Why does MALDI analysis of my oligos that contain one or more Fluorescein-dTs give an incorrect mass even though they give only a single, fluorescent band on a PAGE gel?
RESPONSE:It’s an artifact of the MALDI analysis. The lasers used to ablate the MALDI matrix are generally between 308 and 355 nm, with the most frequently used laser line at 337 nm. When a laser hits a strong absorbance band of a molecule, often photochemistry starts to occur – which often leads to cleavage reactions. The Fluorescein-dT has strong UV absorbance in this region which appears to lead to a 135 m/z fragment is being blown off the molecule in a rather consistent manner. For instance, your oligos D1 and D2, which have 5 and 3 Flu-dTs respectively, the observed mass difference is -676 Da and -406 Da, which nicely fits the loss of a 135 mw fragment: 5 x 135 (675) and 3 x 135 (405). We haven’t seen any papers that identify this 135 m/z fragment – but it’s clear to me that that is what’s occurring. Changing matrix matrix used may help. The matrix 2,4,6-trihydroxyacetophenone has been used successfully to analyze a fluorescein-labeled oligos[1], though the safest bet is to use Ele
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