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1 2 The concept of shotgun proteomics is shown above. Instead of separating the proteins, the entire cell extract is digested with proteases and then the complex mixture is separated. The peptides are eluted from the final separation method, usually reversed-phase chromatography directly into the mass spectrometer where they are automatically subjected to MS/MS analysis. The peptides are identified in a similar way to how proteins are identified. Maybe 10 peptides are entering the mass spectrometer. The MS picks automatically the most intense, isolates it (throwing away the other 9 peptides) and then smashes it into pieces. The mass of the peptide is used to search the database to find all peptides with the same mass. The fragmentation spectra of all these peptides are then calculated and compared to the experimental fragments observed. The best matching peptide sequence is then selected. 3 4 5 6 AC; affinity chromatography. SEC; Size exclusion chromatography. IEC; Ion exchange chromatography. HIC; Hydrophobic interaction chromatography. . HILIC; Hydrophilic interaction chromatography. RPC; Reversed phase chromatography. 7 8 Sfgsdfg 9 10 11 12 13 IEX separates molecules on the basis of differences in their net surface charge. Molecules vary considerably in their charge properties and will exhibit different degrees of interaction with charged chromatography media according to differences in their overall charge, charge density and surface charge distribution. The charged groups within a molecule that contribute to the net surface charge possess different pKa values depending on their structure and chemical microenvironment. Since all molecules with ionizable groups can be titrated, their net surface charge is highly pH dependent. In the case of proteins, which are built up of many different amino acids containing weak acidic and basic groups, their net surface charge will change gradually as the pH of the environment changes i.e. proteins are amphoteric. Each protein has its own unique net charge versus pH relationship which can be visualized as a titration curve. This curve reflects how the overall net charge of the protein changes according to the pH of the surroundings. IEX chromatography takes advantage of the fact that the relationship between net surface charge and pH is unique for a specific protein. In an IEX separation reversible interactions between charged molecules and oppositely charged IEX media are controlled in order to favor binding or elution of specific molecules and achieve separation. A protein that has no net charge at a pH equivalent to its isoelectric point (pI) will not interact with a charged medium. However, at a pH above its isoelectric point, a protein will bind to a positively charged medium or anion exchanger and, at a pH below its pI, a protein will behind to a negatively charged medium or cation exchanger.. 14 When all the sample has been loaded and the column washed so that all non-binding proteins have passed through the column (i.e. the UV signal has returned to baseline), conditions are altered in order to elute the bound proteins. Most frequently, proteins are eluted by increasing the ionic strength (salt concentration) of the buffer or, occasionally, by changing the pH. As ionic strength increases, the salt ions (typically Na + or Cl-) compete with the bound components for charges on the surface of the medium and one or more of the bound species begin to elute and move down the column. The proteins with the lowest net charge at the selected pH will be the first ones eluted from the column as ionic strength increases. Similarly, the proteins with the highest charge at a certain pH will be most strongly retained and will be eluted last. The higher the net charge of the protein, the higher the ionic strength that is needed for elution. By controlling changes in ionic strength using different forms of gradient, proteins are eluted differentially in a purified, concentrated form. A wash step in very high ionic strength buffer removes most tightly bound proteins at the end of an elution. The column is then re-equilibrated in start buffer before applying more sample in the next run. 15 16 17 For molecules such as nucleic acids which carry only negativelycharged groups, an anion exchanger is the obvious choice. However, since the net charge of molecules such as proteins (carrying positively and negatively charged groups) depends on pH, the choice is based on which type of exchanger and pH give the desired resolution within the constraints of sample stability. For example, the slide shows a theoretical protein which has a net positive charge below its isoelectric point and can bind to a cation exchanger. Above its isoelectric point the protein has a net negative charge and can bind to an anion exchanger. However, the protein is only stable in the range pH 5–8 and so an anion exchanger has to be used. 18 19 20 The term “reversed phase” derives from “normal phase” chromatography, a technique utilizing a hydrophilic stationary phase together with mobile phases consisting of organic solvents such as hexane or methylene chloride. In RPC the stationary phase is hydrophobic so that a water/organic solvent mobile phase is used, that is, the stationary phase is more hydrophobic than the mobile phase. RPC media may be referred to as adsorbents while eluent solutions may be referred to as mobile phases. The separation of biomolecules by RPC depends on a reversible hydrophobic interaction between sample molecules in the eluent and the medium. Initial conditions are primarily aqueous, favoring a high degree of organized water structure surrounding the sample molecule. Frequently, a small percentage of organic modifier, typically from 3–5% acetonitrile, is present in order to achieve a “wetted” surface. As sample binds to the medium, the hydrophobic area exposed to the eluent is minimized. Separation relies on sample molecules existing in an equilibrium between the eluent and the surface of the medium. The distribution of the sample depends on the properties of the medium, the hydrophobicity of the sample and the composition of the eluent (mobile phase), as illustrated in the slide. Initially, conditions favor an extreme equilibrium state where essentially 100% of the sample is bound. Since proteins and peptides carry a mix of accessible hydrophilic and hydrophobic amino acids and are rather large, the interaction with the medium has the nature of a multi-point attachment. To bring about elution the amount of organic solvent is increased so that conditions become more hydrophobic. Binding and elution occur continuously as sample moves through the column. The process of moving through the column is slower for those samples that are more hydrophobic. Consequently, samples are eluted in order of increasing hydrophobicity. 