Guidelines for Obtaining the Amino Acid Sequence from a Mass Spectrum

Guidelines for Obtaining the Amino Acid Sequence from a Mass Spectrum 

The task of retrieving the amino acid sequence from a mass spectrum is tricky and time-consuming. It involves connecting certain ions into a sequence ion series. The following steps can be followed to find first either the y- or b-ioxv series  

The first step is to look for the presence of immonium ions at the low-mass end of the spectrum. This step will indicate the presence of certain amino acids. The masses of the amino acid residues along with the masses of their immonium ions are listed in Table 8.3. A hint of the presence of certain amino acids can also be gained from the loss of side-chain masses from the [M -h H]+ ion (also listed in Table 8.3) and also by the losses of neutral molecules such as ammonia and water. 

Ignore the first few ions at the high-mass end of the mass spectrum because they are due to the uninformative (but energetically favorable) losses of neutral molecules, such as H2O, NH3, CO2, and HCOOH, from the [M -H H]+ ion. 

identify pairs of ions that differ in mass by 28 Da. This information indicates putative members of b-aaA a-ion series. 

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