Mobility of Proteins in Porous Substrates under Electrospray Ionization

May 5, 2016 - ... and Chemical Technology, The Hong Kong Polytechnic University, ...... Wong , M. Y. M.; Tang , H. W.; Man , S. H.; Lam , C. W.; Che ,...
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Mobility of Proteins in Porous Substrates under Electrospray Ionization Conditions Bin Hu, and Zhong-Ping Yao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b00894 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 7, 2016

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Analytical Chemistry

Mobility of Proteins in Porous Substrates under Electrospray Ionization Conditions Bin Hu†§, Zhong-Ping Yao†‡§* †

State Key Laboratory for Chirosciences, Food Safety and Technology Research Centre and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China ‡ Key Laboratory of Natural Resources of Changbai Mountain and Functional Molecules (Yanbian University), Ministry of Education, Yanji, 133002, China § State Key Laboratory of Chinese Medicine and Molecular Pharmacology (Incubation) and Shenzhen Key Laboratory of Food Biological Safety Control, Shenzhen Research Institute of The Hong Kong Polytechnic University, Shenzhen, 518057, China ABSTRACT: Proteins are important substances in living organisms and characterization of proteins is an indispensible part for protein study. Analysis of proteins using electrospray ionization mass spectrometry (ESI-MS) with porous substrates was investigated in this study. The results revealed that the ionization process had two stages. At the first stage, mobility and resulting spectra of proteins were similar to those obtained with conventional capillary-based ESI-MS. At the second stage, hydrophobichydrophobic interactions between proteins and the tip surfaces play an important role in mobility and detectability of protein ions, which were size and shape dependent, and a linear relationship could be found between the peak area of selected ion chromatogram and the cross section of protein ions. Preparative separation of proteins could be achieved by collecting the proteins remained on the porous substrates. These results led us to propose that electrospray ionization from porous substrates offer a potential approach for analysis of proteins and investigation of protein structures and conformations.

Proteins play critical roles in life activities and are hot targets of fundamental and applied research in chemistry and biology. Characterization of proteins provides information crucial to understanding the structures and functions of proteins. Techniques such as liquid chromatography (LC),1,2 gel electrophoresis,3,4 capillary electrophoresis (CE)5,6 and capillary electrochromatography (CEC)7,8 are used for separation of proteins, and mass spectrometry (MS),9-13 nuclear magnetic resonance spectroscopy14,15 and X-ray crystallography16,17 are widely used for investigation of protein structures, conformations and interactions. Among these techniques, MS is superior to others in speed, sensitivity and specificity for characterization of proteins. Combination of MS with hydrogen/deuterium exchange18,19 and ion mobility (IM)20-24 have allowed mapping and measurements of different conformations of proteins. Electrospray ionization (ESI) is one of the most powerful ionization techniques for MS characterization of proteins.11,25 Under ESI conditions, a protein is typically detected as a series of multiply charged ions that can be correlated to the protein conformations, and selective deposits of these multiply charged ions of proteins on a surface in vacuo using the soft-landing method has been reported.26 Conventional ESI uses capillary for sample loading and ionization. Upon application of a high voltage, sample solution sprays out of the capillary to form fine droplets. Solvent evaporation and repeated droplet fission ultimately lead to

production of gas-phase ions of analytes. ESI is a very gentle ionization technique and allows maintenance and investigation of protein conformations and interactions. Non-capillary ESI has also been developed and solid substrates such as metal needles, paper and wood tips have been successfully applied for ESI analysis of various samples.27-30 Use of solid substrates for sample loading and ionization much facilitates sample introduction, avoids sample clogging and allows analysis of samples of various forms.31,32 Moreover, new ionization features were found with ESI on solid substrates. For example, it was reported that proteins could be ionized separately from salts or detergents in the solutions with ESI using stainless steel needles,33,34 presumably because the small amount of solution loaded on the hard and smooth tip end could be considered as a large droplet, and proteins, the more hydrophobic components of the solutions, were easier to accumulate on the surface to be ionized. Herein we report that ESI with porous substrates could allow separation of proteins based on their sizes and shapes, including separation of the same proteins with different charge states. The separated proteins can be monitored by the ESI spectra or by analysis of the proteins remained on the substrates, and the latter can also allow preparative separation of proteins. This new observation implies a potential approach for investigation of proteins. The settings of solid-substrate ESI-MS is shown in Figure S1. When a sample solution is loaded onto the solid-substrate

