This Angiotensin example is just a proof-of-concept that the Seven Golden Rules can also identify molecular formulas from known peptides (less than 2000 Da). However the Seven Golden Rules were not designed for that purpose.
Seven Golden Rules
Using data from the LTQ-FT (selected resolving power = 50,000) a m/z of 1126.508420 was measured (ESI positive, Nanomate) and a mass of [M+H] 1126.508650 was calculated with MWTWIN. The mass accuracy was -0.2 ppm and the isotopic abundance errors were: [A+1] error = 1.25; [A+2] error = -3.26; [A+3] error = -1.13.
The Seven Golden Rules (see supplement) calculated 305 formulae within 1 ppm mass accuracy and found 44 formulae within 5% isotopic pattern range. One formula with a match factor of 95.32 was annotated correctly as C50H72N13O15P. The numbers given here might differ as different mass peak integration methods and averages were selected within XCalibur. The Chemical Structure Lookup Servicefound two database entries and an additional one was found manually in PubChem and CAS all pointed to the correct isomer structure. This is an example where simpler formula calculators must fail due to the lack of an annotation databases.
Synonyms (from ChemIDPlus)
Angiotensin II, O-phosphotyrosyl(4)-
Angiotensin II, tyr(P)(4)-
Angiotensin II, 5-L-isoleucine-, dihydrogen phosphate (ester) (9CI)
Plasma desorption mass spectrometry of phosphopeptides: An investigation to determine the feasibility of quantifying the degree of phosphorylation [DOI]