GAS-PHASE STRIPPING OF ALKALI CATIONS FROM BIOMOLECULES VIA REACTION WITH CROWN-ETHERS

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 For example, the spectral intensity of the 2+ charge state of gramicidin S, cyclo[Val-Orn-leu-D-Phe-Pro](2), (GS), withdrawn from a sodium-cont aining water:methanol solution, is distributed over (GS + 2H)(2+), (GS + H + Na)(2+), (GS + 2Na)(2+), (GS-H + 3Na)(2+), (GS-2H + 4Na)(2+), (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 Science B.V. All rights reserved.