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tip, with the application of a high voltage, the solution can be sprayed out of the tip and spectra of the sample can be obtained. In this study, an aliquot of 10 µL solution containing two proteins, alpha-lactalbumin (14.2 kDa) and insulin (5.7 kDa), was loaded onto a porous polyester tip and analyzed with the technique. The spraying lasted for about 2.5 min and the obtained selected ion chromatograms (SICs) of the two proteins are shown in Figure 1a. Two obviously different stages were observed during the ionization period. At duration 1 (T1), both proteins were abundantly observed in the mass spectrum (Figure 1b), which was very similar to the spectrum obtained by conventional ESI (Figure 1d). At duration 2 (T2), however, insulin, the smaller protein, was almost exclusively detected in the spectrum (Figure 1c). Such two-stage ionization and elution order could be reproducibly observed with the same polyester tip (after cleaning and reuse) or with differernt polyester tips (Figure S2), and was assumed to be related to the use of the porous substrate for ionization. As illustrated in Figure S1, when a solution was loaded onto a porous tip for electrospray ionization, initially, the pores on the tip were overwhelmed by the solution and the movement and ionization of the sample solution was mainly a bulk solution behavior, in which all the protein molecules sprayed out together, leading to a spectrum similar to that obtained by capillary-based ESI-MS. The second stage occurred when the solution was consumed to a low level. At this stage, driven by the electrospray, protein molecules migrated along the microchannels at the surface of porous substrate, and the results suggested that smaller molecules were easier to be sprayed out to be detected (Figure 1c).

Figure 1. Analysis of the mixture of insulin and alpha-lactalbumin. a) SICs of the 4+ ion of insulin and the 9+ ion of alphalactalbumin obtained by polyester-tip ESI-MS; b) mass spectrum obtained for duration T1 in a); c) mass spectrum obtained for duration T2 in b); d) mass spectrum obtained by conventional ESI-MS.

To verify the separation effect of the porous tip and detect the proteins remained on the tip, after the ESI process, the front 1.0 mm part of the tip was cut out and extracted with solvent. The spectrum obtained by analysis of the extract using conventional ESI-MS showed that only insulin, the smaller

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protein, was in this most front part (see Figure S3a). Analysis of the other part of the tip revealed that it contained both alpha-lactalbumin and insulin, with alpha-lactalbumin predominated (see Figure S3b), indicating that alphalactalbumin, the larger protein, was more retained in the porous substrate. These results were consistent with spectral results obtained by polyester-tip ESI-MS, and suggested that under the ESI conditions, porous substrates could allow separation of proteins with smaller proteins moving faster to be separately detected. Scanning electron microscope (SEM) was used to examine the properties of the polyester tips in this study. As shown in Figure S4, the SEM images of the surfaces and cross sections of polyster tips revealed the existence of microchannels in the polyster tips. However, the pore size of the microchannels were in the range of tens of micrometers, much larger than the cross sections of proteins, which are typically in the range of nanometers.35 Therefore, the polyster tips should not be able to physically serve as filters for separation of proteins. To better understand the mechanism for the separation, effect of tip materials on the protein separation was investigated in this study. In addition to polyester tips, wooden tips as well as polyethylene tips of different densities were used for analysis of the alpha-lactalbumin/insulin samples. Among these tips, polyethylene tips are most hydrophobic while wooden tips are most hydrophilic, yet wooden tips contain lignins36 that also allow for some hydrophobichydrophobic interactions. As shown in Figure S5, similar twostage ionization and the same elution and ionization orders were observed for these porous tips, as for the polyester tips (Figure 1). However, it could be found that the proteins were more retained by the more hydrophobic tips, and compared to wooden tips and polyester tips, polyethylene tips allowed better separation of protein mixtures (see Figure S5b). This suggested that hydrophobic-hydrophobic interactions between proteins and the substrate surfaces played an important role in the protein mobility and separation. Alpha-lactalbumin, which has a larger size than insulin, had stronger hydrophobichydrophobic interactions with the tip surfaces and was thus more retained in the tips. Such behavior was more significant with the more hydrophobic tips, leading to the better separation effect with the polyethylene tips. Much longer signal duration was observed with the denser polyethylene tips for analysis of the same volume of the samples (Figure S5c), due to the larger surface areas for interactions between analytes and surface as well as the reduced diffusion of the solution. To confirm the role of hydrophobic-hydrophobic interactions on the separation effect, a series of tripeptides (GGG, GGA, GGV, GGF, GGL and GGY) containing varied C-terminal amino acids with increasing nonpolar surface areas (see Figures S6a and S6b) were used. These tripeptides were previously used as model molecules for studying the ionization mechanism of ESI, demonstrating that during the spraying in ESI, the peptides with more nonpolar regions had higher tendency to accumulate on the droplet surface and thus could had higher response.37 In this study, a solution containing equal molar concentration of the six tripeptides was analyzed with polyester-tip ESI-MS. As shown in Figure S6c, a similar two-stage ionization was observed for this peptide

