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Search for linear modification #85
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Hey, As far as I know, those are additive i.e. you are defining a modification on linear peptides only called bs3nh with a deltamass of 155.094619105 AND a modification on all peptides with an NH2 group that has a mass of crosslinker mass+17.026549105. The priority will be given to the one that matches better (but here @lutzfischer is the one that should answer!). If you want to simplify the search, use only
and remove the MODIFICATIONS lines from the BS3 defitinition. If instead you want to consider more possibilities (mods on both linear and crosslinked peptides), use only modification:variable::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y;DELTAMASS:155.094619105 What you are referring to is the fact that the same syntax can be put inside the crosslinker definition instead (https://github.com/Rappsilber-Laboratory/XiSearch#crosslinker-settings)
here with modifications on all peptides
to define Tris as the quencher, you can define a modification for it, again as a separate modification or inside the crosslinker definition (please double check the bs3-tris mass!!)
or
And the same definitions can be given inside the crosslinker as well with the same syntax as above. |
Thank you very much for the explanation! It's still not entirely clear to me. Do all peptides mean the masses of both linear and crosslinked peptides? If MODIFICATIONS are added to the crosslinker definition, any precusor mass matched to the mass of peptide1+crosslinker+modification+peptide2 will be considered as a crosslinked peptide. If we take BS3 and NH2 as an example, where would the NH2 modification be added to peptide1-BS3-peptide2? If loop, 0 is added to the MODIFICATIONS as well, shouldn't a loop link only be considered in a linear peptide? Would it be correct to define loop link like this? If MODIFICATIONS are added to the crosslinker definition and are searched in both linear and crosslinked peptides, would matches to linear peptides show up in the Linear Peptides result table? In the example of modification:linear::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y;DELTAMASS:155.094619105, why is nterm not added to MOIDIFIED? I believe that nterm is interpreted as protein N-terminus not peptide N-terminus in xiSEARCH? In the Linear Peptides result table from a search using the default BS3 setting, one can find both bs3nh and bs3nh2 in the peptide sequences. bs3nh2 is not in the search setting. Could you explain what it is and why it shows up in the sequences? Many many thanks! |
Hello, No problem- sorry I was not clear enough, it is a bit confusing in its flexibility. There are 2 kinds of modifications: variable and linear. "Variable" searches for the modification on all peptides, "Linear" only on linear peptides. You can encode these modifications 2 ways:
If you are encoding a linear modification with 1) indeed what you write:
is correct. The deltamass is the mass change relative to the original amino acid. Then the crosslinker definition should look like this
I.e. not include the modification inside the crosslinker, as they are defined separately. If instead you use 2), then you should not have the separate line that says modification:linear and so on, but rather
In this case, the modification mass is the mass to change the crosslinker by- hence a loop link is 0 (a loop link has the same mass as a crosslink) and an nh2 group is 17.02654915. this is what defines the bs3nh2 that was not in your search. Whether you want a linear or a variable modification in 1) is defined by the choice of
or
In 2) it is defined by using the word
or
Back to your original question- you defined the modification twice (once as a separate line and once in the crosslinker) and xiSEARCH was assigning both, also because they were defined with slightly different masses. Simply choose one and you're good to go. You can edit your config by clicking "as text config" after setting things in the interface to adjust definitions as needed. |
Thank you very much for the further explanation! If I define a monolink or a loop link in a linear peptide as follows modification:linear::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y,nterm;DELTAMASS:155.094619105 I think it is reasonable to include the protein N-terminus as the possible modified amino acid for a monolink. My question is does xiSEARCH treat "nterm" as protein N-terminus or peptide N-terminus in this context? Am I correct that if I add "nterm" to the defintion of a crosslinker "nterm" will be considered as protein N-terminus? |
One more question if I may. In the Github document, the following is written "Similarly, to perform a search with a lot of modifications on a peptide, the value MAX_MODIFICATION_PER_PEPTIDE (default 3) also needs to be adjusted in order to consider combinations of more than 3 modifications. Variable modifications don't count against either limit. Both of these variables reduce the search space. Increasing them leads to a computational cost in terms of memory and search time." Am I right that this limit of default 3 maximum modifications per peptide includes fixed modifications but not variable modifications? Does this limit include the modification:linear as well? If I define linear peptide modifications for bs3nh, bs3oh and bs3loop, would this exceed the total limit of 3 modifications already? |
these are correct provided your crosslinker definition then does not have MODIFICATIONS or LINEARMODIFICATIONS defined in it.
