A Bis-Cyclopentadienyl Ligand-Supported Di-Iron Trihydride Motif as a Synthon for Access to Heterobimetallic Trinuclear Complexes.

Herein, we report the synthesis of a flexible bis-cyclopentadienyl ligand L (the doubly deprotonated form of H2L (1,3-bis(2,4-di-tert-butylcyclopentadienyldimethylsilyl)benzene)), demonstrating its ability to stabilize a series of di-iron hydrido complexes. Notably, this ligand facilitates the isolation of an unprecedented anionic cyclopentadienyl ligand-supported di-iron trihydride complex, LFe2(µ-H)3Li(THF) (2), functioning as a synthon for the [Fe2(µ-H)3]- core and providing access to heterobimetallic complexes 4-6 with coinage metals.

Access to Monomeric Lead(II) Hydrides with Remarkable Thermostability and Their Use in Catalytic Hydroboration of Carbonyl Derivatives.

We report on the remarkable stability of unprecedented, monomeric lead(II) hydrides M+[LPb(II)H]- (M[1-H]), where L = 2,6-bis(3,5-diphenylpyrrolyl)pyridine and M = (18-crown-6)potassium or ([2.2.2]-cryptand)potassium. The half-life of [K18c6][1-H] of ∼2 days in tetrahydrofuran at 25 °C is significantly longer than those reported for dimeric lead(II) hydrides supported by bulky terphenyl ligands (few hours at low temperatures), which are the only examples known for lead(II) hydride compounds. The presence of a Pb-H bond in [1-H]- was unambiguously identified by multinuclear NMR spectroscopy. Remarkably, a 1H resonance of the hydride ligand was found at δ = 41.43 ppm (1JPbH = 1312 Hz). For reactivity study, [1-H]- serves as an excellent hydroboration catalyst with high turnover numbers and turnover frequencies for several carbonyl compounds.

Simple and specific dual-wavelength excitable dye staining for glycoprotein detection in polyacrylamide gels and its application in glycoproteomics.

In this study, a commercially available fluorescent dye, Lissamine rhodamine B sulfonyl hydrazine (LRSH), was designed to specifically stain the glycoproteins in polyacrylamide gels. Through the periodate/Schiff base mechanism, the fluorescent dye readily attaches to glycoproteins and the fluorescence can be simultaneously observed under either 305 nm or 532 nm excitation therefore, the dye-stained glycoproteins can be detected under a regular UV transilluminator or a more elegant laser-based gel scanner. The specificity and detection limit were examined using a standard protein mixture in polyacrylamide gels in this study. The application of this glycoprotein stain dye was further demonstrated using pregnancy urine samples. The fluorescent spots were further digested in gel and their identities confirmed through LC-MS/MS analysis and database searching. In addition, the N-glycosylation sites of LRSH-labeled uromodulin were readily mapped via in-gel PNGaseF deglycosylation and LC-MS/MS analysis, which indicated that this fluorescent dye labeling does not interfere with enzymatic deglycosylation. Hence, the application of this simple and specific dual-wavelength excitable dye staining in current glycoproteome research is promising.

Functional phosphoproteomic profiling of phosphorylation sites in membrane fractions of salt-stressed Arabidopsis thaliana.

BACKGROUND: Under conditions of salt stress, plants respond by initiating phosphorylation cascades. Many key phosphorylation events occur at the membrane. However, to date only limited sites have been identified that are phosphorylated in response to salt stress in plants. RESULTS: Membrane fractions from three-day and 200 mM salt-treated Arabidopsis suspension plants were isolated, followed by protease shaving and enrichment using Zirconium ion-charged magnetic beads, and tandem mass spectrometry analyses. From this isolation, 18 phosphorylation sites from 15 Arabidopsis proteins were identified. A unique phosphorylation site in 14-3-3-interacting protein AHA1 was predominately identified in 200 mM salt-treated plants. We also identified some phosphorylation sites in aquaporins. A doubly phosphorylated peptide of PIP2;1 as well as a phosphopeptide containing a single phosphorylation site (Ser-283) and a phosphopeptide containing another site (Ser-286) of aquaporin PIP2;4 were identified respectively. These two sites appeared to be novel of which were not reported before. In addition, quantitative analyses of protein phosphorylation with either label-free or stable-isotope labeling were also employed in this study. The results indicated that level of phosphopeptides on five membrane proteins such as AHA1, STP1, Patellin-2, probable inactive receptor kinase (At3g02880), and probable purine permease 18 showed at least two-fold increase in comparison to control in response to 200 mM salt-stress. CONCLUSION: In this study, we successfully identified novel salt stress-responsive protein phosphorylation sites from membrane isolates of abiotic-stressed plants by membrane shaving followed by Zr4+-IMAC enrichment. The identified phosphorylation sites can be important in the salt stress response in plants.

Development of a titanium dioxide nanoparticle pipette-tip for the selective enrichment of phosphorylated peptides.

The selective enrichment of specific proteins or peptides on micropipette tips prior to mass spectrometry analysis, which can minimize non-specific interferences as well as sample loss, has been an important issue in current proteomics field. In this paper, we have developed an easy-to-use phosphopeptide-selective pipette tip in which titanium dioxide nanoparticles were embedded in monolithic structure photopolymerized from ethylene glycol dimethacrylate. The simple and convenient fabrication was feasible in a commercial polypropylene pipette tip. Phosphorylated peptides were isolated from non-phosphopeptides by TiO(2) nanoparticle and eluted by 100 mM ammonium phosphate (pH 8.5), which was compatible with 2,5-dihydroxybenzoic acid (DHB)/1% phosphoric acid matrix and allowed for direct analysis of the elution fraction by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) without the necessity of desalting pretreatment. Tryptic digested alpha-casein and beta-casein spiked into bovine serum albumin (BSA) nonphosphorylated peptides (molar ratio 1:1:10) were used to assess the selectivity of TiO(2) tips. The effect of 50 mM ammonium hydrogencarbonate, pH 8 in 50% acetonitrile used as a wash buffer in reduction of nonspecific bound peptide to TiO(2) tip was dramatic. Almost all non-phosphopeptides were not detected by MALDI-MS analysis. The lowest detectable amount of phosphopeptide was estimated at low femtomole level. The easy-to-use TiO(2)-embeded tips operated in combination with the modified wash and elution conditions enable an efficient phosphopeptide enrichment for mass spectrometric analysis.

