Selective dissociation of non-covalent bonds in biological molecules by laser spray

Overcoming aggregation with laser heated nanoelectrospray mass spectrometry: thermal stability and pathways for loss of bicarbonate from carbonic anhydrase II

Variable temperature electrospray mass spectrometry is useful for multiplexed measurements of the thermal stabilities of biomolecules, but the ionization process can be disrupted by aggregation-prone proteins/complexes that have irreversible unfolding transitions. Resistively heating solutions containing a mixture of bovine carbonic anhydrase II (BCAII), a CO2 fixing enzyme involved in many biochemical pathways, and cytochrome c leads to complete loss of carbonic anhydrase signal and a significant reduction in cytochrome c signal above ∼72 °C due to aggregation. In contrast, when the tips of borosilicate glass nanoelectrospray emitters are heated with a laser, complete thermal denaturation curves for both proteins are obtained in <1 minute. The simultaneous measurements of the melting temperature of BCAII and BCAII bound to bicarbonate reveal that the bicarbonate stabilizes the folded form of this protein by ∼6.4 °C. Moreover, the temperature dependences of different bicarbonate loss pathways are obtained. Although protein analytes are directly heated by the laser for only 140 ms, heat conduction further up the emitter leads to a total analyte heating time of ∼41 s. Pulsed laser heating experiments could reduce this time to ∼0.5 s for protein aggregation that occurs on a faster time scale. Laser heating provides a powerful method for studying the detailed mechanisms of cofactor/ligand loss with increasing temperature and promises a new tool for studying the effect of ligands, drugs, growth conditions, buffer additives, or other treatments on the stabilities of aggregation-prone biomolecules.

Observation of dimethylaminoethyl methacrylate-myoglobin binding reaction using laser spray mass spectrometry

Covalent bonds are often created by a reaction between chemicals and protein before causing various adverse effects in a cell. Dimethylaminoethyl methacrylate (DMAEMA), which has moderate toxicity, causes skin inflammation and throat irritation. For this study, we investigated a reaction mechanism between myoglobin and (DMAEMA) using a new analytical tool developed at our laboratory: laser spray mass spectrometry technique. It was found that initially DMAEMA was added to the amino group of protein by the Michael addition mechanism; the added DMAEMA was hydrolyzed to methacrylic acid using an autocatalytic system. The results of this study indicate the feasibility of the laser spray technique in analyses of reaction dynamics.

Measurement of sugars using the laser spray technique with a gold capillary

A gold (Au) capillary has higher thermal conductivity than a stainless steel capillary and can withstand capillary over-heating induced by high CO(2) laser irradiation (over 2.5 W) better than a stainless steel capillary. For this study, a laser spray using an Au capillary was applied for the detection of sugars. The signal of cationized compounds [M+Na](+) can be detected with higher sensitivity than with conventional laser sprays using high laser power (over 2.7 W). Using 3.5 W of laser power, the signal intensity is 15 times higher than the maximum value with stainless steel (2.3 W) in a 10(-5) M maltose aqueous solution. It is considered that almost all the water molecules evaporate by laser irradiation, which is impossible to achieve using a stainless steel capillary.

Thermal unfolding of proteins probed by laser spray mass spectrometry

The stability and conformational changes of cytochrome c (cyt c) at different temperatures and pH have been well examined so far by using various analytical methods. We have found that laser spray mass spectrometry enables much faster and more convenient monitoring of those changes of cyt c compared with other methods. The results correlated well with circular dichroism (CD) experiments under relatively acidic conditions, which destabilize the protein. Laser spray mass spectra of cyt c at various pH were obtained at different levels of laser power. Bimodal charge-state distributions of the protein were observed in laser spray mass spectra, indicating the two-state model of structural change; the lower charges correspond to the folded state, the higher charges to the unfolded state. Based on this result, the presumed denaturation curve of the protein was plotted as a function of laser power, and laser power by which 50% of the protein was assumed to be denatured, E50%, as obtained at each pH. We also examined the melting temperatures, Tm, of cyt c at various values of pH by using CD spectroscopy. The correlation coefficient between E50% and Tm for cyt c was 0.999, demonstrating an excellent correlation. Furthermore, laser spray analysis of ubiquitin, which is found to be more thermally stable than cyt c, gave a higher E50% than cyt c. These results indicate that laser spray mass spectrometry can be an extremely convenient method for probing thermal stabilities and dynamic conformational changes of proteins with subtle structural differences caused by slight changes in pH.

