Gas-phase protonation thermochemistry of arginine.
The gas-phase basicity (GB), proton affinity (PA), and protonation entropy (DeltapS degrees (M)=S degrees (MH+)-S degrees (M)) of arginine (Arg) have been experimentally determined by the extended kinetic method using an electrospray ionization quadrupole time-of-flight (ESI-Q-TOF) mass spectrometer. This method provides GB(Arg)=1004.3+/-2.2 (4.9) kJ.mol(-1) (indicated errors are standard deviations, and in parentheses, 95% confidence limits are given). Consideration of previous experimental data using a fast atom bombardment ionization tandem sector mass spectrometer slightly modifies these estimates since GB(Arg)=1005.9+/-3.1 (6.6) kJ.mol(-1). Lower limits of the proton affinity, PA(Arg)=1046+/-4 (7) kJ.mol(-1), and of the "protonation entropy", DeltapS degrees (Arg)=S degrees (ArgH+)-S degrees (Arg)=-27+/-7 (15) J.mol(-1).K(-1), are also provided by the experiments. Theoretical calculations conducted at the B3LYP/6-311+G(3df,2p)//B3LYP/6-31+G(d,p) level, including 298 K enthalpy correction, predict a proton affinity value of ca. 1053 kJ.mol-1 after consideration of isodesmic proton-transfer reactions with guanidine as the reference base. Computations including explicit treatment of hindered rotations and mixing of conformers confirm that a noticeable entropy loss does occur upon protonation, which leads to a theoretical DeltapS degrees (Arg) term of ca. -45 J.mol(-1).K(-1). The following evaluated thermochemical parameter values are proposed: GB(Arg)=1005+/-3 kJ.mol(-1); PA(Arg)=1051+/-5 kJ.mol(-1), and DeltapS degrees (Arg)=-45+/-12 J.mol(-1).K(-1).
Bouchoux
Guy
G
Laboratoire des Mécanismes Réactionnels, Ecole Polytechnique, 91128 Palaiseau, France. bouchoux@dcmr.polytechnique.fr
Desaphy
Sylvain
S
Bourcier
Sophie
S
Malosse
Christian
C
Bimbong
Rosa Ngo Biboum
RN
D000465:Algorithms; D001120:Arginine / Q000737:chemistry*; D005740:Gases; D019791:Guanidine / Q000737:chemistry; D007700:Kinetics; D008958:Models, Molecular; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics
D005740:Gases; D011522:Protons; D001120:Arginine; D019791:Guanidine