Corey N. Stedwell, Johan F. Galindo, Kerim Gulyuz, Adrian E. Roitberg and Nicolas C. Polfer . Crown Complexation of Protonated Amino Acids: Influence on IRMPD Spectra
J. Phys. Chem. A, Article ASAP, 2012-08-14
Abstract: We report infrared multiple photon dissociation (IRMPD) spectra for a series of crown-adducted, protonated amino acids, generated by electrospray ionization. The tight chelation of 18-crown-6 on the protonated NH3+ moiety results in a considerable red shift of the NH3+ stretch modes, notably the antisymmetric NH3+ stretch. This is rationalized by a distortion of the NH3+ normal mode potential energy surface, as verified by quantum chemical calculations. On the other hand, the local oscillator modes, such as the carboxylic acid OH stretch, indole NH stretch, and phenol OH stretches, remain well-resolved and are subject to minor and predictable blue shifts of 5–15 cm–1. Other chemically diagnostic modes, such as the guanidine NH stretch and alcohol OH stretches, also have discernible band positions. Crucially, some of these diagnostic band positions have little to no overlap with one another and can hence be readily distinguished. In addition, the complexes are often found to efficiently photodissociate by neutral loss of 18-crown-6, particularly for higher-basicity amino acids. This in principle opens the door on multiplexing the IRMPD experiment, where the IR spectra of multiple precursors are recorded simultaneously.