No abstract

The proton affinity of proline analogs, L-azetidine-2-carboxylic acid (Aze), L-proline (Pro), and L-pipecolic acid (Pip), have been measured using the Armentrout modification of the extended kinetic method in a quadrupole ion trap instrument. Experimental values of 223.0 Ϯ 1.5, 224.9 Ϯ 1.6, and 225.6 Ϯ 1.6 kcal/mol have been determined for the 298K proton affinities of Aze, Pro, and Pip respectively. High level theoretical calculations using both MP2 and B3LYP methods at a variety of basis sets were carried out in order to give theoretical predictions for the 298 K proton affinity and gas phase basicity of all three analogs. Recommended values for the gas phase basicity and proton affinity for proline based on our work and other recent determinations are 216 Ϯ 2 and 224 Ϯ 2 kcal/mol. (J Am Soc Mass Spectrom 2002, 13, 72-81)

The proton affinities of lysine (1) and its three homologues ornithine (2), 2,4-diaminobutanoic acid (3), and 2,3-diaminopropanoic acid (4) have been determined using two different variants of the extended kinetic method in an electrospray ionization−quadrupole ion trap instrument. A value of 1004.2 ± 8.0 kJ/mol is recommended for the proton affinity for lysine on the basis of this work and previous experimental measurements and theoretical predictions. Values of 1001.1 ± 6.6, 975.8 ± 7.3, and 950.2 ± 7.1 kJ/mol have been determined for the proton affinities of 2−4. These experimental results are supported by hybrid density functional theory calculations at B3LYP/6-311++G**//B3LYP/6-31+G*. An analysis of the derived entropy terms lends support to the notion that these values can be used as a quantitative prediction for the thermodynamic entropy of protonation provided that appropriate error bars are assigned. Finally, for systems in which this entropy term is large, it is essential that the extended kinetic method be used to derive accurate proton affinities.