Electrospray ionization mass spectrometry: from cluster ions to toxic metal ions in biology

Abstract

<p>In Chapter 1, electrospray ionization mass spectrometry (ESI-MS) was used to examine clusters of protonated amine salt solutions with chloride counter ions in the negative ion mode. These ions have the general formula [(RNH3)xClx+1]-. Primary amines generate a wide cluster distribution with clusters up to 14 mers for methylamine hydrochloride clusters. Secondary and quaternary amines only generate the monomer ion under identical conditions. Collision induced dissociation (CID) of the cluster ions generates cluster ions of lower m/z with the next lower cluster being the most abundant. The product ions from MeNH3Cl 2-, Me2NH2Cl2 - and (MeNH3)2Cl3- have low threshold appearance energies of 1.24 to 2.22 eV center-of-mass frame. Secondary amine monomer ions have lower threshold CID energies than primary amine monomer ions. The amine threshold CID energy decreases as the carbon chain length increases. As an electrospray solvent, isopropyl alcohol (IPA) promotes the formation of counter ions and clustering.;In Chapter 2, salt cluster ions of alkali metal sulfates Mx(SO 4)yz- (M = Li, Na, and Cs) were studied by ESI-MS. Ions with charges up to z = -7, i.e., Na131(SO 4)697- were present in the mass spectra. The effects of capillary temperature, capillary voltage, tube lens voltage, fragmentor voltage, and various solvents were investigated. Collision induced dissociation of the clusters showed that there was an overlap of singly and multiply charged ions at the same m/z. Two different types of instrument interfaces, heated capillary and fragmentor, were utilized in this study. The different cations produced different cluster distributions with lithium sulfate clusters generating the most multiply charged cluster ions. The capillary voltage altered the cluster ion intensity and distribution more than any other parameter on the instrument with the heated capillary interface. The fragmentor voltage produced the largest variation in the cluster ion distribution. Overall, the heated capillary interface produced smaller changes to the cluster ion distribution than the fragmentor interface.;In Chapter 3, solutions comprising two metal chloride compounds were analyzed in negative ion mode by ESI-MS. Various metal clusters, MxCl z- (M = Ce3+, Co2+, Zn 2+ or Ho3+) or CexMyClz - (M = Co2+, Zn2+, or Ho 3+) were present in the mass spectra. No multiply charged ions were observed. Lower heated capillary temperatures favored mixed metal cluster ions. The abundances of the mixed metal clusters increased when a 50/50 isopropyl alcohol/water solution was used compared to a 96/4 isopropyl/water solution. Collision induced dissociation of cerium/cobalt mixed metal clusters revealed that the Co2+ ion generally leaves as part of a neutral fragment, and the negatively charged fragment retains the Ce3+ ion.;In Chapter 4, ESI-MS was used to evaluate toxic metal binding to the [Gln11]-amyloid beta-protein fragment (1-16). Cd and Pb bound to the amyloid fragment; Hg did so weakly. Collision induced dissociation (CID) studies found that Pb and Cd attached to the same binding site as the essential element Zn. Competition studies found that Pb and Cd have a higher affinity for the binding site than Zn. The signal ratio (Cd + peptide)/(Zn + peptide) was 1.39, and the ratio (Pb + peptide)/(Zn + peptide) was 1.85. Cadmium and lead displaced about 80 percent of the bound Zn ions, but an excess of Zn did not remove the bound toxic metals.</p>