Dissociation of heme from gaseous myoglobin ions studied by infrared multiphoton dissociation spectroscopy and Fourier-transform ion cyclotron resonance mass spectrometry.

Detachment of heme prosthetic groups from gaseous myoglobin ions has been studied by collision-induced dissociation and infrared multiphoton dissociation in combination with Fourier-transform ion cyclotron resonance mass spectrometry. Multiply charged holomyoglobin ions (hMbn+) were generated by electrospray ionization and transferred to an ion cyclotron resonance cell, where the ions of interest were isolated and fragmented by either collision with Ar atoms or irradiation with 3 mum photons, producing apomyoglobin ions (aMbn+). Both charged heme loss (with [Fe(III)-heme]+ and aMb(n-1)+ as the products) and neutral heme loss (with [Fe(II)-heme] and aMbn+ as the products) were detected concurrently for hMbn+ produced from a myoglobin solution pretreated with reducing reagents. By reference to Ea = 0.9 eV determined by blackbody infrared radiative dissociation for charged heme loss of ferric hMbn+, an activation energy of 1.1 eV was deduced for neutral heme loss of ferrous hMbn+ with n = 9 and 10.