Solution and gas-phase H/D exchange of protein-small-molecule complexes: Cex and its inhibitors.

The properties of noncovalent complexes of the enzyme exo-1,4-ß-D-glycanase ("Cex") with three aza-sugar inhibitors, deoxynojirimycin (X(2)DNJ), isofagomine lactam (X(2)IL), and isofagomine (X(2)IF), have been studied with solution and gas-phase hydrogen deuterium exchange (H/Dx) and measurements of collision cross sections of gas-phase ions. In solution, complexes have lower H/Dx levels than free Cex because binding the inhibitors blocks some sites from H/Dx and reduces fluctuations of the protein. In mass spectra of complexes, abundant Cex ions are seen, which mostly are formed by dissociation of complexes in the ion sampling interface. Both complex ions and Cex ions formed from a solution containing complexes have lower cross sections than Cex ions from a solution of Cex alone. This suggests the Cex ions formed by dissociation "remember" their solution conformations. For a given charge, ions of the complexes have greater gas-phase H/Dx levels than ions of Cex. Unlike cross sections, H/Dx levels of the complexes do not correlate with the relative gas-phase binding strengths measured by MS/MS. Cex ions from solutions with or without inhibitors, which have different cross sections, show the same H/Dx level after 15 s, indicating the ions may fold or unfold on the seconds time scale of the H/Dx experiment. Thus, cross sections show that complexes have more compact conformations than free protein ions on the time scale of ca. 1 ms. The gas-phase H/Dx measurements show that at least some complexes retain different conformations from the Cex ions on a time scale of seconds.

Mass spectra and ion collision cross sections of hemoglobin.

Mass spectra of commercially obtained hemoglobin (Hb) show higher levels of monomer and dimer ions, heme-deficient dimer ions, and apo-monomer ions than hemoglobin freshly prepared from blood. This has previously been attributed to oxidation of commercial Hb. Further, it has been reported that that dimer ions from commercial bovine Hb have lower collision cross sections than low charge state monomer ions. To investigate these effects further, we have recorded mass spectra of fresh human Hb, commercial human and bovine Hb, fresh human Hb oxidized with H(2)O(2), lyophilized fresh human Hb, fresh human Hb both lyophilized and chemically oxidized, and commercial human Hb oxidized with H(2)O(2). Masses of α-monomer ions of all hemoglobins agree with the masses expected from the sequences within 3 Da or better. Mass spectra of the ß chains of commercial Hb and oxidized fresh human Hb show a peak or shoulder on the high mass side, consistent with oxidation of the protein. Both commercial proteins and oxidized fresh human Hb produce heme-deficient dimers with masses 32 Da greater than expected and higher levels of monomer and dimer ions than fresh Hb. Lyophilization or oxidation of Hb both produce higher levels of monomer and dimer ions in mass spectra. Fresh human Hb, commercial human Hb, commercial bovine Hb, and oxidized commercial human Hb all give dimer ions with cross sections greater than monomer ions. Thus, neither oxidation of Hb or the difference in sequence between human and bovine Hb make substantial differences to cross sections of ions.

Carbamino group formation with peptides and proteins studied by mass spectrometry.

At high pH and in the presence of dissolved CO(2), the N-terminus and epsilon-amino groups of amino acids, peptides, and proteins can form carbamino adducts with CO(2), R-NH(2) + CO(2) <--> R-NHCOO(-) + H(+). We report the first study of carbamino group formation by electrospray ionization (ESI) mass spectrometry (MS). Angiotensin II, bradykinin, substance P, and insulin have been studied. A careful optimization of the instrumental parameters was necessary to allow the transfer of the fragile adducts into vacuum for mass analysis. Particularly, dissociation of the adducts in the ion sampling process and pH changes in ESI must be minimized. With these precautions, levels of carbamino group formation of angiotensin II and bradykinin determined from mass spectra agree with those expected to be in solution, calculated from literature equilibrium constants. Thus, ESI MS can quantitatively measure ratios of carbamino adduct to total peptide concentration in solution. Values of equilibrium constants for carbamino group formation with substance P (pK(c) = 4.77 +/- 0.18) and insulin (pK(c) = 4.99 +/- 0.05) are reported for the first time.

Structural characterization of hemoglobins from Monilifera and Frenulata tubeworms (Siboglinids): first discovery of giant hexagonal-bilayer hemoglobin in the former "Pogonophora" group.

