Electrospray ionization tandem mass spectrometric studies of copper and iron complexes with tobramycin

Tobramycin is an aminoglycoside antibiotic that shows antimicrobial activity against a wide spectrum of aerobic bacteria. This paper describes the electrospray ionization mass spectrometric analysis of metallo (Cu(II) and Fe(III))-tobramycin complexes. The Cu(II) (1.5 x 10(-4) M) and Fe(III) (1.5 x 10(-4) M) ions co-ordinate with tobramycin (1.5 x 10(-4) M) at 1:1 molar ratio for 30 min at room temperature. ESI mass spectrum of tobramycin shows ions at m/z 468.1, 934.8, and 234.8 corresponding to mono-protonated ([M+H](+)), dimer ([2M+H](+)) and di-protonated ([M+2H](2+)), respectively. The Cu(II) and Fe(III) complexes show preferentially different molar ratio as 1:1, 1:2 and 1:3 such as [Cu-63+(M-2H)+H](+), [Cu-65 + (M-2H) + H](+), [Cu-63 + (M-2H) + 2H(2)O + H](+), [Cu-65 + (M-2H) + 2H(2)O + H](+) [Fe + (M-3H) + 2H(2)O + H], [Fe + (2M-3H) + 2H(2)O + H](+), [Fe + (M-3H) + H2O + H](+) and [Fe + (M-3H) + H](+), respectively. Therefore, ESI-MS provides a simple, accurate and reproducible method for the identification of Cu(II)- and Fe(III)-tobramycin complexes ions than the other conventional methods (the method of continuous variations, the mole-ratio method, the slope-ratio method, computer-based curve-flitting methods). (C) 2012 Elsevier B.V. All rights reserved.