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mixture. At duration 1 (T1), the response of the six tripeptides was in agreement with their nonpolar surface areas, i.e., more hydrophobic peptides were detected with higher intensities (see Figure S6d). This was consistent with the result obtained with capillary-based ESI,37 and demonstrated that ionization at T1 was a conventional ESI process. At duration 2 (T2), however, a very different response pattern of the six tripeptides was observed (see Figure S6e). Comparison of the response of the tripeptides at T1 and T2 indicated that larger differences in response were observed for more hydrophobic peptides, e.g., GGF, and smaller difference in response were observed for less hydrophobic peptides, e.g., GGG. The response of the peptides at T2 was affected not only by the ionization efficiency of the peptides during the spray ionization but also by the interactions of the peptides with the tip surface. To quantitatively assess the effect of the interaction of the peptides with the tip surface, the ionization efficiency of the six tripeptides was calibrated to be the same by normalizing their individual SIC responses at T1 to the same value (See Figure S6f). Under such conditions, the differences in SICs of the six pepetides at T2 were caused by the difference in their interactions with the tip surface, and the SIC area of each peptide ion was proportional to the eluted and ionized amount of the species and thus can be used as an indicator for the mobility or elution tendency of the species in the substrate. The SIC area of each peptide ion at T2 was calculated (see Figure S7 for the calculation details) and plotted against the nonpolar surface area value of the peptide ion. A linear relationship with a R2 of 0.9961 and a slope of 0.5903 was found between SIC area of each peptide ion and its area of non-polar character (Figure S6g), clearly indicating that more hydrophobic compounds were more retained by the polyester tips at T2. Wooden tips and polyethlene tips were used for porous-tip ESI-MS analysis of the six tripepetides as well. Similar two-stage ionization and linear relationship between SIC area of each peptide ion at T2 and its area of non-polar character were observed for these two tips (Figure S6h-k). It could be found that the linear curves for more hydrophobic tips had larger slope values, indicating that more hydrophobic tips enabled better separation of the peptides and that hydrophobic-hydrophobic interactions were crucial for the mobility of the peptides in the tips. To gain more insight into the effect of protein conformation on its mobility in the porous substrate, a solution of bovine cytochrome c in acetonitrile/water/acetic acid (25/75/0.1, v/v/v) was investigated with polyester-tip ESI-MS. Cytochrome c has been well studied with conventional ESIMS, and the results revealed that the larger charge state of the protein observed in the ESI-MS had larger collision cross section (CCS).35 The total ion chromatogram (TIC) obtained with polyester-tip ESI-MS analysis of the cytochrome c sample is shown in Figure 2a, together with the SICs of the 7+ and 17+ ions of the protein. It can be seen that in the first minute (T1), the 7+ and 17+ ions had very similar abundances and the spectrum obtained (Figure 2b) was very similar to that obtained by conventional ESI-MS (Figure 2d), indicating that these two charge state ions had similar tendency to be eluted and ionized. These spectra (Figures 2b and 2d) indicated that under the present conditions, the protein existed in two conformations with charges centered at 8+ and 14+, respectively, in the spectra. For a later spraying time (T2),

