Structures of cationized proline analogues: evidence for the zwitterionic form.
The structures of lithiated and sodiated alpha-methyl-proline (alpha-Me-Pro) and structural isomers, both with and without a water molecule, are investigated using blackbody infrared radiative dissociation (BIRD) and density functional theory. From the BIRD kinetic data measured as a function of temperature, combined with master equation modeling of these data, threshold dissociation energies for the loss of a water molecule from these clusters are obtained. These energies are 77.5 +/- 0.5 and 53 +/- 1 kJ/mol for lithiated and sodiated alpha-Me-Pro, respectively. For the nonzwitterionic isomer, proline methyl ester, these values are 3.0-4.5 kJ/mol higher. These results provide compelling experimental evidence that alpha-Me-Pro is zwitterionic in these clusters. Theory at the temperature corrected B3LYP/6-311++G**//B3LYP/6-31++G** level indicates that the salt-bridge or zwitterionic forms of lithiated and sodiated alpha-Me-Pro are between 17 and 23 kJ/mol lower in energy than the nonzwitterionic or charge-solvated forms and that attachment of a single water molecule does not significantly change the structure or the relative energies of these clusters. The proton affinity of proline is 8 kJ/mol higher than that of alpha-Me-Pro, indicating that lithiated and sodiated singly hydrated proline should also be zwitterionic.
Lemoff
Andrew S
AS
Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
Bush
Matthew F
MF
Williams
Evan R
ER
D000838:Anions; D002412:Cations; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D011392:Proline / Q000031:analogs & derivatives / Q000737:chemistry*; D011789:Quantum Theory; D014867:Water / Q000737:chemistry
D000838:Anions; D002412:Cations; D014867:Water; D011392:Proline