I am sorry, I made a typo there. FIXED modifications do not count against the limit. Meaning, for example, if you have carbamidomethylation on C, and a peptide is GKLCMKR, the Ccm will not count against the modification limit in the search.
No- what that means is in the step of generating theoretical peptides in the search, all those mods will be included (you can define as many as you like, but that explodes your memory and search time), but theoretical peptides will be defined with at most with MAX_MODIFICATION_PER_PEPTIDE. For example, if you define 2 mods, "a" that can go on K/S/T/Y and "b" that can go on K/L, and set MAX_MODIFICATION_PER_PEPTIDE to 3. for the peptide
Only theoretical peptides with up to 3 modifications will be considered. This generates a lot of combinations! Hence there is also the number MAX_PEPTIDES_PER_PEPTIDE to tweak how many different peptides are generated per peptide |
"nterm" is protein N terminus and xiSEARCH automatically considers the possibility of trimming of n-terminal methionine |
Thank you very much for the speedy replies! There seems to be one problem. When I defined a linear peptide modification including "nterm" like this modification:linear::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y,nterm;DELTAMASS:155.094619105 all the linear peptide modifications previously detected using MODIFIED:K,S,T,Y were gone from the linear peptide result table. Only linear peptides with a protein N-terminal modification were in the table. If I switched the modification definition back to modification:linear::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y;DELTAMASS:155.094619105 then all those linear peptides with the a modication showed up again in the table. I've tried a couple of different datasets. This behaviour appeared consistent. Could this problem be solved? |
which version of xiSEARCH are you using? I will try to reproduce this. Could you also attach your config file? |
xiSEARCH_1.7.6.7 is the version I'm using. I used MODIFIED:K,S,T,Y,nterm for all the bs3 monolink and loop link linear peptide modification search. Many thanks. Sorry, I was searching for where to attached a file... here it is. |
could you attach it as a text file (just change the extension to .txt) so that it doesn't take the markdown formatting |
could you check with this configuration: one line for mods on residue, the other line on any amino acid and specifying position
|
We could reproduce the bug whenever "nterm" is in the definition of the modification. The workaround of defining the n-terminal modification separately as above seems to solve the issue |
Thank you! Indeed, with two separate deifintions for KSTY and nterm, it seems to have solved the issue. Now I have 6 defined modifications (bs3nh, bs3nhn, bs3oh, bs3ohn, bs3loop, bs3loopn) instead of 3 (bs3nh, bs3oh, bs3loop), do you think I should increase the value of MAX_MODIFICATION_PER_PEPTIDE (default 3)? Or rather not, since the protein N-terminal peptide is unlikely to carry more than 3 of these different modifications? |
It's hard to know, that depends on the number of proteins in your database. I would start with default values and see what memory and search times come out. If your database is small, rather than increasing MAX_MODIFICATION_PER_PEPTIDE, I would increase MAX_MODIFIED_PEPTIDES_PER_PEPTIDE to for example 100, if your memory and search time allows for it. Half those mods can only occur on n-terminal peptides, so i don't think number of modifications per peptide is what is limiting you, rather how many possibilities of the different combinations to consider in site assignment. |
as an aside- we strongly recommend searching with multiple crosslinker and including noncovalent associations in the search |
Do you mean adding unrelated crosslinkers together into the search? Is this practice more for in-cell crosslink data or also for recombinant proteins? Could you point to a publication showing the comparison to a regular single crossliner search? That would be great. Thank you. |
No- I mean to select "multiple crosslinkers" in the interface and select the crosslinker you used and "NonCovalent" which handles non-covalent association of peptides in the gas phase. This helps removing false positive identifications. https://github.com/Rappsilber-Laboratory/xisearch#crosslinker-selection This defines an additional crosslinker with mass 0 called "NonCovalent" (see the end of this section https://github.com/Rappsilber-Laboratory/xisearch#crosslinker-settings on how to add it to the config file). Publication: https://pubs.acs.org/doi/10.1021/acs.analchem.8b04037 (see fig.5 for an example of why this is useful to add). |
I see. Thank you very much for you help and all the speedy replies! |
If I may ask again, with "NonCovalent" added, I saw an extra table "NAPS". Have these PSMs been automatically excluded from the result of the Links table? |
I'm afraid I've encountered another problem. If I included crosslinker:NonCovalentBound:Name:NonCovalent additionally, the search crashed with the following error: Search Crashed:null Peptides:DDDDK, DDDDK What might be the problem? |
The NAPS table reports noncovalently associated peptides and the links table your crosslinks. If you open the links table in excel, you will see the "crosslinker" column and check that no "NonCovalent" are there. MzIdentML will contain both reporting which psm is which, and xiview.org will show only crosslinkers that aren't from noncovalent association.