however, the signal of the 17+ ion was significantly reduced, while the signal of the 7+ ion maintained relatively abundant (Figure 2c), leading to the protein ions with lower charge states much predominated in the spectrum. To investigate whether the observed spectral differences between T1 and T2 were caused by the solvent evaporation, which could lead to reduced ratio of the most volatile solvent and enhanced concentrations of the analytes during the spray process and thus possible change in the resulting spectral profiles, cytochrome c solutions with different compositions of acetonitrile, the most volatile solvent, and with different concentrations in the same solvents were analyzed by conventional ESI-MS. As shown in Figures S9 and S10, no significant changes to the spectral profile of the protein were observed when the composition of acetonitrile was reduced from 25% to 5% and the concentration of cytochrome c was enhanced from 1 µM to 100 uM, indicating that solvent evaporation did not play a major role in the observed effect. The results shown in Figure 2 suggested that the protein species with higher charge states, i.e., with larger CCSs,35 were more retained in the porous substrate. These were consistent with those previously obtained for the protein mixtures in this study, since higher charge states and larger CCSs meant more unfolded states of the protein and thus more hydrophobic-hydrophobic interactions of the protein with the tip surface, leading to more retaintion of the protein by the tip. The observation that proteins of higher charge states did not have higher mobility also indicated that electrophoretic migration played a less important role in porous-tip ESI, consistent with

Figure 2. Analysis of the cytochrome c sample. a) TIC of and SICs of the 7+ and 17+ ions; b) mass spectrum obtained for duration T1 in a); c) mass spectrum obtained for duration T2 in b);

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d) mass spectrum obtained by conventional ESI-MS. e) Plot of CCSs of the protein ions with different charge states against their SIC areas.

the previous results obtained with paper spray.38 Similarly, the SIC area of the protein ion at each charge state was calculated and plotted against the CCS value of the protein ion.35 A linear correlation, as shown in Figure 2e, was obtained, indicating that the mobility of the protein in the porous substrate was size and shape dependent and the present technique could be used to measure the cross sections of proteins and to study protein size, shape and conformations. Note that according to the literature,35 more than one CCS values were used for some charge states of cytochrome c. This might, to some extent, reduce the linearity of the plot. The solution of bovine cytochrome c was also analyzed using porous tips of different properties, including wooden tips and polyethylene tips of different densities. Similar two-stage ionization and the same elution order of different charge states of cytochrome c were observed with these tips (Figure S8). It can be seen that more hydrophobic tips or porous tips with larger surface areas allowed more retention of the proteins and better separation of the proteins with different charge states, again demonstrating that hydrophobic-hydrophobic interactions of proteins and the tip surfaces were the key for the protein elution and separation in porous-tip ESI-MS.

Figure 3. Analysis of the myoglobin sample. a) Spectrum obtained by conventional ESI-MS. Ions of apo-myoglobin are labelled with hollow triangle (∆) and ions of holo-myoglobin are labelled with filled triangle (▲). b) Spectrum obtained for the front 1.0 mm part of the polyester tip after the polyester-tip ESI-MS analysis. c) Spectrum obtained for the other part of the polyester tip after the polyester-tip ESI-MS analysis

In addition to preparative separation of smaller proteins from larger proteins as demonstrated with polyester tips (see Figure S3), separation of different conformations of the same protein could also be achieved by porous-tip ESI. In this study, a myoglobin solution (20 µM in 12.5% acetonitrile), which contained both intact holo-myoglobin and denatured apomyoglobin as well as heme (as indicated in Figure 3a),39 was analyzed using polyester-tip ESI-MS. Similar to the study with insulin/ alpha-lactalbumin, after the electrospray, the polyester tip was cut into two parts to determine the residues on the tip.

