#proteomics

UV-VIS spectroscopy

Which method is the most sensitive label-free quantification for proteins?

Label-free quantification avoiding dyes, radioisotopes or antibodies requires intrinsic chromophores. Infrared spectroscopy demands high concentration and drying, mass spectrometry needs ionization, extensive calibration and often tagging. Fluorescence sensitivity depends on quantum yield, environment, quenching and usually extrinsic labeling. Ultraviolet absorption at 280 nm for aromatic residues and 260 nm for nucleic acids offers sensitive, non-destructive, immediate measurement in microgram range using Beer-Lambert law and known molar absorptivities, recovering sample intact. Instruments are inexpensive and prevalent, making UV spectroscopy gold standard for routine rapid estimation in molecular biology, biochemistry and diagnostics laboratories.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

What kind of fragments dominate in peptide MS spectra?

During low-energy collision-induced dissociation of protonated peptides, proton mobility model dictates that proton migrates along backbone and weakens peptide bonds, producing mainly b and y type ions. Among these, y ions containing C-terminal portion often dominate spectra because N-terminal fragments may further fragment or undergo cyclization. C-terminal lysine or arginine in tryptic peptides strongly sequesters proton, enhancing stability of y ions. While a-ions accompany b ions, x-ions are rare in CID, and c/z ions appear in electron-based dissociation methods, making y-ion series most informative for database searching in proteomics experiments.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

Which technique can sequence a 25-residue peptide directly?

Determining amino acid order of a 25-residue peptide requires fragmentation along peptide backbone to generate overlapping ions. Circular dichroism assesses secondary structure, SDS-PAGE estimates molecular size, and X-ray diffraction requires crystals unsuitable for short peptides. Tandem mass spectrometry, MS/MS, isolates chosen precursor ion and fragments it via collision-induced dissociation, producing b and y ion series covering sequence. Modern Orbitrap or quadrupole-time-of-flight instruments achieve sufficient resolution and mass accuracy to read full sequence directly, enabling rapid peptide sequencing without Edman degradation in proteomics workflows for biological samples.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

Which amino acid has monoisotopic mass of 147.07?

Aromatic amino acids exhibit larger masses due to phenyl rings and contribute distinctive residues in peptide spectra. Phenylalanine molecular weight is 165.07 Da, residue mass after dehydration is 147.07 Da after subtracting water. Tyrosine residue mass is 163 Da, tryptophan 186 Da, histidine 137 Da. Observing mass difference of 147 Da between successive fragment ions indicates phenylalanine at that position during de novo sequencing. High-resolution mass spectrometry distinguishes phenylalanine from oxidized methionine which is isobaric near 147.03 Da, requiring accurate mass measurement for confident assignment in proteomics, critical for protein identification accuracy.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

Which amino acid has a monoisotopic residue mass of 99 Da?

Amino acid residue mass equals monoisotopic molecular mass minus water lost during peptide bond formation. Valine molecular mass is 117.07 Da; subtracting 18.0106 Da for H2O yields 99.06 Da residue mass. Lysine residue is 128 Da, glycine 57 Da, serine 87 Da. This 99 Da value creates diagnostic mass difference between consecutive b or y ions in tandem mass spectra when valine is present in sequence. Knowledge of residue masses enables manual de novo sequencing, validation of proteomic database search results, and understanding of mass spectrometry fragmentation ladders in peptide identification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

A mass shift of +203 Da suggests presence of:

Glycosylation adds monosaccharide residues with defined masses to proteins, altering function and stability. N-acetyl glucosamine, GlcNAc, exists as common modification in N-linked and O-linked glycans and O-GlcNAc signaling. Residue mass of HexNAc after dehydration during glycosidic bond formation is 203.079 Da, observed as +203 Da increment on peptide mass spectra. Detecting this shift indicates single GlcNAc attachment, distinct from 162 Da for hexose, 146 Da for fucose, or 291 Da for sialic acid. Recognition of +203 Da aids mapping of glycosylation sites in proteomics studies.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

Which property is essential for MALDI matrix?

MALDI requires matrix compounds that isolate analyte molecules, absorb energy from pulsed laser typically 337 nm or 355 nm, and facilitate proton transfer. Strong ultraviolet absorption ensures efficient energy uptake, leading to rapid sublimation of matrix crystals and entrainment of analyte into gas phase without direct photochemical damage. High volatility would evaporate prematurely, non-reactivity alone insufficient, and fluorescence indicates emission rather than absorption. Common matrices like CHCA, DHB, and sinapinic acid all possess aromatic systems with high molar absorptivity at laser wavelength, enabling gentle desorption and ionization of peptides.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

Mass Spectroscopy

A shift of +42 Da in MS indicates:

Lysine residues undergo enzymatic acetylation where an acetyl group CH3CO replaces hydrogen on epsilon amino group, neutralizing positive charge and regulating chromatin structure and metabolism. Addition of C2H2O minus H corresponds to monoisotopic mass increment of 42.0106 Da, observed as +42 Da shift in mass spectra. Detecting this modification on peptides helps identify epigenetic regulatory marks and metabolic regulation. Differentiation from trimethylation which also adds ~42 Da requires high resolution or diagnostic fragment ions, but in typical low-resolution proteomics workflows +42 Da strongly suggests acetylation event on proteins.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details