Mass spectrometric and quantum-chemical study on the structure, stability, and chirality of protonated serine dimers

Stability and structure of homo- and heterochiral protonated serine (Ser) dimers were investigated by a combination of mass spectrometry and ab initio quantum chemical calculations. This established that the energy difference between the most stable homo- and heterochiral forms is very small: tandem mass spectrometry with Cooks' kinetic method yielded a negligible difference in Gibbs free energy (0.2+/-0.2 kJ mol(-1)). The various isomeric forms of (Ser)2 H+ and their energetics were determined by extensive electronic-structure calculations, which yielded homo- and heterochiral forms of the isomers with distinctly different relative energies. The most stable homochiral isomer is stabilized by two hydrogen bonds and is far more stable than any other homochiral isomer. The most stable heterochiral isomer has completely different features, and it is characterized by a salt-bridge structure. This clearly shows that salt-bridge structures do exist in the gas phase even in comparatively small molecules and in the absence of particularly basic or acidic functional groups.