Surface-enhanced laser desorption/ionization

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Surface-enhanced laser desorption/ionization
AcronymSELDI
Analytes Biomolecules
Other techniques
Related Matrix-assisted laser desorption/ionization
Soft laser desorption
Surface-assisted laser desorption/ionization

Surface-enhanced laser desorption/ionization (SELDI) is a soft ionization method in mass spectrometry (MS) used for the analysis of protein mixtures. It is a variation of matrix-assisted laser desorption/ionization (MALDI). [1] [2] In MALDI, the sample is mixed with a matrix material and applied to a metal plate before irradiation by a laser, [3] whereas in SELDI, proteins of interest in a sample become bound to a surface before MS analysis. The sample surface is a key component in the purification, desorption, and ionization of the sample. SELDI is typically used with time-of-flight (TOF) mass spectrometers and is used to detect proteins in tissue samples, blood, urine, or other clinical samples, however, SELDI technology can potentially be used in any application by simply modifying the sample surface. [1] [2]

Contents

Sample preparation and instrumentation

SELDI Mechanism Surface enhanced laser desorption ionization.png
SELDI Mechanism

SELDI can be seen as a combination of solid-phase chromatography and TOF-MS. The sample is applied to a modified chip surface, which allows for the specific binding of proteins from the sample to the surface. Contaminants and unbound proteins are then washed away. After washing the sample, an energy absorbing matrix, such as sinapinic acid (SPA) or α-Cyano-4-hydroxycinnamic acid (CHCA), is applied to the surface and allowed to crystallize with the sample. [1] [2] Alternatively, the matrix can be attached to the sample surface by covalent modification or adsorption before the sample is applied. [4] The sample is then irradiated by a pulsed laser, causing ablation and desorption of the sample and matrix. [1] [2]

SELDI-TOF-MS

Samples spotted on a SELDI surface are typically analyzed using time-of-flight mass spectrometry. An irradiating laser ionizes peptides from crystals of the sample/matrix mixture. The matrix absorbs the energy of the laser pulse, preventing destruction of the molecule, and transfers charge to the sample molecules, forming ions. The ions are then briefly accelerated through an electric potential and travel down a field-free flight tube where they are separated by their velocity differences. The mass-to-charge ratio of each ion can be determined from the length of the tube, the kinetic energy given to ions by the electric field, and the velocity of the ions in the tube. The velocity of the ions is inversely proportional to the square root of the mass-to-charge ratio of the ion; ions with low mass-to-charge ratios are detected earlier than ions with high mass-to-charge ratios. [5]

SELDI surface

The binding of proteins to the SELDI surface acts as a solid-phase chromatographic separation step, and as a result, the proteins attached to the surface are easier to analyze. The surface is composed primarily of materials with a variety of physico-chemical characteristics, metal ions, or anion or cation exchangers. Common surfaces include CM10 (weak cation exchange), H50 (hydrophobic surface, similar to C6-C12 reverse phase chromatography), IMAC30 (metal-binding surface), and Q10 (strong anion exchange). SELDI surfaces can also be modified to study DNA-protein binding, antibody-antigen assays, and receptor-ligand interactions. [2]

Additional surface methods

The SELDI process is a combination of surface-enhanced neat desorption (SEND),surface-enhanced affinity-capture (SEAC), and surface-enhanced photolabile attachment and release (SEPAR) mass spectrometry. With SEND, analytes can be desorbed and ionized without adding a matrix; the matrix is incorporated into the sample surface. In SEAC, the sample surface is modified to bind the analyte of interest for analysis with laser desorption/ionization mass spectrometry (LDI-MS). [1] [4] [6] SEPAR is a combination of SEND and SEAC; the modified sample surface also acts as an energy absorbing matrix for ionization. [4]

History

Genesis 2000 robot preparing Ciphergen SELDI-TOF protein chips for proteomic pattern analysis Protein pattern analyzer.jpg
Genesis 2000 robot preparing Ciphergen SELDI-TOF protein chips for proteomic pattern analysis

SELDI technology was developed by T. William Hutchens and Tai-Tung Yip at Baylor College of Medicine in 1993. [7] Hutchens and Yip attached single-stranded DNA to agarose beads and used the beads to capture lactoferrin, an iron-binding glycoprotein, from preterm infant urine. The beads were incubated in the sample and then removed, washed, and analyzed with a MALDI-MS probe tip. This research led to the idea that MALDI surfaces could be derivatized with SEAC devices; the technique was later described by Hutchens and Yip in 1998. [1] [7]

