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#monoisotopic mass

2 public questions tagged with this topic.

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.

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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.

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