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Zwitterions
- Sodiated Oligoglycines
 The
shape of sodiated oligoglycines depends very strongly on whether the peptide
assumes a zwitterion structure or not. The effect is particularly pronounced
for sodiated pentaglycine and is nicely demonstrated by the figures below.
The figure on the left-hand side shows a typical representative for a
charge solvation structure found by molecular
mechanics simulations. The expected cross section for this structure
is ~111 Å2 in close agreement with a value of ~108 Å2
obtained in experiments using our MALDI-sector
instrument. Sodiated zwitterion structures are much more compact with
a theoretical cross section of ~98 Å2 (figure on the
right-hand side), which is close to the cross section measured for protonated
pentaglycine (~97 Å2). Protonated pentaglycine structures
are predicted by theory to be compact (~96 Å2) in agreement
with experiment. As oligoglycines have only one basic site (the N-terminus)
the protonated form cannot be a zwitterion and provides therefore an excellent
calibration experiment without any structural ambiguity.
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