Kinetic and thermodynamic considerations of the bracketing method: Entropy‐driven proton‐transfer reactions in a Fourier transform mass spectrometer

Several reports of experimentally derived proton affinity values and gas‐phase basicity values for amino acids and peptides have recently appeared in the literature. Here, we show that the thermodynamic quantity that is measured by the Fourier transform mass spectrometry proton transfer bracketing of amino acids and peptides is gas‐phase basicity and not proton affinity. Both experimental and theoretical evidence supports this conclusion. The difference between the values determined by proton transfer bracketing measurements for lysine versus leucine is consistent with a difference in gas‐phase basicity rather than proton affinity. The rate of proton transfer from protonated lysine to a series of reference compounds have been measured. Entropy‐driven, endothermic proton transfer is found to occur at the collision rate. Recent ab initio and semi‐empirical calculations of the proton affinity of lysine are found to agree with the value that is derived from bracketing studies when one assumes that gas‐phase basicity is measured. While entropy‐driven reactions have been observed previously in high‐pressure mass spectrometers, this is the first evidence for such reactions at low pressure in a Fourier transform mass spectrometer.