T. Gregory Schaaff

Transition from nanoparticle to molecular behavior: a femtosecond transient absorption study of a size-selected 28 atom gold cluster

AbstractTheultrafastelectrondynamicsofchemicallypreparedgoldnanoclusterswitha28atomgoldcoresurroundedby16glutathionemoleculeswereinvestigated.Afterexcitationwithfemtosecondlaserpulsestheseclustersshowaninducedtransientabsorptioninthevisiblefrom2.58to1.65eV(480–750nm)withamaximumaround2.07eV(600nm).Theexcitedstaterelaxationshowsabiexponentialdecaywithasubpicosecondandalongernanoseconddecaytimein-dependentofthelaserpumppower.Theseresultsaredifferentfromthoseobservedpreviouslyforlargergoldnano-particles,whichsuggeststhattheAu 28 –glutathionesystemshowsmolecularproperties. 2002ElsevierScienceB.V.Allrightsreserved. 1. IntroductionRecentlythechemicalsynthesisandstructuralcharacterizationofaverystable,water-soluble28atomgoldclusterwhichiscappedbyamonolayerof16tripeptideglutathioneunits(GSH¼c–Glu–Cys–Gly)havebeenreported[1,2].Theopticalabsorption spectrum of these clusters is wellstructuredwithadistinctabsorptiononsetnear1.31eV(950nm)correspondingtoanelectronicgapbetweenthehighestoccupied(molecular)or-bitalandthelowestunoccupiedorbital(HOMO–LUMOgap)[1].Afirststrongabsorptionmaxi-mumisseenat1.84eV(675nm)followedbyacontinuousincreaseintheabsorptionintensityathigherenergies.Inarecentluminescencestudy[3]itwasfurtherfoundthattheseclustershavetwoseparateluminescencebandswithmaximaat1.5and1.15eV(800and1100nm)[3].Thetotalquantumyieldoftheluminescenceasmeasuredatambient temperature was approximated to beaboutð3:5 1:0Þ 10

Gas phase H/D exchange kinetics: DI versus D2O

The gas phase H/D exchange reactions of bradykinin (M + 3H)3+ ions with D2O and DI were monitored in a quadrupole ion trap mass spectrometer. The H/D exchange kinetics of both chemical probes (D2O and DI) indicate the presence of two noninterconverting reactive gas phase ion populations of bradykinin (M + 3H)3+ at room temperature. The H/D exchange involving DI, however, generally proceeds faster than that involving D2O. The rate observations described here can be rationalized on the basis of the “relay mechanism” (see Campbell et al. J. Am. Chem. Soc.1995, 117, 12840–12854) recently proposed to account for H/D exchange between D2O and gaseous protonated polypeptides. The higher exchange rate with DI is believed to arise primarily as a result of its lower gas-phase acidity relative to that of D2O and, secondarily, as a result of the longer bond length of DI relative to that of OD in D2O.

Hydroiodic acid attachment kinetics as a chemical probe of gaseous protein ion structure: bovine pancreatic trypsin inhibitor.

The kinetics of attachment of hydroiodic acid (HI) to the (M + 6H)6+ ions of native and reduced forms of bovine pancreatic trypsin inhibitor (BPTI) in the quadrupole ion trap environment are reported. Distinctly nonlinear (pseudo first-order) reaction kinetics are observed for reaction of the native ions, indicating two or more noninterconverting structures in the parent ion population. The reduced form, on the other hand, shows very nearly linear reaction kinetics. Both forms of the parent ion attach a maximum of five molecules of hydroiodic acid. This number is expected based on the amino acid composition of the protein. There is a total of 11 strongly basic sites in the protein (i.e., six arginines, four lysines, and one N-terminus). An ion with protons occupying six of the basic sites has five available for hydroiodic acid attachment. The kinetics of successive attachment of HI to the native and reduced forms of BPTI also differ, particularly for the addition of the fourth and fifth HI molecules. A very simple kinetic model describes the behavior of the reduced form reasonably well, suggesting that all of the neutral basic sites in the reduced BPTI ions have roughly equal reactivity. However, the behavior of the native ion is not well-described by this simple model. The results are discussed within the context of differences in the three-dimensional structures of the ions that result from the presence or absence of the three disulfide linkages found in native BPTI. The HI reaction kinetics appears to have potential as a chemical probe of protein ion three-dimensional structure in the gas phase. Hydroiodic acid attachment chemistry is significantly different from other chemistries used to probe three-dimensional structure and hence, promises to yield complementary information.