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Extraction and analysis with aqueous ammonium acetate allowed maintenance of the protein conformations. As shown in Figure 3b, only non-denatured holo-myoglobin with compact conformation, indicated by a narrow charge state distribution in higher mass range in the spectrum, was found in the front 1 mm part of the tip; while the spectrum obtained for the remainder part of the tip (Figure 3c) was predominated with heme and a broad charge state distribution in lower mass range, which corresponded to denatured apo-myoglobin with loosen conformations. These results demonstrated that the technique could be used for preparative separation of proteins and isolation of non-denatured forms from denatured forms of proteins. A real application of the preprative seperation of proteins was also conducted in this study by analyzing a crude extract of proteins from fresh porcine heart using polyester-tip ESI-MS. The results showed that preparative separation of αchain and β-chain of hemeglobin in the crude extract could be achieved by the technique (see Figure S12). In conclusion, we demonstrated that porous-tip ESI-MS could allow separation of proteins on porous substrates, followed by ESI-MS detection. The separation was size and shape dependent, and allowed separation of different conformations and charge states of the same proteins and measurements of their cross sections. These features are not available with LC-MS approaches, including those involving use of nanospray emitters or self-packed capillary emitters, which are commonly used for protein separation and detection. Compared to electrochromatographic techniques that could separate proteins based on chromatographic retention and electrophoretic migration under high voltages, the high voltage applied in the present technique is mainly to induce electrospray and bulk solution movement, and electrophoretic migration has been demonstrated to have little effect on protein separation in porous-tip ESI-MS; some unique fectures, e.g.; seperation and detection of the same proteins with differernt charge states, which are useful for studying protein conformations,40 were observed with porous-tip ESI-MS. Compared to IM-MS that can also separate and detect proteins based on their sizes and shapes, the protein separation in the present technique is caused by the mobility differences of proteins in microchannels of substrates in solution phase, while the separation in IM-MS is caused by the differences in collision with drift gas in the gas phase. Therefore, the information obtained by the present technique is directly about the proteins in the solution phase, and the present technique could be better for maintaining and studying intact proteins and protein complexes. Our results showed that the technique could also be used for preparative separation of proteins. These together indicated the high potential of the technique for separating proteins and studying protein structures and conformations. However, it could be seen that the resolution and efficiency of protein separation by the present technique were still limited. Further investigation on effects of various factors and developing the technique for more efficient protein analysis and studies is on-going. ASSOCIATED CONTENT Supporting Information

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Materials and experimental methods, experimental procedures, and supplementary figures. This material is available free of charge via the Internet at http://pubs.acs.org.

(26) Ouyang, Z.; Takats, Z.; Blake, T. A.; Gologan, B.; Guymon, A. J.; Wiseman, J. M.; Oliver, J. C.; Davisson, V. J.; Cooks, R. G. Science 2003, 301, 1351-1354. (27) Hiraoka, K.; Nishidate, K.; Mori, K.; Asakawa, D.; Suzuki, S. Rapid

AUTHOR INFORMATION

Commun Mass Spectrom 2007, 21, 3139-3144.

Corresponding Author *Tel.: +852 3400 8792. Fax: +852 2364 9932. E-mail: [email protected] (Z. P. Yao).

(28) Wang, H.; Liu, J. J.; Cooks, R. G.; Ouyang, Z. Angew Chem Int Ed 2010, 49, 877-880. (29) Hu, B.; So, P. K.; Chen, H. W.; Yao, Z. P. Anal Chem 2011, 83, 82018207.

ACKNOWLEDGMENT

(30) Wong, M. Y. M.; Tang, H. W.; Man, S. H.; Lam, C. W.; Che, C. M.; Ng, K.

We thank Miss Choi Yi Ching for the assistance in the SEM measurements. This work was supported by Hong Kong Research Grants Council (CRF Grant No. C5031-14E and GRF Grant No. 5029/13P), Natural Science Foundation of China (Grants No. 81373369 and No. 21405127) and The Hong Kong Polytechnic University.

M. Rapid Commun Mass Spectrom 2013, 27, 713-721. (31) Hu, B.; So, P. K.; Yao, Z. P. J Am Soc Mass Spectrom 2013, 24, 57-65. (32) So, P. K.; Hu, B.; Yao, Z. P. Mass Spectrom. 2014, 3, S0028. (33) Mandal, M. K.; Chen, L. C.; Hiraoka, K. J Am Soc Mass Spectrom 2011, 22, 1493-1500. (34) Mandal, M. K.; Chen, L. C.; Yu, Z.; Nonami, H.; Erra-Balsells, R.; Hiraoka, K. J Mass Spectrom 2011, 46, 967-975.

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