SELDI technology was first commercialized by Ciphergen Biosystems in 1997 as the ProteinChip system, and is now produced and marketed by Bio-Rad Laboratories. [6]

Applications

SELDI technology can potentially be used in any application by modifying the SELDI surface. [1] SELDI-TOF-MS is optimal for analyzing low molecular weight proteins (<20 kDa) in a variety of biological materials, such as tissue samples, blood, urine, and serum. This technique is often used in combination with immunoblotting and immunohistochemistry as a diagnostic tool to aid in the detection of biomarkers for diseases, and has also been applied to the diagnosis of cancer and neurological disorders. [8] [9] SELDI-TOF-MS has been used in biomarker discovery for lung, breast, liver, colon, pancreatic, bladder, kidney, cervical, ovarian, and prostate cancers. [2] SELDI technology is most widely used in biomarker discovery to compare protein levels in serum samples from healthy and diseased patients. [9] [10] [11] [12] Serum studies allow for a minimally invasive approach to disease monitoring in patients and are useful in the early detection and diagnosis of diseases and neurological disorders, such as amyotrophic lateral sclerosis (ALS) and Alzheimer's. [9] [10]

SELDI-TOF-MS can also be used in biological applications to detect post-translationally modified proteins and to study phosphorylation states of proteins. [8]

Advantages

A major advantage of the SELDI process is the chromatographic separation step. While liquid chromatography-mass spectrometry (LC-MS) is based on the elution of analytes in the separated sample, separation in SELDI is based on retention. Any sample components that interfere with analytical measurements, such as salts, detergents, and buffers, are washed away before analysis with mass spectrometry. Only the analytes that are bound to the surface are analyzed, reducing the overall complexity of the sample. As a result, there is an increased probability of detecting analytes that are present in lower concentrations. [10] Because of the initial separation step, protein profiles can be obtained from samples of as few as 25-50 cells. [8]

In biological applications, SELDI-TOF-MS has a major advantage in that the technique does not require the use of radioactive isotopes. Furthermore, an assay can be sampled at multiple time points during an experiment. [8] Additionally, in proteomics, the biomarker discovery, identification, and validation steps can all be done on the SELDI surface. [1]

Limitations

SELDI is often criticized for its reproducibility due to differences in the mass spectra obtained when using different batches of chip surfaces. [2] While the method has been successful with analyzing low molecular weight proteins, consistent results have not been obtained when analyzing high molecular weight proteins. [8] There also exists a potential for sample bias, as nonspecific absorption matrices favor the binding of analytes with higher abundances in the sample at the expense of less abundant analytes. [2] While SELDI-TOF-MS has detection limits in the femtomolar range, [10] the baseline signal in the spectra varies and noise due to the matrix is maximal below 2000 Da, with Ciphergen Biosystems suggesting to ignore spectral peaks below 2000 Da. [13]

See also

Related Research Articles

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<span class="mw-page-title-main">Peptide mass fingerprinting</span>

Peptide mass fingerprinting (PMF) is an analytical technique for protein identification in which the unknown protein of interest is first cleaved into smaller peptides, whose absolute masses can be accurately measured with a mass spectrometer such as MALDI-TOF or ESI-TOF. The method was developed in 1993 by several groups independently. The peptide masses are compared to either a database containing known protein sequences or even the genome. This is achieved by using computer programs that translate the known genome of the organism into proteins, then theoretically cut the proteins into peptides, and calculate the absolute masses of the peptides from each protein. They then compare the masses of the peptides of the unknown protein to the theoretical peptide masses of each protein encoded in the genome. The results are statistically analyzed to find the best match.

<span class="mw-page-title-main">Matrix-assisted laser desorption/ionization</span> Ionization technique

In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.

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<span class="mw-page-title-main">MALDI imaging</span>

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<span class="mw-page-title-main">Protein mass spectrometry</span> Application of mass spectrometry

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Desorption electrospray ionization (DESI) is an ambient ionization technique that can be coupled to mass spectrometry (MS) for chemical analysis of samples at atmospheric conditions. Coupled ionization sources-MS systems are popular in chemical analysis because the individual capabilities of various sources combined with different MS systems allow for chemical determinations of samples. DESI employs a fast-moving charged solvent stream, at an angle relative to the sample surface, to extract analytes from the surfaces and propel the secondary ions toward the mass analyzer. This tandem technique can be used to analyze forensics analyses, pharmaceuticals, plant tissues, fruits, intact biological tissues, enzyme-substrate complexes, metabolites and polymers. Therefore, DESI-MS may be applied in a wide variety of sectors including food and drug administration, pharmaceuticals, environmental monitoring, and biotechnology.