21 Sample is applied under conditions that favor binding, typically using an aqueous solution containing an ion-pairing agent, such as trifluoroacetic acid (TFA), to enhance the hydrophobic interaction and a low concentration of organic modifier such as 5% acetonitrile. After application, and when all non-bound molecules have passed through (i.e., the UV signal has returned to baseline), conditions are altered in order to elute the bound sample. Elution begins by increasing the concentration of organic modifier, such as acetonitrile. Molecules with the lowest hydrophobicity will elute first. By controlling the increase in organic modifier, molecules are eluted differentially. Those molecules with the highest degree of hydrophobicity will be most strongly retained and eluted last. A wash step removes most of the tightly bound molecules at the end of elution. 22 23 Ion-pairing agents A common way to increase the hydrophobicity of charged components, enhance binding to the medium, and so alter retention time, is to add ion-pairing agents to the eluent. These agents bind via ionic interactions with charged groups and thereby suppress their influence on overall hydrophobicity. Since most proteins and peptides are slightly basic, ion-pairing agents are often acids such as trifluoroacetic acid (TFA) whereas a base such as triethylamine is used for negatively charged molecules. 24 Ion-pairing agents A common way to increase the hydrophobicity of charged components, enhance binding to the medium, and so alter retention time, is to add ion-pairing agents to the eluent. These agents bind via ionic interactions with charged groups and thereby suppress their influence on overall hydrophobicity. Since most proteins and peptides are slightly basic, ion-pairing agents are often acids such as trifluoroacetic acid (TFA) whereas a base such as triethylamine is used for negatively charged molecules. 25 To prevent the OH groups from binding to peptides in a hydrophilic mode, the OH groups are often blocked or end capped to give acetyl groups. 26 Water is a good solvent for polar substances, but a poor solvent for non-polar substances. In liquid water a majority of the water molecules occur in clusters due to hydrogen bonding between them- selves (Figure 3). Although the half-life of water clusters is very short, the net effect is a very strong cohesion between the water molecules, reflected, for example, by a high boiling point. At an air-water interface, water molecules arrange themselves into a strong shell of highly ordered structure. Here, the possibility to form hydrogen bonds is no longer in balance, but is dominated by the liquid side of the interface. This gives rise to an ordered structure that manifests itself as a strong surface tension. Anything that influences the stability of the water shell also affects the surface tension. When a hydrophobic substance such as a protein or hydrophobic ligand is immersed in water some- thing analogous to the surface tension phenomenon happens. The water molecules cannot “wet” the surface of the hydrophobic substance. Instead they form a highly ordered shell around the substance, due to their inability to form hydrogen bonds in all directions. Minimizing the extent of this shell leads to a decrease in the number of ordered water molecules, that is, a thermodynamically more favorable situation in which entropy increases. In order to gain entropy, hydrophobic substances are forced to merge to minimize the total area of such shells. Thus hydrophobic interaction depends on the behavior of the water molecules rather than on direct attraction between the hydrophobic molecules. The hydrophobic ligands on HIC media can interact with the hydrophobic surfaces of proteins. In pure water any hydrophobic effect is too weak to cause interaction between ligand and proteins or between the proteins themselves. However, certain salts enhance hydrophobic interactions, and adding such salts brings about binding (adsorption) to HIC media. For selective elution (desorption), the salt concentration is lowered gradually and the sample components 27 28 The hydrophobic ligands on HIC media can interact with the hydrophobic surfaces of proteins. The ligands immobilised onthe beads are, in order of increasing hydrophobicity: hydroxypropyl < methyl < benzyl = propyl < iso-propyl < phenyl < pentyl! These ligands are much more less hydrophobic than C8 and C18 ligands. In pure water any hydrophobic effect is too weak to cause interaction between ligand and proteins or between the proteins themselves. However, certain salts enhance hydrophobic interactions, and adding such salts brings about binding (adsorption) to HIC media. For selective elution (desorption), the salt concentration is lowered gradually and the sample components elute in order of hydrophobicity. HIC media are composed of ligands containing alkyl or aryl groups coupled to an inert matrix of spherical particles. The matrix is porous, in order to provide a high internal surface area, while the ligand plays a significant role in the final hydrophobicity of the medium. Interaction between the protein and the medium is promoted by moderately high salt concentrations, typically 1–2 M ammonium sulfate or 3 M NaCl. The type of salt and the concentration required in the start buffer are selected to ensure that the proteins of interest bind to the medium and that other less hydrophobic proteins and impurities pass directly through the column. Antichaotropic salts are employed, eg. (NH4)2SO4. Chaotropic salts (NaCl, NH4SCN) do not retain solutes 29 30 Interaction between the protein and the medium is promoted by moderately high salt concentrations, typically 1–2 M ammonium sulfate or 3 M NaCl. The type of salt and the concentration required in the start buffer are selected to ensure that the proteins of interest bind to the medium and that other less hydrophobic proteins and impurities pass directly through the column. 31 32 33 Binding: buffer conditions are optimized to ensure that the target molecules interact effectively with the ligand and are retained by the affinity medium as all other molecules wash through the column. Elution: buffer conditions are changed to reverse (weaken) the interaction between the target molecules and the ligand so that the target molecules can be eluted from the column. Wash: buffer conditions that wash unbound substances from the column without eluting the target molecules or that re-equilibrate the column back to the starting conditions (in most cases the binding buffer is used as a wash buffer). Ligand coupling: covalent attachment of a ligand to a suitable pre-activated matrix to create an affinity medium. Pre-activated matrices: matrices which have been chemically modified to facilitate the coupling of specific types of ligand. 34 The basis for purification of IgG, IgG fragments and subclasses is the high affinity of protein A and protein G for the Fc region of polyclonal and monoclonal IgG-type antibodies. 35 36 37 38 39 40 41