"Gas-phase Basicities
for Ions from Bradykinin and Its des-Arginine Analogues",
N.P. Ewing, G.A. Pallante, X. Zhang, and C.J. Cassady, J. Mass
Spectrom. 36, in press (2001).
Apparent
gas-phase basicities (GBapps) for [M+H]+
of bradykinin, des-Arg1-bradykinin and des-Arg9-bradykinin
have been assigned by deprotonation reactions of [M+2H]2+
in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer.
With a GBapp of 225.8 ± 4.2 kcal/mol, bradykinin
[M+H]+ is the most basic of the ions studied. Ions
from des-Arg1-bradykinin and des-Arg9-bradykinin have
GBapp values of 222.8 ± 4.3 kcal/mol and 214.9
± 2.3 kcal/mol, respectively. A purpose of this work was
to determine a suitable reaction efficiency "break point"
for assigning GBapp values to peptide ions using the bracketing
method. An efficiency value of 0.1 (i.e., approximately 10% of
all collisions resulting in a deprotonation reaction) was used
to assign GBapps. Support for this criterion is provided by the
fact that our GBapp values for des-Arg1-bradykinin
and des-Arg9-bradykinin are identical, within experimental
error, to literature values obtained using a modified kinetic
method. However, the GBapps for bradykinin ions from the two studies
differ by 10.3 kcal/mol. The reason for this not clear, but may
involve conformation different produced by experimental conditions.
The results may be influenced by salt-bridge conformers and/or
by conformational changes caused by the use of a proton-bound
heterodimer in the kinetic method. Factors affecting the basicities
of these peptide ions are also discussed and molecular modeling
is used to provide information on protonation sites and conformations.
The presence of two highly basic arginine residues on bradykinin
results in its high GBapp, while the basicity of des-Arg1-bradykinin
ions is increased by the presence of two proline residues at the
N-terminus. The proline residue in the second position folds the
peptide chain in a manner that increases intramolecular hydrogen
bonding to the protonated N-terminal amino group of the proline
at the first position.