Hi, all,
Is it possible to definitely say that two molecules, a protein and a
peptide, associate non-covalently using MALDI or electrospray? We are
looking at some data obtained on a Voyager DE-STR MALDITOF (work not done
here, as, alas, we only have the Voyager DE) with a protein (MW approx
27,000) and a peptide (MW approx 3,000). Each species alone shows the
singly and doubly charged species, as well as dimers (matrix sinapinic
acid in 80% acetonitrile 0.1% TFA). [We realize that dimers are well
documented in MALDI and are definitely not proof of dimerization under
physiological conditions.]
The real point the researcher wants to make is that when the protein and
peptide are incubated together, MALDI then shows a species that is the sum
of the protein and peptide molecular weights (along with the individual
species and their dimers, again), and that this is evidence of
at least non-covalent association of the two species. Having been though
my practice lab at PE Biosystems where we looked at an unknown containing
BSA and insulin--and saw beautiful individual peaks for theses two species
and a mass that perfectly matched the sum of their masses--I don't see
this as strong evidence of non-covalent association between the peptide
and the protein.
The protein-peptide mixture was run on LCMS electrospray and no mass
matching the complex was found. The next thing that is being done is to
run electrospray on the "complex" without the LC. As in MALDI, is it
possible to observe "complexes" by electrospray that are not "real"? Or
if the mass of the complex is found is this clear evidence of association?
(Do non-covalent interactions, electrostatic or hydrophobic, persist in
electrospray?)
Deb McMillen
Institute of Molecular Biology
University of Oregon
Eugene OR 97403
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