ESI ionization for protein/peptide

[littlewenwen]

Dear All

I have a question about the ionization of peptide/protein in LC-ESI-MS.

the question is: what determines the ionization mode (positive or negative) and ionization state ( singly, doubly or multiply chagrged)?

for example, if we have two peptides (assume they exist):

peptide A: KKKKKKK

peptide B: EEEEEEE

if the pH=7, doe this mean that: for peptide A, only positively charged ionization is possible, and the maximal chagred state is 7+?

and for peptide B, only negatively charged ionization is possible, and the maximal charged state is 7-?

[littlewenwen]

another question is, if we have a peptide C:

peptide C: EEEKKK

at pH 7, what will be the ionization mode (positive /negative) for this peptide?

[Triple Quad]

Dear All

I have a question about the ionization of peptide/protein in LC-ESI-MS.

the question is: what determines the ionization mode (positive or negative) and ionization state ( singly, doubly or multiply chagrged)?

for example, if we have two peptides (assume they exist):

peptide A: KKKKKKK

peptide B: EEEEEEE

if the pH=7, doe this mean that: for peptide A, only positively charged ionization is possible, and the maximal chagred state is 7+?

and for peptide B, only negatively charged ionization is possible, and the maximal charged state is 7-?

For peptide A at pH=7 you will very likely get some ions via electrospray at pH=7, conditions aren't ideal, therefore you won't get the best signal but you'll likely get some ions. Positive ESI mode is best but you will still get ions for peptide A in negative mode.

For any peptide you can get some ions in positive and negative ESI.

[Triple Quad]

another question is, if we have a peptide C:

peptide C: EEEKKK

at pH 7, what will be the ionization mode (positive /negative) for this peptide?

I would chose positive mode for that peptide, but again you could work with it and get negative mode ESI to work as well.

[Kostas Petritis]

Actually peptide charge state in the gas phase will be alsoy be determined by its size. The larger the peptide, the higher the charge state.

In the examples that you mention, the theoretical charge state should be 8+ and 8- respectively as the N terminus Lysine and C terminus glutamic acid will have two ionizable charges.

Finally, generally, positive ion mode is prefered due to characteristic fragmentation but in your case if you do not care about fragmentation or if you do not have very complex mixtures you can do either (depending of course of their structure). I would probably go for A and C in positive and B in negative...

[littlewenwen]

Thank you all for suggestions.