Siboglinids are symbiotic polychete annelids having hemoglobins as essential oxygen- and sulfide-carriers for their endosymbiotic bacteria. We analyzed the structure of the hemoglobins from two species of siboglinids: the monilifera Sclerolinum contortum and the frenulata Oligobrachia webbi (i.e. haakonmosbiensis) from Norwegian cold seeps. Measured by Multi-Angle Laser Light Scattering (MALLS), Sclerolinum shows a 3190+/-50 kDa hexagonal bilayer hemoglobin (HBL-Hb) and a 461+/-46 kDa ring-Hb, just as vestimentifera, whereas Oligobrachia has a 409+/-3.7 kDa ring-Hb only. Electrospray Ionization-Mass Spectrometry (ESI-MS) showed Sclerolinum HBL-Hb composed of seven monomeric globins (15-16 kDa), three disulfide-bonded globin heterodimers and three linkers. The heterodimers always contain globin-b (15814.4+/-1.5 Da). Sclerolinum ring-Hb is composed of globins and dimers with identical masses as its HBL-Hb, but lacks linkers. Oligobrachia ring-Hb has three globin monomers (14-15 kDa) only, with no disulfide-bonded dimers. Comparison of Sclerolinum hemoglobins between Storegga and Haakon Mosby Mud Volcano, using the normalized height of deconvoluted ESI-MS peaks, shows differences in globin monomers abundances that could reflect genetic differences or differential gene expression between distinct seep populations. The discovery of HBL-Hb in Sclerolinum is a new element supporting the hypothesis of monilifera being phylogenetically more closely related to vestimentifera, than to frenulata.

Structural study of Carcinus maenas hemocyanin by native ESI-MS: interaction with L-lactate and divalent cations.

The interaction of L-lactate and divalent cations with Carcinus maenas hemocyanin has been probed by electrospray ionization mass spectrometry under conditions preserving noncovalent interactions (native ESI-MS). C. maenas native hemocyanin in the hemolymph occurs mainly as dodecamers and to a lesser extent as hexamers. A progressive acidification with formic acid after alkaline dissociation resulted in the preferential recruitment of the two lightest subunits into light dodecamers, a molecular complex absent from native hemolymph, in addition to regular dodecamers and hexamers. Addition of L-lactic acid also induced the recruitment of these subunits, even at alkaline pH. A dodecamer-specific subunit is needed to enable aggregation over the hexameric state. Experiments with EDTA suggested the existence of different binding sites and association constants for divalent cations within hexameric structures and at the interface between two hexamers. L-lactic acid specific interaction with the lightest subunits was not inhibited by removal of the divalent cations.

APCI interface for LC- and SEC-MS analysis of synthetic polymers: advantages and limits.

The main advantage of the APCI interface for the LC-MS analysis of synthetic polymers resides in its compatibility with the main chromatographic modes: reversed-phase liquid chromatography, normal-phase liquid chromatography, and size exclusion chromatography in organic phase, with the usual flow rates. Moreover, APCI can be used in positive or negative modes. Representative applications are described to highlight benefits and limitations of the LC-APCI-MS technique with the analysis of industrial polymers up to molecular masses of 5 kDa: polyethers; polysiloxanes; and copolymers of siloxanes. Results are discussed in regard to those obtained by more classical techniques: SEC and MALDI-MS. The use of an APCI interface in LC-MS and SEC-MS coupling applied to synthetic polymers is efficient up to 2000-4500 Da. The main drawback of the APCI interface is the in-source decomposition that is observed above m/z = 2000-3000 and can induce an underestimation of average molecular weights. However, APCI allows detection on a wide range of polarity of sample/solvent and appears to be complementary to ESI.

Noncovalent complexes between DNA and basic polypeptides or polyamines by MALDI-TOF.

MALDI-MS was evaluated as a method for the study of noncovalent complexes involving DNA oligonucleotides and various polybasic compounds (basic polypeptides and polyamines). Complexes involving single-stranded DNA were successfully detected using DHAP matrix in the presence of an ammonium salt. Control experiments confirmed that the interactions involved basic sites of the polybasic compounds and that the complexes were not formed in the gas phase but were pre-existing in the matrix crystals. Moreover, the pre-existence in solution was probed by isothermal titration calorimetry at concentration and ionic strength similar to those used for mass spectrometry. Spectra showed no important difference between negative and positive ion modes. The influence of nature and size of DNA and polybasic compound on the relative intensities and stoichiometries of the complexes was investigated. Despite the fact that relative intensities can be affected by ionization yields and the gas-phase stabilities of the different species, numerous trends observed in the MALDI study were consistent with the expected in-solution behaviors. Experimental conditions related to sample preparation were investigated also. Complex abundance generally decreased when increasing the ammonium acetate concentration. It was dramatically decreased when using ATT instead of DHAP. Penta-L-arginine is an exception to these observations. Lastly, in the case of complexes involving DNA duplex, the ATT matrix was shown to favor the observation of specific DNA duplex but not that of its complex with polybasic compounds. Inversely, DHAP was appropriate for the conservation of DNA-polybasic compound interaction but not for the transfer of intact duplex.

Macrotricycles featuring a pi-basic tetrahedral cavity: preference for NH4+ detected by electrospray ionization mass spectrometry.