Denaturation of alpha-lactalbumin and ubiquitin studied by electrospray and laser spray

Electrospray and laser spray mass spectra of human alpha-lactalbumin and bovine ubiquitin were studied, with an emphasis on the denaturation induced by laser spray. There were no remarkable differences in the electrospray and laser spray mass spectra for acidic and basic aqueous solutions of alpha-lactalbumin in positive and negative modes of operations. This originates from the fact that this protein is tightly folded with four disulfide bonds. For ubiquitin, however, denaturation was induced by laser spray for the positive mode of operation and the [M+nH](n+) with a maximum of n = 13 was observed, i.e., all the acidic amino acid residues are fully neutralized (protonated). In contrast, the laser-induced denaturation was not observed for the negative mode of operation, i.e., denaturation of ubiquitin is largely suppressed in the negatively charged liquid droplets. The marked difference observed in the positive and negative modes of operations for ubiquitin is ascribed to the difference in the susceptibility of side-chain/main-chain interactions in the positive-ion excess and in the negative-ion excess liquid droplets. That is, the interactions between the basic residues and main-chain amide carbonyl groups (-NH(3) (+)***O=C< or -NH(2)***O=C<) which play an important role in stabilizing the protein structures are not so affected in the negative mode of operation but are weakened in the positive mode of operation.

Investigation of molecular interaction within biological macromolecular complexes by mass spectrometry

The advent of electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) has accelerated structural studies of biological macromolecular complexes. At present, mass spectrometry can provide accurate mass values not only of individual biological macromolecules but also of their assemblies. Furthermore, it can also give information on the interface sites of the biological macromolecular complexes. The present article focuses on the role of mass spectrometry in the investigation of biological molecular interactions, such as protein-protein, protein-DNA, and protein-ligand interactions, which play essential roles in various biological events.

Structural and Functional Characterization of Biological Macromolecules by Mass Spectrometry

Mass spectrometry has widely been used as a tool for identification of proteins in the research fields of biochemistry and clinical chemistry because it can provide accurate information on molecular masses of biological molecules with a small amount of sample in a short time. If mass spectrometry is properly used, it can also give information on the tertiary structure or on the molecular interactions of biological macromolecules. The present paper focuses on the role of mass spectrometry as a tool for the investigation on the tertiary structure of proteins and on the biological molecular interactions that play essential roles in various biological events.

Stability analysis for double-stranded DNA oligomers and their noncovalent complexes with drugs by laser spray

Laser spray, which is a newly developed ionization technique, can characterize the stability of noncovalent complexes in the solution phase. By using this advantage, laser spray has been applied to probe the intrinsic stability of double-stranded DNA (dsDNA) sequences and their binding affinities with various drugs in the solution phase. Systematic experiments were carried out using six 16-mer and three 22-mer dsDNA oligomers, together with the complexes of the 16-mer dsDNA with minor groove binders: berenil, Hoechst 33342, DAPI, and netropsin. Dissociation curves for each dsDNA or each complex were plotted as a function of laser power. The laser power (E50%), where 50% of each dsDNA or each complex was dissociated, was compared with its melting temperature (Tm) determined by UV spectroscopy. Linear correlations between E50% and Tm were obtained not only for the dsDNA oligomers (correlation factor r = 0.9835) but also for the 16-mer dsDNA complexes with minor groove binders (r = 0.9966). In addition, laser spray has successfully clarified the binding affinities of a 16-mer dsDNA with two intercalators: daunomycin and nogalamycin. In the case of the dsDNA-daunomycin complex, by changing the molar ratio of dsDNA : drug from 1 : 1 to 1 : 5, the concentration-dependent stability of the complex was confirmed by laser spray. The present results demonstrate that laser spray mass spectrometry can be a powerful and convenient method to investigate the relative binding affinities of dsDNA-ligand complexes in the solution phase, which could be applied to the early stage of high-throughput screening of drugs targeting for dsDNA.

Evaluation of binding affinity of protein-mutant DNA complexes in solution by laser spray mass spectrometry

We have applied laser spray mass spectrometry developed by Hiraoka et al. to investigate the binding affinity of protein-mutant DNA complexes. The results were compared with our previous data of collision-induced dissociation (CID) experiments using electrospray ionization mass spectrometry (ESI-MS). Systematic experiments were carried out on the complexes of the c-Myb DNA binding domain (c-Myb DBD) bound to eight kinds of 16- or 22-mer point mutant double-stranded DNA (dsDNA), whose solution K(d) values are different in the range from 10(-9) M to 10(-7) M. The dissociation curve as a function of laser power was plotted for each complex, and the laser power where 50% of complex was dissociated (E(50%)) in population was obtained. The correlation coefficient between E(50%) and the relative binding free-energy change (DeltaDeltaG) of each complex formation in solutions was 0.9808, which is much better than the coefficient obtained by the previous ESI-CID experiments that was 0.859. In addition, complexes of the c-Myb DBD with five other mutant dsDNA were also examined to confirm that laser spray can be used to estimate the K(d) values of a DNA-protein complex in solutions if an appropriate calibration curve is available. In the process of laser spray, dissociations of these noncovalent complexes occur in solutions, but not in the gas phase. This differs greatly from ESI-CID. Laser spray mass spectrometry has been found to be better than ESI-CID in evaluating binding affinity of a protein to various mutant DNA.