Magnetic bead-based hydrophilic interaction liquid chromatography for glycopeptide enrichments.

Purification of glycopeptides prior to the analysis by mass spectrometry (MS) is demanded due to ion suppression effect during ionization caused by the co-presence of non-glycosylated peptides. Among various purification methods, hydrophilic interaction liquid chromatography (HILIC) has become a popular method in recent years. In this work, we reported a novel magnetic bead-based zwitterionic HILIC (ZIC-HILIC) material which was fabricated by coating a zwitterionic polymer synthesized by spontaneous acid-catalyzed polymerization of 4-vinyl-pyridinium ethanesulfonate monomer on iron oxide magnetic nanoparticles. The resulting magnetic ZIC-HILIC nanoparticles were shown to provide high specificity and high recovery yield (95-100%) for the enrichment of glycopeptides from a standard glycoprotein, fetuin, using a simple magnetic bar. In addition, we proposed a two-step HILIC enrichment strategy using magnetic ZIC-HILIC nanoparticles for a large scale analysis of glycoproteins in complex biological samples. Using this approach, we identified 85 N-glycosylation sites in 53 glycoproteins from urine samples. Two novel glycosylation sites on N513 of uromodulin and N470 of lysosomal alpha-glucosidase which have not yet been reported were identified by two-step HILIC approach. Furthermore, all these identified sites were confirmed by studies conducted using PNGase F deglycosylation and 18O enzymatic labeling.

Assignment of disulfide-linked peptides using automatic a1 ion recognition.

We present a novel approach for the assignment of peptides containing disulfide linkages. Dimethyl labeling is introduced to generate labeled peptides which exhibit enhanced a1 ion signals during MS/MS fragmentation. For disulfide-linked peptides, multiple a1 ions can be observed due to multiple N-termini. This distinct feature allows sieving out the disulfide-linked peptides; meanwhile, the N-terminal amino acids can be identified. With such information, the number of possible peptide combinations involved in a disulfide bond dramatically narrows down. Furthermore, we developed a computational algorithm to perform target a1 ion screening followed by molecular weight matching of cysteine-containing peptides with specific amino acids at the N-termini. Once the protein sequence and the peak list from a LC-MS/MS survey scan of labeled peptides are imported, the identities of disulfide-linked peptides can be readily obtained. The presented approach is simple and straightforward, offering a valuable tool for protein structural characterization.

Enhanced a1 fragmentation for dimethylated proteins and its applications for N-terminal identification and comparative protein quantitation.

In this work, dimethyl labeling at the protein level was developed to assist the fragmentation of intact proteins using the Q-TOF instrument. It was shown that a1 ions were favorably enhanced upon collision-induced dissociation for dimethylated proteins with molecular mass below 20 kDa and without N-terminal modifications. This method is helpful in confirming proteolytic sites located at the N-terminus of proteins. Moreover, this labeling could be incorporated with stable isotopes for comparative profiling at the protein level, in which the heavily labeled and lightly labeled a1 ions were generated from the corresponding proteins upon high-voltage collisions in a broad mass region that covered all of the charge states of the proteins. Using hemoglobin as an example, a linear dynamic range from 1:1 to 1:20 was satisfactorily obtained with an R2 value greater than 0.99. This approach appears to be promising for top-down proteomics.

Photopolymerized microtips for sample preparation in proteomic analysis.

We demonstrate a novel method for the fabrication of disposable plastic microtips, which we name "EasyTip", by a photopolymerization technique. C18 reversed-phase (C18) and ion metal affinity chromatography (IMAC) beads were immobilized on a plastic pipette tip, made of polypropylene materials, by photo-initiated polymerization. The fabricated EasyTips can be manipulated using commercial pipettes for wash/elution of minute amount of biological samples (< 10 microL) and can be applied for mass spectrometry (MS)-based proteomic analysis, in which the detection sensitivity depends critically on the optimal sample preparation. The recovery of a sample of 25 fmol of tryptic hemoglobin digest loaded in a C18 EasyTip was near 100% and we estimated the loading capacity to be around 0.4-2.0 microg of total proteins or peptides, which is well above a sufficient quantity for MS analysis. The effectiveness of the C18 EasyTips in enhancing the detection sensitivity of matrix-assisted laser desorption/ionization (MALDI)-MS signal, and thus providing a greater sequence coverage, was also demonstrated by the analysis of hemoglobin digest and the in-gel digested epidermal growth factor receptor (EGFR) protein from A431 cell lysate. We also demonstrated the usefulness of the immobilized IMAC EasyTips in extracting the signal of tryptic phosphopeptides of beta-casein (10 pmol) having one and four phosphorylation sites by using an IMAC EasyTip prior to off-line analysis by MS. The combination of IMAC EasyTips and MALDI-MS allowed the unambiguous identification of phosphopeptides based on the phosphatase assay as well as the post-source decay. Compared to other miniaturized devices, this fabrication method is simple, cheap, and requires less human intervention. Moreover, the method of manipulating the EasyTips is straightforward and can be automated readily by a robotic system for high-throughput analysis.