Cation-pi interactions play an important role in biology. The title compounds are C3-symmetric macrotricycles built from resorcinol, a pi electron-rich arene. They were prepared in up to 18% yield by intramolecular cyclization of 1,3,5-trisubstituted benzene tripods bearing pendant resorcinol groups, with methylene acetal bridges. Positive ESI-MS showed that these receptors recognize NH4+ over K+, and poorly respond to the large t-BuNH3+ cation, suggesting that they bind NH4+ intramolecularly, presumably via cation-pi interactions.

A study of noncovalent complexes involving single-stranded DNA and polybasic compounds using nanospray mass spectrometry.

Noncovalent complexes involving a single-stranded DNA oligonucleotide and a polybasic compound (spermine, penta-L-lysine, penta-L-arginine, or polydisperse poly-L-lysine) were detected by nanospray-MS. Several control experiments tended to show that these complexes preexisted in solution and that the interactions were initially ionic ones between oligonucleotide phosphates and protonated basic sites of the polybasic compound. Collision-induced dissociation (CID) experiments carried out with these complexes allowed us to identify some differences in the nature of the interactions between the solution and the gas phase, arising from possible proton transfers. Different dissociation pathways were observed according to the nature of the polybasic compound and to the initial charge state of the complex. The complex involving spermine dissociated by cleavage of noncovalent bonds leading to the separation of the two components, whereas the one involving penta-L-arginine underwent fragmentations of covalent bonds. Both behaviors were independent of the initial charge state of the complex. On the other hand, the dissociation pathway of the complex involving penta-L-lysine has been shown to be clearly charge state dependent. Noncovalent dissociation (separation of the two components) driven by coulomb repulsion occurred for the higher charged complexes, whereas fragmentation of covalent bonds was the main pathway of the lower charged complexes. In the latter case, differences in CID behavior were observed for different lengths of poly-L-lysine.

Block lengths and block sequence of linear triblock and glycerol derivative diblock copolyethers by electrospray ionization--collision-induced dissociation mass spectrometry.

Chemical properties of ethylene oxide (EO) and propylene oxide (PO) block copolymers are strongly dependent on their sequence. Useful information about copolymer sequence can be obtained by tandem mass spectrometry (MS/MS). In this work, collision-induced dissociation (CID) of ammonium adducts of various linear triblock and glycerol derivative diblock copolyethers produced by electrospray ionization was studied under low-energy conditions. At first, homopolymers MS/MS spectra enabled us to identify the nature of the product ions and to suggest decomposition pathways. Then, it was shown that copolyethers with the same composition in each repeat unit but with inversed block sequences (i.e., PEO-b-PPO-b-PEO vs PPO-b-PEO-b-PPO and gPEO-b-PPO vs gPPO-b-PEO) can be easily distinguished with characteristic fragment ions. In the case of linear copolymers, CID spectra gave pertinent information about block lengths.

Analysis of poly(oxyethylene) and poly(oxypropylene) triblock copolymers by MALDI-TOF mass spectrometry.

Triblock copolymers of ethylene oxide (EO) and propylene oxide (PO) are widely used in the chemical industry as nonionic surfactants. Triblock copolymers can be arranged in a EO-PO-EO or PO-EO-PO sequence. This arrangement results in an amphiphilic copolymer, in which the block sequence and block length determine the properties of the copolymer. MALDI-TOF MS was used to analyze various triblock copolyethers: EO-PO-EO (Mn =2000 g.mol(-1)), PO-EO-PO (Mn = 2000 g.mol(-1)), and a random copolymer EO/PO (Mn = 2500 g.mol(-1)). Data treatment was assisted by using a homemade software allowing a picture of monomer composition of oligomers from the mass spectra. MALDI-TOF mass spectra of EO/PO copolymers were shown to depend strongly on the number of laser shots, relative proportions of polymer/salt, and the nature of the matrix. An unsaturated byproduct was detected. Its presence was demonstrated by prefractionation of copolymers by SEC before MALDI-TOF analysis, and its content was estimated by 1H NMR. The formation of layers inside the MALDI deposit was evidenced by varying the number of laser shots. Lighter oligomers of the copolymer, unsaturated byproduct, or both would be in the core of the deposit, coated with heavier oligomer. The layer formation depends on the nature of the matrix and the quantity of added salt. DHB matrix with a relative high sodium salt content induces layer formation inside the deposit, whereas dithranol matrix or low salt content does not. Consequently, an optimization of experimental parameters in order to estimate the lighter oligomers or unsaturated byproduct content or to obtain the actual representation of the monomer contribution in the copolymers from the MS data only seems obviously critical. MALDI-TOF mass spectrometry is obviously a powerful technique to analyze copolymers, but a careful survey of the experimental parameters is required. The combination of MALDI-TOF MS with separations techniques and NMR brings precious complementary information.