Fig. 8
Schematic representation of the tandem MS-spectrum of the signal at m/z 616.2. a: Spectrum of the signal obtained from the Se0-nanoparticle samples isolated from cultures of Rba. capulatus: Red bars. Comparison with the spectrum of the reference BPheide a: Green bars. 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-spectra. b: Proposed structure for the molecule yielding this MS-signal with indication of bond cleavage locations. It is based on: 1. The results of demetalation showing that it represented a metal-free molecule. 2. Similarities between its MS/MS fragmentation pattern with that of BPheide a (see Table 5). 3. The low difference between the measured mass (616.212 m. u.) and the theoretical mass (616.217 m. u.) of the proposed structure. 4. The low intensity of most of the tandem MS-signals of m/z 616.2 compared to the corresponding signals obtained for the referemce BPheide a. This effect is proposed to result from the extension of the bacteriochlorine π-resonance system to the C7-O and C18-O bonds of m/z 616.2, leading to a lower flexibility of the molecule. 5. The low fractional part of the molecular mass of this MS-signal compared to that of BPheide a, indicating that it contains a relatively large number of oxygen atoms and a relatively low number of hydrogen atoms (see text)