High Susceptibility of Histidine to Charge Solvation Revealed by Cold Ion Spectroscopy

Abstract

Histidine remained the last aromatic amino acid for which the intrinsic spectroscopic properties and structures were obscure. We measured the UV and IR spectra of protonated histidine, isolated in the gas phase, using photofragmentation cold ion spectroscopy. Unexpectedly, the UV absorption appears strongly redshifted relative to that of the cation in aqueous solutions. In investigating this phenomenon, we solved the geometries of all abundant conformers using IR conformer-selective spectroscopy and ab initio quantum chemical calculations. In all of the structures, the proton resides on the imidazole ring. The measured UV spectra of protonated methylimidazole, histamine and histidine, together with calculations of the electronic spectra for the latter, suggest that, in comparison with other aromatic amino acids, such location of proton makes UV absorption of histidine highly sensitive to the local environment of its side chain.

Binding Motifs in the Naked Complexes of Target Amino Acids with an Excerpt of Antitumor Active Biomolecule: An Ion Vibrational Spectroscopy Assay

“…The IRMPD spectrum of protonated His has been reported both in the 600–1800 cm −1 fingerprint range [50] and in the 3200–3700 cm −1 range [51] and ascribed to imidazole protonated species, which is the major contribution due to a conformer stabilized by an imidazole NH⋅⋅⋅NH 2 ⋅⋅⋅O=C hydrogen‐bond network [50] . Also, conformer‐selective IR spectra of protonated His, by using cold‐ion spectroscopy, reveal structures in which the proton resides on the imidazole ring and is engaged in hydrogen bonding with the amino group [52] . In contrast, if protonated His is complexed with [18]crown‐6 ether (18C6), IRMPD spectroscopy and theoretical calculations indicate that 18C6 binding shifts the competition towards amino group protonation [53, 54] .…”

“…[50] Also, conformer-selective IR spectra of protonated His, by using cold-ions pectroscopy,r eveal structures in which the proton resides on the imidazole ring and is engagedi nh ydrogen bonding with the amino group. [52] In contrast, if protonated His is complexed with [18]crown-6 ether (18C6), IRMPD spectroscopy and theoretical calculations indicate that 18C6 binding shifts the competition towards amino group protona-tion. [53,54] This outcomei ss uggested to be due to the presence of three, strong, and nearly equivalent NH + •••O interactions occurring in the complexed a-aminop rotonated species.…”

Cold ion spectroscopy for structural identifications of biomolecules

“…The internal temperature of the ions in ESI, typically, does not exceed T=500K. 15 Taking this value and assuming that for a reliable detection the spectroscopic transitions due to minor conformers should be, at least, of 10% intensity (ni) relative to the similar peaks of the lowest energy conformer (n0), one can evaluate the maximum relative potential energy of the conformers that can be detected as: 16 kcal/mol.…”

“…Figure 1.9 compares the gas-phase UV absorptions of the four protonated aromatic amino acids, produced by electrospray ionization from their equimolar solution mixture and measured by photodissociation spectroscopy. 16 Figure 1.9. Photofragmentation spectra of four protonated aromatic amino acids recorded by integrating all major charged fragments, which were detected by an Orbitrap mass analyzer under similar settings of the cold ion spectrometer and DUV/UV OPO.…”

“…Reproduced with permission from ref. 16 Among these aromatics, tryptophan is by far the strongest UV absorber in the first absorption band, followed by Tyr, His and Phe. Despite this arrangement of absorption intensities, most of the published works on IR cold-ion spectroscopy of biomolecules was done with Phe and, less often, with Tyr as a chromophore in peptides.…”