Ion trap collisional activation of protonated poly(propylene imine) dendrimers: generations 1–5

Abstract Ions derived from electrospray ionization of poly(propylene imine) dendrimers (generations 1–5, synthesized from a 1,4-diaminobutane core) were subjected to ion trap tandem mass spectrometry. In some cases, ion/ion proton transfer reactions were used to form low charge state parent ions from higher charge state ions generated by electrospray. In addition, ion/ion proton transfer reactions performed on product ions formed by ion trap collisional activation of multiply charged parent ions facilitated interpretation of tandem mass spectrometry (MS/MS) spectra. Almost all of the products derived from dendrimer parent ions could be rationalized based on a set of dissociation processes involving a previously noted intramolecular nucleophilic attack by a nearest-neighbor nitrogen atom on a carbon alpha to a protonation site. For a given protonation site, attack can occur either from the adjacent nitrogen closer to the interior or closer to the exterior of the dendrimer. The processes that lead to the dominant products are highly dependent upon parent ion charge state. Singly charged ions fragment primarily by processes directed by protonation at a diaminobutane nitrogen whereas highly charged parent ions fragment largely by processes directed by protonation closer to the periphery of the dendrimer. MS/MS data for singly charged ions of series of synthesis failure products in the fourth and fifth generation dendrimers were collected. The results show that ion trap tandem mass spectrometry can provide information about the composition of mixtures of isomeric products resulting from side reactions that occur during the course of the multistep syntheses of the dendrimers. The data show, for example, that synthesis failures tend to occur on one side of the dendrimer.

A secondary ion microprobe ion trap mass spectrometer

An ion trap mass analyzer has been attached to an organic secondary ion microprobe. A pressure differential >100 can be maintained between the ion trap and microprobe. The well-focused secondary ion beam can transit a small (2 mm) diameter tube, but gas flow from ion trap to microprobe is impeded. This pressure differential allows the microprobe to retain imaging capability. Ion trap and microprobe data systems are integrated by taking advantage of the highly reproducible periodicity of the ion trap operating in resonant ejection mode and asynchronous signal and data acquisition afforded by commercially available interface cards. Secondary ion mass spectra and images obtained indicate an approximately 10-fold improvement in sensitivity, although preliminary evidence indicates low (<1%) trapping efficiency. Image data acquisition using the ion trap for mass analysis requires at least 10 times as much time compared to using a quadrupole mass filter because the mass-selected instability mode is used for mass analysis, i.e., mass resolution in the ion trap is not continuous as it is in the quadrupole.

Reactions of angiotensin ions with hydroiodic acid

Abstract The kinetics of HI attachment to gaseous angiotensin-related ions were determined in a quadrupole ion trap mass spectrometer. The (M + H) + and (M + 2H) 2+ ions of peptides with sequences of DRVYIHPFHL, NRVYVHPFHL, and RVYIHPFHL and the (M + H) + ions of DRVYIHPF and NRVYVHPF and their respective methyl esters were studied. For many of the ions studied here, multiple reactive conformations were resolved by their differing reactivities. The kinetics of the attachment of HI to the singly charged ions are consistent with structural models that are generated by molecular mechanics calculations, which suggest a high degree of intraionic interactions between the protonation site and other basic sites in the ion. The incorporation of HI can also disrupt the inherent intraionic interactions in the ions, which is not only reflected in the reaction kinetics, but also consistent with the interactions suggested by the molecular mechanics simulations. These results confirm that HI attachment kinetics can be used as a probe of three dimensional ion structure and provide important new information regarding the utility of this molecular probe.