In nano-ESI MS, the qualitative and quantitative characteristics of mass spectra vary considerably upon the use of different spraying conditions, i.e., aperture of the spraying needle and the voltage applied. The major parameters affected by the aperture size is the liquid flow rate which determines the initial droplet size and the current emitted upon the spray process, as described by different models of the ESI process. In the present study, the effect of flow rate on ion signals was studied systematically using mixtures of compounds with different physicochemical properties (i.e., detergent/oligosaccharide and oligosaccharide/peptide). For these model systems, the functional dependence of certain analyte-ion ratios upon the flow rate can be correlated to changes in analyte partition during droplet fission prior to ion release.

The comparison between electrospray ionization (ESI) mass spectra from NaCl solutions with and without analyte obtained under ionspray and nanospray conditions reveals different mass spectral behavior of the two ESI techniques. This can be attributed to the different initial droplet sizes which are in the microns range for ionspray, while in nanospray they are believed to be about one order of magnitude smaller. In the context of the widely accepted uneven-fission model, nanospray would then enter one fission generation later; in addition, a higher initial droplet surface charge density in nanospray results in early fissions without extensive evaporation and thus increase in sample and salt concentration. This rationalizes that ionspray spectra closely resemble nanospray spectra from solutions with about one order of magnitude higher salt concentrations, showing a higher tolerance of nanospray towards salt contamination. When the analyte is a peptide (in a solution containing a high molar surplus of salt), molecule ion formation effectively competes with salt cluster ion formation; when the analyte is a sugar, it is detectable beside a high salt concentration only with nanospray, indicating the supporting effect of surface activity on ion release in the case of peptides. A model is presented which explains the different mass spectral behaviour of ionspray and nanospray by suggesting different "predominant fission pathways" depending on the size of the initial droplets.

The gas phase reactivity of perhalogenated closo-dodecaborate clusters [B(12)X(12)](2-) (X = F, Cl, Br, I) with N-tetraalkylated ammonium counter ions was investigated by electrospray ionization ion trap mass spectrometry (ESI-IT-MS). Collisions with the background gases introduced a broad variety of gas phase reactions. This study represents the first experimental approach to a new class of boron-rich boron clusters that are not accessible in the condensed phase. The anionic ion pair [B(12)X(12) + N(C(n)H(2n+1))(4)](-) is generally found as the ion of highest mass. Its reaction sequence starts with an alkyl transfer from the ammonium ion to the dodecaborate cluster. Subsequently, the alkylated intermediate [B(12)X(12) + C(n)H(2n+1)](-) decomposes to give very reactive ions of the general formula [B(12)X(11)](-). These ions possess a free boron vertex and immediately bind to the residual gases N(2) and H(2)O in the ion trap by formation of the corresponding adducts [B(12)X(11) + N(2)](-) and [B(12)X(11) + H(2)O](-). Subsequent fragmentations of the water adduct repetitively substitute halogen atoms by hydroxyl groups. The fragmentation process of the free anion [B(12)X(12)](2-) depends on the applied excitation energy and on the halogen substituent X. A radical dehalogenation of the B(12) unit is observed for X = I, whereas for X = Cl or F the loss of small molecules (mainly BX(3)) dominates. The different reaction behavior is explained by the different electron affinity of the halogens and the strength of the boron-halogen-bonds. Surprisingly, isolation of the fragment ion [B(12)I(9)](-) in the ion trap yields the highly stable [B(24)I(18)](2-) dianion. This observation suggests a reaction between two negative ions in the gas phase.

Isomeric linear and rectangular trinuclear metal-free and zinc(II) phthalocyanine complexes connected by common annulated benzene rings have been synthesized by the mixed cyclotetramerization of a 1,2,4,5-tetracyanobenzene derivative and 4,5-bis(2,6-dimethylphenoxy)phthalonitrile. Their electronic-absorption and fluorescence spectra have been studied in comparison with the corresponding mono-and dinuclear phthalocyanines. For the zinc complexes, the molec-

Biosynthesis of heme d1, the essential prosthetic group of the dissimilatory nitrite reductase cytochrome cd1, requires the methylation of the tetrapyrrole precursor uroporphyrinogen III at positions C‐2 and C‐7. We produced Pseudomonas aeruginosa NirE, a putative S‐adenosyl‐l‐methionine (SAM)‐dependent uroporphyrinogen III methyltransferase, as a recombinant protein in Escherichia coli and purified it to apparent homogeneity by metal chelate and gel filtration chromatography. Analytical gel filtration of purified NirE indicated that the recombinant protein is a homodimer. NirE was shown to be a SAM‐dependent uroporphyrinogen III methyltransferase that catalyzes the conversion of uroporphyrinogen III into precorrin‐2 in vivo and in vitro. A specific activity of 316.8 nmol of precorrin‐2 h−1·mg−1 of NirE was found for the conversion of uroporphyrinogen III to precorrin‐2. At high enzyme concentrations NirE catalyzed an overmethylation of uroporphyrinogen III, resulting in the formation of trimethylpyrrocorphin. Substrate inhibition was observed at uroporphyrinogen III concentrations above 17 μm. The protein did bind SAM, although not with the same avidity as reported for other SAM‐dependent uroporphyrinogen III methyltransferases involved in siroheme and cobalamin biosynthesis. A P. aeruginosa nirE transposon mutant was not complemented by native cobA encoding the SAM‐dependent uroporphyrinogen III methyltransferase involved in cobalamin formation. However, bacterial growth of the nirE mutant was observed when cobA was constitutively expressed by a complementing plasmid, underscoring the special requirement of NirE for heme d1 biosynthesis.