Method of identifying peptides
Methods of identifying polypeptides have been developed using a de novo sequencing technique. Methods use photodissociation and low-energy fragmentation and the spectra of peptide ions obtained therefrom, such as obtained by post-source decay (PSD), have been developed. The methods include photodissociation and the spectra therefrom obtainable from treating ions with predetermined wavelengths of radiation in the vacuum ultraviolet range of the electromagnetic spectrum. The confidence of amino acid assignments based on x-type ions is evaluated by observing complementary y-, v- and w-type ions that provide additional constraints to sequence identification.
1. A method for de novo sequencing of a peptide or a protein by fragmentation and mass spectrometry comprising:
low-energy fragmentation of the peptide or protein into first product ions selected from the group consisting of b- and y-type ions and obtaining first mass spectral data of the first product ions;
high-energy fragmentation of the same peptide or the same protein into second product ions selected from the group consisting of a-, b-, c-, d-, v-, w-, x-, and y-type ions and obtaining second mass spectral data of the second product ions;
identifying fragment ion types by adding and subtracting masses of the second product ions to establish the presence of related peaks associated with predetermined mass spacings of ion types and pairing a first mass from the first product ions and a second mass from the second product ions;
and de novo sequencing the peptide or protein, wherein de novo sequencing comprises subtracting masses of the second product ions after identifying the fragment ion types to establish the presence of related peaks associated with amino acid molecular weights.
2. The method of claim 1 wherein low-energy fragmentation comprises post-source decay (PSD) following one of matrix assisted laser desorption ionization (MALDI) and collision-induced dissociation (CID) following electrospray ionization (ESI).
3. The method of claim 1 wherein high-energy fragmentation comprises photodissociation.
4. The method of claim 3 wherein photodissociation comprises vacuum ultraviolet photodissociation.
5. The method of claim 1 wherein de novo sequencing comprises exploiting a number of similarities arising from the presence of related peaks associated with predetermined mass differences and predetermined mass sums in the second mass spectral data.
6. The method of claim 5 wherein comprising adding a first value from the second mass spectral data to a second value from the second mass spectral data to obtain a predetermined mass sum, the predetermined mass sum being one of the sum of:
(i) an a n fragment and an x N-n fragment;
(ii) a b n fragment and an x N-n fragment;
(iii) an a n fragment and a v N-n+1 fragment; and
(iv) a b n fragment and a v N-n+1 fragment,
where n is the number of residues from a C terminus of the peptide or protein and N is the number of residues in the peptide or protein.
7. The method of claim 6 wherein the predetermined mass sum is one of M−1.01 Da, M+26.99 Da, M+28.02 Da, M+56.01 Da, M+27.02 Da, M+41.02 Da, M+55.02 Da, and M+69.02, where M is a measured peptide ion mass.
8. The method of claim 5 comprising subtracting a first value from the second mass spectral data from a second value from the second mass spectral data to obtain a predetermined mass difference, the predetermined mass difference being one of the difference between:
(i) an x n fragment and a y n fragment;
(ii) a v n+1 fragment and an x n fragment;
(iii) a w n+1 fragment and an x n fragment;
(iv) a v n+1 fragment and a y n fragment; and
(v) a w n+1 fragment and a y n fragment,
where n is the number of residues from a C terminus of the peptide or protein.
9. The method of claim 8 wherein the predetermined mass difference is 25.98 Da.
10. The method of claim 8 wherein the predetermined mass difference is one of 29.03 Da, 28.03 Da, 42.04 Da, 55.01 Da, 54.01 Da, or 68.02 Da.
11. The method of claim 1 further comprising guanidinating a peptide.
12. The method of claim 1 wherein de novo sequencing of a peptide or a protein by fragmentation and mass spectrometry comprises de novo sequencing of a tryptic peptide from a digestion of a protein.
13. The method of claim 1 further comprising assigning arginine as a C-terminal residue if 175.12 Da is in the second mass spectral data.
14. The method of claim 1 further comprising assigning guanidinated lysine as a C-terminal residue if 189.13 Da is in the second mass spectral data.
15. The method of claim 1 further comprising comparing an amino acid residue molecular weight to a difference between ion types from the second mass spectral data whether or not the ion types are in the first mass spectral data.
16. The method of claim 1 wherein the method for de novo sequencing is capable of differentiating leucine and isoleucine.
17. The method of claim 1 further comprising differentiating leucine and isoleucine.