FREEZE-DRIED BIOMOLECULES - FT-ICR STUDIES OF THE SPECIFIC SOLVATION OF FUNCTIONAL-GROUPS AND CLATHRATE FORMATION OBSERVED BY THE SLOW EVAPORATION OF WATER FROM HYDRATED PEPTIDES AND MODEL COMPOUNDS IN THE GAS-PHASE
Sw. Lee et al., FREEZE-DRIED BIOMOLECULES - FT-ICR STUDIES OF THE SPECIFIC SOLVATION OF FUNCTIONAL-GROUPS AND CLATHRATE FORMATION OBSERVED BY THE SLOW EVAPORATION OF WATER FROM HYDRATED PEPTIDES AND MODEL COMPOUNDS IN THE GAS-PHASE, Journal of the American Chemical Society, 120(45), 1998, pp. 11758-11765
Solvent evaporation from extensively hydrated peptides and selected mo
del compounds formed by electrospray ionization has been examined usin
g an external ion source Fourier transform ion cyclotron resonance (FT
-ICR) mass spectrometer. Water evaporation from the clusters, formed a
t room temperature by appropriate operation of an electrospray ion sou
rce, is initially rapid and results in evaporative cooling of the clus
ters to a temperature; around 130-150 K, determined by the balance bet
ween evaporative cooling and heating by background blackbody radiation
. In this ''freeze-drying'' process, it is observed that the kinetics
of solvent evaporation and the cluster size distributions are highly d
ependent on the nature of the core ion in the cluster. In agreement wi
th earlier studies of the hydrated proton, pure water clusters exhibit
special stability characteristic of clathrate formation where, for ex
ample, a hydronium ion is encapsulated by a pentagonal dodecahedron of
twenty water molecules. Similar clustering of water occurs around pro
tonated primary alkylamines where the protonated amine replaces one of
the water molecules in the clathrate structures, which encapsulate on
e or more neutral water molecules. This observation supports the conje
cture that the doubly protonated cyclic decapeptide gramicidin S with
40 water molecules attached,the most significant magic number observed
in mass spectra at various delay times, has both protonated ornithine
residues solvated by pentagonal dodecahedron clathrate structures. Ot
her peptides such as doubly protonated bradykinin do not exhibit any s
pecific solvation during the freeze-drying process. Studies of model c
ompounds are presented which reveal other interesting aspects of water
structure around singly and multiply charged ions with low extents of
hydration, including the observation of neutral clathrates attached t
o a ''spectator'' ion.