<span class="mw-page-title-main">Time-of-flight mass spectrometry</span> Method of mass spectrometry

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Sample preparation for mass spectrometry is used for the optimization of a sample for analysis in a mass spectrometer (MS). Each ionization method has certain factors that must be considered for that method to be successful, such as volume, concentration, sample phase, and composition of the analyte solution. Quite possibly the most important consideration in sample preparation is knowing what phase the sample must be in for analysis to be successful. In some cases the analyte itself must be purified before entering the ion source. In other situations, the matrix, or everything in the solution surrounding the analyte, is the most important factor to consider and adjust. Often, sample preparation itself for mass spectrometry can be avoided by coupling mass spectrometry to a chromatography method, or some other form of separation before entering the mass spectrometer. In some cases, the analyte itself must be adjusted so that analysis is possible, such as in protein mass spectrometry, where usually the protein of interest is cleaved into peptides before analysis, either by in-gel digestion or by proteolysis in solution.

<span class="mw-page-title-main">Laser spray ionization</span>

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<span class="mw-page-title-main">Capillary electrophoresis–mass spectrometry</span>

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<span class="mw-page-title-main">Matrix-assisted ionization</span>

In mass spectrometry, matrix-assisted ionization is a low fragmentation (soft) ionization technique which involves the transfer of particles of the analyte and matrix sample from atmospheric pressure (AP) to the heated inlet tube connecting the AP region to the vacuum of the mass analyzer.

References

  1. 1 2 3 4 5 6 7 8 Tang N, Tornatore P, Weinberger SR (2004). "Current developments in SELDI affinity technology". Mass Spectrometry Reviews. 23 (1): 34–44. Bibcode:2004MSRv...23...34T. doi:10.1002/mas.10066. PMID   14625891.
  2. 1 2 3 4 5 6 7 8 Muthu, Manikandan; Vimala, A.; Mendoza, Ordetta Hanna; Gopal, Judy (2016-02-01). "Tracing the voyage of SELDI-TOF MS in cancer biomarker discovery and its current depreciation trend – need for resurrection?". TrAC Trends in Analytical Chemistry. 76: 95–101. doi:10.1016/j.trac.2015.10.004.
  3. ↑ Karas, Michael; Krüger, Ralf (2003-02-01). "Ion Formation in MALDI: The Cluster Ionization Mechanism". Chemical Reviews. 103 (2): 427–440. doi:10.1021/cr010376a. ISSN   0009-2665. PMID   12580637.
  4. 1 2 3 Merchant, Maggie; Weinberger, Scot R. (2000-04-01). "Recent advancements in surface-enhanced laser desorption/ionization-time of flight-mass spectrometry". Electrophoresis. 21 (6): 1164–1177. doi:10.1002/(sici)1522-2683(20000401)21:6<1164::aid-elps1164>3.0.co;2-0. ISSN   1522-2683. PMID   10786889. S2CID   1893004.
  5. ↑ Dass, Chhabil (2007). Fundamentals of Contemporary Mass Spectrometry - Dass - Wiley Online Library. doi:10.1002/0470118490. ISBN   9780470118498.
  6. 1 2 Lomas, Lee O.; Weinberger, Scot R. (2008-01-01). Surface-Enhanced Laser Desorption/Ionization (SELDI) Technology. John Wiley & Sons, Ltd. doi:10.1002/9780470061565.hbb128. ISBN   9780470061565.
  7. 1 2 Hutchens TW and Yip TT. "New desorption strategies for the mass spectrometric analysis of macromolecules." Rapid Commun Mass Spectrom 7: 576-580 (1993).
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  11. ↑ Jr GW, Cazares LH, Leung SM, Nasim S, Adam BL, Yip TT, Schellhammer PF, Gong L, Vlahou A (1999). "Proteinchip(R) surface enhanced laser desorption/ionization (SELDI) mass spectrometry: a novel protein biochip technology for detection of prostate cancer biomarkers in complex protein mixtures". Prostate Cancer and Prostatic Diseases. 2 (5/6): 264–276. doi: 10.1038/sj.pcan.4500384 . PMID   12497173.
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  13. ↑ Henderson, N.A.; Steele, R.J.C. (2005). "SELDI-TOF proteomic analysis and cancer detection". The Surgeon. 3 (6): 383–390. doi:10.1016/s1479-666x(05)80048-4. PMID   16353858.
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