Calculation of m/z for Polypeptides in Mass Spectrometry
Calculation of m/z for Peptides in Mass Spectrometry
In mass spectrometric analysis, the mass-to-charge ratio (m/z) is a fundamental parameter for identifying ions in a spectrum. Each peak represents an ion, and its m/z value is derived by dividing the mass of the ion by its charge. Typically, the charge (z) is +1 or -1, as most ions are singly charged. However, larger molecules, such as proteins and peptides, may exhibit multiple charges, significantly affecting their ionization.
To calculate m/z for peptides, it is essential to consider the constituent amino acid residues and any modifications. The mass of each amino acid is available in standard mass tables, while modification masses should be sourced from pertinent literature. Additionally, the ion type is crucial, with b ions and y ions being common; these result from cleavage at the peptide’s N-terminal and C-terminal, respectively.
Procedure for Calculating Peptide m/z
First, determine the peptide's mass by summing the masses of all amino acid residues and any modifications. This value represents the uncharged mass. To find m/z, adjust this mass by adding or subtracting the charge, typically the mass of a proton (~1.007 Da).
For example, consider a tripeptide composed of serine (S, 87.078 Da), tyrosine (Y, 163.176 Da), and phenylalanine (F, 147.177 Da), forming a b ion from N-terminal cleavage. The ion's mass totals 87.078 + 163.176 = 250.254 Da. If it holds a single positive charge, the m/z is 250.254 + 1.007 = 251.261. Note that this simplified approach omits the complexities encountered in practical mass spectrometry, where factors like ionization technique and fragment ions necessitate more sophisticated calculations.
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