Crown Complexation of Protonated Amino Acids: Influence on IRMPD Spectra

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 NH(3)(+) moiety results in a considerable red shift of the NH(3)(+) stretch modes, notably the antisymmetric NH(3)(+) stretch. This is rationalized by a distortion of the NH(3)(+) 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.

Hypervalent radical formation probed by electron transfer dissociation of zwitterionic tryptophan and tryptophan‐containing dipeptides complexed with Ca2+ and 18‐crown‐6 in the gas phase

“…Geometry optimization was carried out by density functional theory (DFT) calculations at the B3LYP/6‐31++G(d,p) level using GAUSSIAN 09 . In this study, the optimization for each species was performed from several initial configurations according to the conformers reported for similar systems . Vibrational analyses were carried out for each optimized structure to confirm the minima on the potential energy surfaces.…”

“…Previously, PID and reactivity of TrpH + and Trp‐containing protonated peptides complexed with 18‐crown‐6 (18C6) were reported, in which 18C6 bonded to the NH 3 + moiety of TrpH + , as shown in Fig. . Enantioselective photodissociation of cold TrpH + enantiomers on a chiral crown ether was examined and applied to the quantitative chiral analysis .…”

“…1. [28] Enantioselective photodissociation of cold TrpH + enantiomers on a chiral crown ether was examined and applied to the quantitative chiral analysis. [29,30] The photo-induced electron transfer, followed by proton transfer, proceeded the C α -C β bond cleavage of D-TrpH + on the chiral crown ether.…”

“…[34] In this study, the optimization for each species was performed from several initial configurations according to the conformers reported for similar systems. [20,28,[35][36][37] Vibrational analyses were carried out for each optimized structure to confirm the minima on the potential energy surfaces.…”

Gas-phase conformations of small polyprolines and their fragment ions by IRMPD spectroscopy

“…As a result, manual definition of input geometries using chemical intuition alone rapidly becomes difficult when considering peptides of sizes larger than 2 and 3 residues. In order to more completely explore the potential energy surface of these peptides, we have therefore employed a molecular mechanics/molecular dynamics (MM/MD) approach using AMBER 12 [41], approximately following a procedure described in more detail previously [42]. Initial guess geometries were first optimized in the Gaussian09 package [43] at the B3LYP/6-31++G(d,p) level of theory.…”