Transition metal(II) complexes of histidine-containing tripeptides: Structures, and infrared spectroscopy by IRMPD

Complexes of divalent metal ions (Cu2+ and Ni2+) with histidine tripeptide complexes (HAA, AHA and AAH) are interesting gas-phase models for some of the most widely observed patterns of metal ion binding to peptides and proteins. Gas-phase structures were characterized using infrared multiple photon dissociation (IRMPD) spectroscopy in ion-trapping mass spectrometers, along with density functional theory (DFT) computations. Ground states are square-planar with two deprotonated amide nitrogens bound to the metal ion via a double iminol rearrangement (IM binding mode), but contrary to expectations based on solution behavior, the histidine imidazole group is not bound to the metal, but instead is hydrogen bonded remote from the metal ion. The alternative “charge-solvated” (CS) binding mode (amide carbonyl oxygens binding the metal ion) lies higher in energy, but in many cases was observed to be present as conformationally unrelaxed ions with an abundance (relative to the IM conformation) that was dependent on instrument configuration and source and trap conditions. Taking advantage of the ability to form and trap both IM and CS conformations for a few of the complexes, the infrared spectroscopy of both conformations was explored in the fingerprint (1000–1800 cm−1) and hydrogen-stretching (3200–3800 cm−1) regions. For the fingerprint region, agreement is very good between the observed IRMPD spectra and the spectra predicted by DFT calculations at the B3LYP/6–311 + +g(d,p) level. Agreement in the H-stretching region is not perfect, but the characteristic IM and CS spectral patterns are evident.