Taking peptides (in CHCA matrix) as an example, why would most ions produced carries an 'overall net' +1 charge?
As a peptide must contain different combinations of acidic and basic amino acids (hence side chains are of different charges). How would all these charges add-up / behave (were they shielded?).
I am curious as in most database searches (e.g. Mascot), in essentially all cases, we set the peptide charge parameter to +1 and it works fine every time (i.e. most peaks in the spectrum could be matched in no problem).
Thanks.
I would like to add a few words to this topic.
There are two main theories for the ionization mechanism in UV MALDI:
- Dr. Karas' "Lucky Survivor" Theory
- Dr. Knochenmuss " Coupled Photophysical and Chemical Dynamics (CPCD)" model
The first (thanks to Thomas for referencing it), explain quite well where part of the signal we observe comes from. You have ions in the sample, therefore, you observe them when you desorb them. The very same paper also contains proof that this is NOT the only mechanism. It is not even clear how much it contributes, for different analytes give rise to different results.
The second theory offer an explanation for how laser irradiation interact with the matrix and produces primary ions and radicals (first step) which then, while the plume is still dense, collide with analyte molecule producing the ions we see (second step). Although it has not the same degree of empiric evidences as Dr. Karas theory, it is a very robust representation of what might happen.
Both theories explain the presence of singly charged species as the most "stable" (favorable) ones. Knochenmuss theory mostly accounts for singly charged ions because of the total energy involved. Karas theory is based on neutralization.
Moreover, he also addressed the question directly ( "Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors", J. Mass Spectrom. 35, 1–12 (2000)).
The presence of collision induced neutralization is the best explanation we have right now.
Moreover, if you look at the work of Trimpin and co-worker, you will notice that multiply charged ions produce from solid state sample are observable. Especially when there is NO plume (no high density, no extreme rate of collision etc...)
Not always, since I have seen also doubly and triply-charged ions in MALDI, but it is true that most ions produced are +1. Why? I would like to know, too. As far as I know there is no proper answer yet.
As far as I know, the matrix used in MALDI is only capable to ionize the amine group, that is, the N-terminal portion of the protein/peptide. The double/triple charges can be observed when the ionization also occurs in the side groups of the amino acids, but this reaction is more difficult to happen.
Well it good question. Generally we mostly observed multiple charges in ESI-MS. But in MALDI-MS i also observed multiple charges. I was read about it recently one Lecture .
According that Lecture (http://www.hopkinsmedicine.org/mams/mams/middleframe_files/teaching_files/me330.884/2005/ms2005-lecture-2-basic-maldi-esi.pdf)
• multiply-charged ions less stable in gas phase, though charge-state rises
with increasing mass
• multiply-charged ions less easily desorbed from the surface because of
electrostatic attraction
• charge-state of desorbed ions are reduced by collision in expanding
plume. We see the surviving ions.
Maybe of interest ...
Article Compelling Evidence for Lucky Survivor and Gas Phase Protona...
I would like to add a few words to this topic.
There are two main theories for the ionization mechanism in UV MALDI:
- Dr. Karas' "Lucky Survivor" Theory
- Dr. Knochenmuss " Coupled Photophysical and Chemical Dynamics (CPCD)" model
The first (thanks to Thomas for referencing it), explain quite well where part of the signal we observe comes from. You have ions in the sample, therefore, you observe them when you desorb them. The very same paper also contains proof that this is NOT the only mechanism. It is not even clear how much it contributes, for different analytes give rise to different results.
The second theory offer an explanation for how laser irradiation interact with the matrix and produces primary ions and radicals (first step) which then, while the plume is still dense, collide with analyte molecule producing the ions we see (second step). Although it has not the same degree of empiric evidences as Dr. Karas theory, it is a very robust representation of what might happen.
Both theories explain the presence of singly charged species as the most "stable" (favorable) ones. Knochenmuss theory mostly accounts for singly charged ions because of the total energy involved. Karas theory is based on neutralization.
Moreover, he also addressed the question directly ( "Ionization in matrix-assisted laser desorption/ionization: singly charged molecular ions are the lucky survivors", J. Mass Spectrom. 35, 1–12 (2000)).
The presence of collision induced neutralization is the best explanation we have right now.
Moreover, if you look at the work of Trimpin and co-worker, you will notice that multiply charged ions produce from solid state sample are observable. Especially when there is NO plume (no high density, no extreme rate of collision etc...)
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