at what stage of the search? could you paste more of the log? Have you tried rerunning allocating more memory if possible? |
Allocating more memory didn't help. It stopped about midway during the search. Here is the error log of the latest try. Aug 14, 2023 2:36:27 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process |
and the search runs otherwise through to the end without the NonCovalent line? Could you attach your current config file? Apologies for the many questions, just trying to understand. |
I tried a different dataset and got the following error log if I included NAP search. Without the NonCovalent line, it always went through, also the previous one. INFO: Search Thread Search_22 finished (Search got stoped through config) |
can you post the full log - or email it to me? From what you send, a error occurred somewhere and as a result xisearch stopped the search. Just that the actual error is not in the part of the log you posted - need to see why that is the case - INFO: Search Crashed:null Peptides:DDDDK, DDDDK should have been the error reported again. I hope in the full log there is somewhere the actual error. |
I've just re-run the search with apl files using the same config file. Here is the error log. Thank you. Aug 22, 2023 5:49:24 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:24 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:24 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Link site is < 0 Aug 22, 2023 5:49:25 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:25 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:25 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:25 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:25 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:26 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:28 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:29 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:29 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:29 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:29 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:29 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process java.lang.ArrayIndexOutOfBoundsException Aug 22, 2023 5:49:30 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:30 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:30 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:30 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:31 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:31 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:31 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:31 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:31 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:32 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:32 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:32 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:32 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:32 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:33 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:33 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:33 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:33 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:33 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:34 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:34 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:34 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:34 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:34 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:35 PM rappsilber.ui.LoggingStatus setStatus Aug 22, 2023 5:49:35 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:35 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:35 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:35 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:36 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:36 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:36 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:36 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:36 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:37 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:37 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:37 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:37 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:37 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:38 PM rappsilber.ui.LoggingStatus setStatus Aug 22, 2023 5:49:39 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:39 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:39 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:39 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:40 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:40 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:40 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:40 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:40 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:41 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:41 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:41 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:41 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:41 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:42 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:42 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:42 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:42 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:43 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:43 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:43 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:43 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:43 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:44 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:44 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:44 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:44 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:45 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:46 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:46 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:46 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:47 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:48 PM rappsilber.ui.LoggingStatus setStatus Aug 22, 2023 5:49:49 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:49 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:49 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:50 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:50 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:50 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:50 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:51 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:51 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:51 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:51 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:52 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:52 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:52 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:52 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:53 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:53 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:53 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:54 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:54 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:54 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:54 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:54 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process Aug 22, 2023 5:49:55 PM rappsilber.applications.SimpleXiProcessMultipleCandidates process |
hello, |
I only used the sil0.apl files and I can confirm that every spectrum in there has a known precursor charge state. In the NAP paper, I tried to find an answer to why QE produced a higher proportion of NAP peptides compared to Velos or even to Lumos which uses a quad similar to QE's as an ion filter. I could not find it. Maybe I missed it in the paper. Could you comment on this? |
I am not aware that there is any official reason given for that behaviour. But the reason for more NAPs in QE products MIGHT be down to how the ions get forwarded into the machine. The "lens" (is named somewhat differently in different machines) is different and might lead to peptides not as easily (in relative terms) disassociate for QE than in others. But that is only an very untested hypothesis. When we looked at crosslink identifications from an early qexactive instrument - without considering non-covalent peptides at the time - we had an surprising number of overlength and in fact made us in turn aware of the problem of non-covalent associated peptides. |
Dear Lutz
If I understood correctly, in the default setting, the following line defines the search for BS3amidated linear peptide
modification:linear::SYMBOLEXT:bs3nh;MODIFIED:K,S,T,Y;DELTAMASS:155.094619105
In the defaut BS3 crosslinker setting, there is also the MODIFICATIONS:NH2,17.026549105
From which one of these two does xiSEARCH use to find the linear modification? Does one take precedence over the other? If BS3 is quenched by an amine-based reagent other than ammonia, e.g. Tris, where should the modifiation be defined to find the corresponding monolinks?
In the Linear Peitides result table from a search using the default BS3 setting, one can find both bs3nh and bs3nh2 in the peptide sequences. bs3nh2 is not in the search setting. Could you explain what it is and why it shows up in the sequences?
Many thanks!
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