ABSTRACT
Optically active 2-thiazolidinecarboxylic acid (2-THC), a substrate for D-amino acid oxidase in animal kidney, is known to undergo racemization quickly in solution. The association of (+)- and (-)-2-THC with antimony potassium tartrate K(2)[Sb(2)(L or D-tart)(2)] was studied by electrospray ionization mass spectrometry (ESI-MS). We observed that relative intensities of associated ions in acetonitrile/water solution were changing as the racemization progressed. For [Sb(2)(L-tart)(2)](2-), the intensities of the associated ions increased as (+)-2-THC underwent racemization to a (-)-isomer; on the other hand, the intensity of the associated ion decreased as (-)-2-THC underwent racemization to a (+)-isomer. In the case of [Sb(2)(D-tart)(2)](2-), an opposite effect on the intensities of the associated ions was observed. The change in the intensities of associated ions can be used for chiral recognition of (+)-2-THC and (-)-2THC. Stereochemical models of the association of the optical isomers with [Sb(2)(L- or D-tart)(2)](2-) were constructed from the consideration of both hydrogen bonding of NH-O functions and HSAB (hard and soft acids and bases) interaction of S and Sb atoms. Comparison of the stereochemical models with the ESI-MS results enabled us to predict the absolute configurations of the 2-THC isomers.
ABSTRACT
Self-assembled monolayers (SAMs) of three ruthenium complexes, [Ru(L)(2)](PF(6))(2), [Ru(L)(tpyPO(3))](PF(6))(2), and [Ru(L18)(tpyPO(3))](PF(6))(2), were prepared on evaporated gold films on glass or stainless steel plates; where L = 2, 6-bis(benzimidazoyl)pyridine, tpyPO(3) = 2,6-bis(2,2':6', 2"-terpyridyl)pyridine phosphanate, and L18 = 2, 6-bis(N-octadecylbenzimidazoyl)pyridine. Structures of these SAM complexes were studied by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The SAMs were either prepared by direct binding of Ru-complexes to Au films by alkanethiol or by the multilayer method. In the multilayer method 1,4-thiobutylphosphate was used to form a base layer on an Au film, and the base layer was then chemically bridged to the Ru-complexes by zirconium phosphate. MALDI-TOFMS of SAM1, that had been prepared by direct binding of [Ru(L)(2)](PF(6))(2) to the Au film by an octanethiol group, showed cleavage at the S-Au linkages and elimination of the counter anion to yield a molecular ion and its dimeric ion. On the other hand, SAM2 and SAM3, which had been prepared by bridging Ru-complexes [Ru(L)(tpyPO(3))](PF(6))(2) or [Ru(L18)(tpyPO(3))](PF(6))(2) to the base layers with zirconium phosphate, showed dissociation from the base layers and elimination of the counter anion to give ions of the Ru complex molecules and their fragmentation ions. No molecular ion containing the base layer resulting from the S-Au bond cleavage was observed. Copyright 2000 John Wiley & Sons, Ltd.
ABSTRACT
Self-assembly of 1,1'-ferrocenedicarboxylic acid in solution was detected by electrospray mass spectrometry (ESI-MS). The ESI mass spectra showed the acid was self-assembled as a cyclic tetramer in methanol and acetonitrile, and the tetramer was found to complex more strongly with the sodium ion than with any other alkali metal ion. The result was supported by semiempirical molecular orbital calculations, which indicate that the tetramer possesses a cyclic structure like a pseudo-crown ether, and its internal diameter is consistent with the diameter of a sodium ion.
ABSTRACT
Two copper proteins azurin-1 and azurin-2 were isolated from denitrifying bacteria Alcaligenes xylosoxidans GIFU1051, and the mass spectrometric analysis of the proteins were carried out by both matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) and electrospray ionization (ESI). The mass spectrometric analysis was also carried out with the recombinant zucchini protein mavicyanin, which was obtained by expression in Escherichia coli. All the proteins were detected as positive ions with the copper atom being eliminated. The molecular weights were determined as 14,017.6 for azurin-1, 13,807.6 for azurin-2 and 11,808.8 for mavicyanin. The observed molecular weight of azurin-1 agrees within two daltons with that calculated from the amino acid composition. Azurin-2 was found to have one different amino acid residue when compared with the known azurin-2 isolated from A. xylosoxidans NCIB11015. The measured molecular weight for the recombinant mavicyanin agrees within two daltons with that of calculated from the amino acid composition of the native protein; therefore, the recombinant mavicyanin is identical to the native protein.
Subject(s)
Azurin/analysis , Metalloproteins/analysis , Plant Proteins/analysis , Escherichia coli/chemistry , Mass Spectrometry , Molecular Weight , Spectrometry, Mass, Matrix-Assisted Laser Desorption-IonizationABSTRACT
Phenylethanolamine N-methyltransferase (PNMT) catalyzes the production of epinephrine from norepinephrine using S-adenosyl-L-methionine as a methyl donor. Previous studies of chemical modification of the PNMT with reagents specific to Cys residues showed that the enzyme contains a Cys residue essential for its activity. Each of the six Cys residues in human PNMT was changed to Ser by PCR-based site-directed mutagenesis, and each mutant PNMT was expressed in Escherichia coli to identify the functionally important Cys residue. The six mutants (C48S, C60S, C91S, C131S, C139S, and C183S) and the wild-type enzyme were expressed at almost the same levels as revealed by Western blotting analysis. Kinetic parameters (apparent Km and Vmax) of C48S, C60S, C91S, C131S, and C139S for the substrates, norepinephrine and S-adenosyl-L-methionine, showed similar values to those of the wild-type enzyme. However, C183S exhibited markedly reduced enzyme activity with less than 3% of the wild-type Vmax and with ca. sixfold increased apparent Km values for both substrates. These results suggested that Cys183 plays an important role in the activity of human PNMT.
