center dot center dot center dot Na+ and amide -N=C-O- center dot center dot center dot center dot Na+ bonds. Ions of G were totally suppressed by the presence of A and B in aqueous solution.