Fig. 7
Schematic representation of the tandem MS-spectrum of the signal at m/z 595.3. a: Spectrum of the signal obtained from the Se0-nanoparticle samples isolated from cultures of Rba. capulatus. The bond cleavage(s) corresponding to the fragment loss(es) are indicated in parentheses on top of each bar representing molecular ions obtained in these MS/MS-spectra. No reference representing a similar compound was available. b: Proposed structure for the molecule yielding this MS-signal with indication of bond cleavage locations. It is base on: 1. The results of demetalation showing that this MS-signal represented a metal-free compound. 2. The low difference between the measured mass (595.302 m. u.) and the theoretical mass (595.284) of the proposed structure. 3. The absence of signal for the loss of an acetoxy group [-CO-CH3], indicating that it represented a metabolite of BChl biosynthesis containing a C3-vinyl group. 4. The large number of double fragment losses corresponding to the loss of 104 m. u. (m/z 491.3) and the large number of signals proposed to represent the loss of three fragments. These results were consistent with the presence of substituents bound not only to the C13 and C17 carbon atoms, but also to the C15 carbon atom of the tetrapyrrole ring, thus representing a porphyrin instead of a bacteriochlorin molecule. 5. The odd m/z value obtained for this signal, which corresponds to a protonated molecular ion as obtained for the references PPIX (m/z 563.3) and PPIX-O-CH3 (m/z 577.3), and is consistent with the proposed porphyrin structure