ASMS 2008 Poster: • The gas-phase acidities of eighteen cysteinepolyglycine peptides were determined using the extended kinetic method. The entropy factor is important in these systems. • The gas-phase acidity of the cysteine residue increases systematically with the increase in the length of the peptide. • It is worth mentioning that the CGn peptides are more acidic than the corresponding GnC peptides.

1. Determination of the Gas-Phase Acidities of the Cysteine-Polyglycine Peptides
Kiran Kumar Morishetti, John Tan, Betty Huang and Jianhua Ren*
Department of Chemistry, University of the Pacific, Stockton, CA
INTRODUCTION CID Experiments The Kinetic Method Computational Studies
Knowledge of thermochemical properties of Proton bound deprotonated heterodimers [A ] -
∆acidHi - ∆acidHavg ∆acidHi - ∆acidHavg ∆(∆S) The geometries of the neutral and deprotonated
ln = - -
poly-functional macromolecules is an essential between the reference acid and the compound of [Ai-] RTeff RTeff R peptides were optimized using the AM1
path in understanding the biophysical/ interest are generated in the ESI source, [ref- method. The frequency calculations were
biochemical processes. In connection with the •H•SCGn-]¯, and fragmented at several collision CG4 First Plot
performed at the same level followed by single
investigations of the unusual acidities of the energies. The CID product ion intensities are 1 point energy calculations at B3LYP/6-31+G(d)
cysteine residue at the active sites of the recorded for each heterodimer at each collision
0 level to yield the thermochemical quantities of
_
ln(IG4C-H/Iref)
thioredoxin family of enzymes, recently, in our energy.
the peptides. The theoretical gas-phase
}
HSCGn + ref -1 1.0 ev
lab we have studied the gas phase acidities of a _ CID 1.5 ev acidities of the peptides were derived from the
series of cysteine-polyalanine peptides.1 As an [ref H SCGn] -2 2.0 ev Ecm
_ 2.5 ev
isodesmic proton transfer reaction.
extension of this work we have studied the gas SCGn + Href -3
phase acidities of cysteine-polyglycine peptides, CG4 G4C -4
using the kinetic method and Quantom Chemical TFA-
TFA- [G4CS--H-TFA-]- -4 -2 0 2 4 HSCGn + CH3S- -
SCGn + CH3SH
calculations. TFA SCG4 -
∆acidHi-∆ acidHavg ∆Hrxn = ∆Hacid(HSCGn) - ∆Hacid(CH3SH)
[SCG4--H-TFA-]- CG4 second plot ∆Hacid, kcal/mol
Cys-(Gly)n-NH2 (Gly)n-Cys-NH2 1.2
G4CS-
(HSCGn) n = 3-9 (GnCSH)
[(∆H(CG4)-∆Havg)/RT eff-
y = -1.0104x + 1.6505 HSCG5 G5CSH
1.15
R2 = 0.9914
50 125 200 275 350 425 500 50 125 200 275 350 425 500
∆(∆)S/R]
Solid Phase Peptide Synthesis DBA- [G4CS--H-DBA-]-
1.1
DBA SCG4-
O O O O O O 1.05
H2N CH C NH CH2 C N CH2 C N CH2 C N CH2C N CH2 C N G4CS-
CH H H H H H
2
1
SH [SCG4--H-DBA-]- -SCG
Rink Amide Resin DBA-
0.45 0.5 0.55 0.6 0.65 5
Peptide Sequence Determination 1/RTeff
a1 b1 c1 a2 b2 c2 a3 b3 c3 a4 b4 c4 a5 b5 c5 100 200 300 400 500 600 100 200 300 400 500 600
CG5H+
O [SCG5--H-TFA-]- TFA-
H+ O O O O O
H2N CH C N CH C N CHC N CHC N CHC N CH C NH2
m/z= 406.2
CG5 G5C [G5CS--H-TFA-]-
Compound Experimental Quantum Chemical
CH2 H H
∆H (kcal/mol) ∆H (kcal/mol)
H H H H H H H
SH
H
m/z TFA SCG5- G5CS- G5CS-
x5 y5 z5 x4 y4 z4 x3 y3 z3 x2 y2 z2 x1 y1 z1 406.2
y3
CG3 325.7 314.3
GGG
m/z= 189.1 y4
m/z= 246.1
TFA- Conclusions
m/z=172.1 b5
m/z= 332.1
CG4 324.6 309.7 • The gas-phase acidities of eighteen cysteine-
b2
m/z=161.0 CG5 323.8 306.9 polyglycine peptides were determined using
a2 50 150 250 350 450 550 50 150 250 350 450 550
m/z= 133.0
SCG5- G5CS- [G5CS--H-DBA-]- CG6 322.8 303.8 the extended kinetic method. The entropy
factor is important in these systems.
DBA [SCG5--H-DBA-]-
CG7 ~322 305.2
DBA- • The gas-phase acidity of the cysteine residue
CG8 <322 - increases systematically with the increase in
Methods and Apparatus CG9 <322 - the length of the peptide.
DBA-
ESI-Triple Quadrupole G 3C 328.0 320.5 • It is worth mentioning that the CGn peptides are
150 300 450 600 150 300 450 600
G 4C 326.9 336.0 more acidic than the corresponding GnC
Co
∆H (kcal/mol)
Q1 llis
io
n
ce Reference Acid peptides.
Q0
ll
G 5C 325.0 322.5
HexaFluoroButyric Acid (HFBA) 321.9 Acknowledgements
Cap
illar
y
TriFluoroAcetic Acid (TFA) 323.8 G 6C 324.5 330.9
G 7C ~323 314.9 • ACS-PRF type G grant
Q3
PentafluoroBenzoic Acid (PFBA) 326.2
DiBromoAcetic Acid (DBA) 328.3 G 8C <323 - • Dean’s undergraduate research award (Tan and
DiChloroAcetic Acid (DCA) 328.4 Huang)
G 9C <323 -
DiFluoroAcetic Acid (DFA) 331.0
Varian 1200 LCMS 1John Tan and Jianhua Ren J. Am. Soc. Mass Spectom., 2007, 18, 188-194