Gas phase stripping of alkali cations from biomolecules via reaction with crown ethers

Electrospray mass spectra of biomolecules produced from salty solutions can exhibit a large number of alkali adducts. Often the number of attached 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 For example, the spectral intensity of the 2+ charge state of gramicidin S, cyclo[Val-Orn-leu-D-Phe-Pro](2), (GS), withdrawn from a sodium-containing water:methanol solution, is distributed over (GS + 2H)(2+), (GS + H + Na)(2+), (GS + 2Na)(2+), (GS-H + 3Na)(2+), (GS-2H + 4Na)(2+),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 adducts must involve displacement of protons from non-traditional basic sites. The character of these coordination sites must be either "salt-like" 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 stripping reaction, the desalted biomolecule of reduced charge and the alkali-metal-attached crown, are observed in the FTICR mass spectra. For gas phase, ''hyper-metallated'' peptides, in which the number of adducted alkali metal ions exceeds the net charge, we suggest that some of the attached cations replace amide protons by coordinating as a salt or zwitterion to a tautomerized, deprotonated amide link in the backbone of the peptide. Alkali adduction to these non-traditional base sites leads to slightly higher stripping rates, probably by removing the secondary constraints of transannular NH ... O = C hydrogen bonds, which makes attached alkali cations more accessible to the stripping reagent. Stripping of K+ is faster than stripping of Na+, as may be expected for electrostatically bound cations. (C) 1998 Published by Elsevier Science B.V. All rights reserved.