Rm. Pope et al., GAS-PHASE STRIPPING OF ALKALI CATIONS FROM BIOMOLECULES VIA REACTION WITH CROWN-ETHERS, International journal of mass spectrometry and ion processes, 175(1-2), 1998, pp. 179-186
Citations number
33
Categorie Soggetti
Spectroscopy,"Physics, Atomic, Molecular & Chemical
Electrospray mass spectra of biomolecules produced from salty solution
s can exhibit a large number of alkali adducts. Often the number of at
tached alkali metal atoms exceeds the net charge state of the ion. For
example, the spectral intensity of the 2+ charge state of gramicidin
S, cyclo[Val-Orn-leu-D-Phe-Pro](2), (GS), withdrawn from a sodium-cont
aining water:methanol solution, is distributed over (GS + 2H)(2+), (GS
+ H + Na)(2+), (GS + 2Na)(2+), (GS-H + 3Na)(2+), (GS-2H + 4Na)(2+), (
GS-3H + 5Na)(2+), and (GS-4H + 6Na)(2+). Each additional metal cation
displaces a proton without affecting the net charge of the biomolecule
. Hence, in (GS-2H + 4Na)(2+), at least two of the alkali metal adduct
s must involve displacement of protons from non-traditional basic site
s. The character of these coordination sites must be either ''salt-lik
e'' or zwitterionic. Alkali adducts of trapped polypeptide ions may be
removed via gas phase ion-molecule ''stripping'' reactions with crown
ethers 12-crown-4, 15-crown-5, or 18-crown-6. Both products of the st
ripping reaction, the desalted biomolecule of reduced charge and the a
lkali-metal-attached crown, are observed in the FTICR mass spectra. Fo
r gas phase, ''hyper-metallated'' peptides, in which the number of add
ucted alkali metal ions exceeds the net charge, we suggest that some o
f the attached cations replace amide protons by coordinating as a salt
or zwitterion to a tautomerized, deprotonated amide link in the backb
one of the peptide. Alkali adduction to these non-traditional base sit
es leads to slightly higher stripping rates, probably by removing the
secondary constraints of transannular NH ... O = C hydrogen bonds, whi
ch makes attached alkali cations more accessible to the stripping reag
ent. Stripping of K+ is faster than stripping of Na+, as may be expect
ed for electrostatically bound cations. (C) 1998 Published by Elsevier
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