Step 3: Scoring mefhod. Once an amino acid sequence
fits the defined mass tolerance, the sequence is evaluated
by using several different criteria. First, the number
(n,) of predicted fragment ions that match ions
observed in the spectrum within + 1 u and their aburdances
(i,) are summed. The continuity of an ion
series is considered by incrementing a component of
the score (p) for each consecutive fragment ion
matched. If an immonium ion for the ammo acids His,
Tyr, Trp, Met, and Phe is present in the spectrum
along with the associated amino acid, then an additional
component of the score ( p) is incremented. If the
amino acid is not present in the sequence, then p is
decreased. The values used for B and p are 0.075 and
0.15, respectively. The total number of predicted sequence
ions is also noted (n,). A score is calculated for
each amino acid sequence by using the relationship