Denaturation of lysozyme and myoglobin in laser spray

In laser spray, the tip of an electrospray capillary is irradiated with a continuous CO(2) laser beam. Here, we report results from a modified laser spray method that employs a relatively low laser irradiance level. With a laser power of approximately 2 W and a focal spot size ( approximately 0.3 mm), which covered the entire front surface of the electrospray capillary, the irradiance was approximately 3 x 10(3) W/cm(2). This resulted in a quiescent and smooth vaporization of aqueous solutions. This "evaporation-mode" laser spray method yielded the best results so far obtained in our laboratory with laser-irradiated electrospray, producing higher and more stable signals. The method was applied to the analysis of aqueous solutions of lysozyme and myoglobin. Mass spectra were obtained as a function of laser power from 0 W (electrospray) to approximately 2 W. The spray generated at the tip of the stainless steel capillary was observed with a CCD camera. With increase of laser power, the droplets in the spray became finer and the Taylor cone became progressively smaller. The strongest ion signals were recorded when the sample solution protruded only slightly from the tip of the capillary. A broadening of the lysozyme charge-state distribution, attributable to protein unfolding, was observed with a laser power of 2 W. No denaturation of myoglobin took place up to a laser power of 1.6 W. However, a sudden onset of denaturation was observed at 1.8 W as a broadening of the myoglobin charge distribution and the appearance of apo-myoglobin peaks. These findings demonstrate that laser spray is capable of dissociating the noncovalent complexes selectively without breaking covalent bonds.

Evaluation of protein-DNA binding affinity by electrospray ionization mass spectrometry

Binding affinity of complexes between a DNA-binding domain (DBD) of a transcription factor, c-Myb, and several double-stranded DNA (dsDNA) were evaluated by collision-induced dissociation (CID) of the multiply protonated molecules generated by electrospray ionization mass spectrometry (ESI-MS). Complexes of the c-Myb DBD and dsDNA were prepared in solution and analyzed by ESI-MS. Multiply protonated molecules of a high-affinity complex, the c-Myb DBD and dsDNA with a specific sequence, were clearly observed in ESI mass spectrum. Protonated molecules of the complex were quite stable in the gas-phase, and not easily dissociated even if high cone voltage was applied in the first vacuum chamber source when the sample was prepared in 10 mM ammonium acetate. As for the sample prepared in buffer with higher concentration of ammonium acetate, such as 500 mM ammonium acetate, protein-dsDNA complexes could easily be dissociated with an increase in the cone voltage, giving multiply protonated molecules of free c-Myb DBD and some DNA fragments. Systematic CID experiments were carried out on seven complexes between the c-Myb DBD and 22-mer dsDNA with different solution-Kd values in the range of 10(-9) M to 10(-7) M. For each complex dissociation curve as a function of cone voltage was plotted, and the cone voltage where 50% of the complex was dissociated (V(50%)) was calculated. Consequently, positive correlation was obtained between V(50%) and relative binding free energy change (DeltaDeltaG) in complex formation in solution. This suggests that ESI-CID experiments can provide quantitative evaluation of the stability of protein-DNA complexes based on proper calibration.

Current literature in mass spectrometry

In order to keep subscribers up‐to‐date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of mass spectrometry. Each bibliography is divided into 11 sections: 1 Books, Reviews & Symposia; 2 Instrumental Techniques & Methods; 3 Gas Phase Ion Chemistry; 4 Biology/Biochemistry: Amino Acids, Peptides & Proteins; Carbohydrates; Lipids; Nucleic Acids; 5 Pharmacology/Toxicology; 6 Natural Products; 7 Analysis of Organic Compounds; 8 Analysis of Inorganics/Organometallics; 9 Surface Analysis; 10 Environmental Analysis; 11 Elemental Analysis. Within each section, articles are listed in alphabetical order with respect to author (5 Weeks journals ‐ Search completed at 8th. Sept. 2004)