Gas-phase solvation of protonated amino acids by methanol.
The enthalpy and entropy changes for the formation of the 1:1 complexes of methanol with various gaseous protonated amino acids have been measured using pulsed-ionization high-pressure mass spectrometry. The enthalpy changes for formation of the clusters Gly(MeOH)H(+), Ala(MeOH)H(+), Val(MeOH)H(+), Leu(MeOH)H(+), Ile(MeOH)H(+), and Pro(MeOH)H(+) have been determined to be -92.0, -83.3, -82.4, -79.5, -78.7, and -73.6 kJ mol(-1), respectively. These values agree very well with the energetic values computed at the MP2(full)/6-311++G(2d,2p)//B3LYP/6-311+G(d,p) level of theory for the lowest energy adducts in each system. Both experimental observations and computational determinations of the potential energy surface for the glycine system suggest that a mixture of low-lying isomers may be present for each of the cluster systems examined. The primary structural motif for these clusters is the coordination of the methanol molecule to the ammonium group of the protonated amino acid via a strong ionic hydrogen bond. For the amino acids studied here, computational results reveal that one methanol molecule does not sufficiently stabilize any zwitterionic structure such that no appreciable extent of proton transfer from the amino acid to methanol was observed.
Department of Chemistry, University of Waterloo , Waterloo, Ontario, Canada N2L 3G1.
Kris
K
Eldridge
Ronghu
R
Wu
Jonathan K
JK
Martens
Terry B
TB
McMahon
D000596:Amino Acids / Q000737:chemistry*; D019277:Entropy; D005740:Gases / Q000737:chemistry; D000432:Methanol / Q000737:chemistry*; D011522:Protons; D011789:Quantum Theory; D012995:Solubility
D000596:Amino Acids; D005740:Gases; D011522:Protons; D000432:Methanol