Ricerc@Sapienza

The binding motifs in the halide adducts with tyrosine ([Tyr + X]-, X = Cl, Br, I) have been investigated
and compared with the analogues with 3-nitrotyrosine (nitroTyr), a biomarker of protein nitration, in a
solvent-free environment by mass-selected infrared multiple photon dissociation (IRMPD) spectroscopy
over two IR frequency ranges, namely 950–1950 and 2800–3700 cm-1. Extensive quantum chemical
calculations at B3LYP, B3LYP-D3 and MP2 levels of theory have been performed using the 6-311++G(d,p)
basis set to determine the geometry, relative energy and vibrational properties of likely isomers and
interpret the measured spectra. A diagnostic carbonyl stretching band at B1720 cm-1 from the intact
carboxylic group characterizes the IRMPD spectra of both [Tyr + X]- and [nitroTyr + X]-, revealing that
the canonical isomers (maintaining intact amino and carboxylic functions) are the prevalent structures.
The spectroscopic evidence reveals the presence of multiple non-covalent forms. The halide complexes
of tyrosine conform to a mixture of plane and phenol isomers. The contribution of phenol-bound
isomers is sensitive to anion size, increasing from chloride to iodide, consistent with the decreasing
basicity of the halide, with relative amounts depending on the relative energies of the respective
structures. The stability of the most favorable phenol isomer with respect to the reference plane
geometry is in fact 1.3, -2.1, -6.8 kJ mol-1, for X = Cl, Br, I, respectively. The change in p-acidity by ring
nitration also stabilizes anion–p interactions yielding ring isomers for [nitroTyr + X]-, where the anion is
placed above the face of the aromatic ring.