Method for identifying peptides using tandem mass spectra by dynamically determining the number of peptide reconstructions required
View Patent ↗A method for identifying peptides using tandem mass spectrometry takes the spectrum for a peptide to be analyzed and uses a scoring function to score a match between the spectrum and each candidate peptide in a peptide database. The scoring function has a value corresponding to a number of fragment peaks in the spectrum that match fragment peaks in a spectrum of the candidate peptide. Using the match scores, a generating function of the spectrum is computed to determine the number of peptide reconstructions at each value of the scoring function. The generating function is then used to determine the number of candidate peptides for each match score and the probability of a peptide having a given match score to the spectrum. A spectral probability can be determined by calculating the total probability of all peptides with scores equal to or larger than the given match score.
1. A method for identifying peptides using tandem mass spectrometry, comprising:
obtaining an experimental spectrum for a peptide to be analyzed, the experimental spectrum comprising a plurality of fragment peaks and having a parent mass, each fragment peak corresponding to a mass within the experimental spectrum;
using a scoring function, scoring a match between the experimental spectrum and a spectrum corresponding to each candidate peptide of a plurality of candidate peptides to generate a match score for each candidate peptide, wherein the scoring function has a value corresponding to a number of fragment peaks in the experimental spectrum matching fragment peaks in a spectrum of the candidate peptide;
using the match scores, computing, using a computer comprising a processor and a memory, a generating function of the experimental spectrum to determine a number of candidate peptides at each value of the scoring function, wherein the generating function is determined according to the relationship:
∑
all
_
peptides
_
P
ⅇ
-
Energy
(
P
,
S
)
=
∑
t
x
(
t
)
·
ⅇ
-
t
wherein Energy(P,S) is an energy score, x(t) is the number of peptides with energy t and S is the experimental spectrum;
determining a score distribution of match scores of all candidate peptides for the experimental spectrum;
using the score distribution, identifying a subset of the candidate peptides to be reported as possible explanations of the experimental spectrum; and
generating a report for output to a display device or a memory device comprising a listing of the identified subset of candidate peptides and their score distributions.
2. The method of claim 1 , further comprising:
determining an energy score for each pairing of the experimental spectrum and a candidate peptide, wherein the energy score comprises a difference between a match score of the pairing and a match score of a best matching candidate peptide.
3. The method of claim 2 , wherein the match score of a best matching candidate peptide is estimated using a de novo sequencing algorithm to define a plurality of peptide reconstructions.
4. The method of claim 3 , wherein the number of peptide reconstructions is counted for each energy score to determine a total number of peptide reconstructions to be reported for the experimental spectrum.
5. The method of claim 4 , wherein the total number of peptide reconstructions is computed by summing all of the candidate peptides from the subset of candidate peptides for all mass values within a pre-determined window around the parent mass.
6. A method for identifying peptides using tandem mass spectrometry, comprising:
obtaining an experimental spectrum for a peptide to be analyzed, the experimental spectrum comprising a plurality of fragment peaks and having a parent mass, each fragment peak corresponding to a mass within the experimental spectrum;
using a scoring function, scoring a match between the experimental spectrum and a spectrum corresponding to each candidate peptide of a plurality of candidate peptides to generate a match score for each candidate peptide, wherein the scoring function has a value corresponding to a number of fragment peaks in the experimental spectrum matching fragment peaks in a spectrum of the candidate peptide;
using the match scores, computing, using a computer comprising a processor and a memory, a generating function of the experimental spectrum to determine a number of candidate peptides at each value of the scoring function, wherein the generating function is a weighted generating function and further comprising determining a probability of a peptide having a given match score, and wherein the weighted generating function is determined according to the relationship:
∑
all
peptides
P
prob
(
P
)
·
ⅇ
-
Energy
(
P
,
S
)
=
∑
t
y
(
t
)
·
ⅇ
-
t
,
wherein Energy(P,S) is an energy score, y(t) is the overall probability of all peptides with energy t and S is the experimental spectrum;
determining a score distribution of match scores of all candidate peptides for the experimental spectrum;
using the score distribution, identifying a subset of the candidate peptides to be reported as possible explanations of the experimental spectrum; and
generating a report for output to a display device or a memory device comprising a listing of the identified subset of candidate peptides and their score distributions.
7. The method of claim 6 , wherein the spectral probability comprises a total probability of all peptides with scores equal to or larger than the given match score.