Steric effect in the threshold dissociative ionizations of serine conformers [CH2OH-CalphaH(NH2)-CbetaOOH] is revealed by high-level ab initio calculations combined with our newly developed infrared laser desorption / tunable VUV photoionization mass
spectrometry. We find that near the ionization thresholds the Calpha-Cbeta and Calpha-C bonds are selectively broken for the respective cationic conformers, yielding the different fragments. Novel dynamic processes, proton transfer and reorientation between the predissociative fragments, are involved in the threshold photoionization dissociations.
