Vibrational Signatures of Conformer-Specific Intramolecular Interactions in Protonated Tryptophan

Abstract

Because of both experimental and computational challenges, protonated tryptophan has remained the last aromatic amino acid for which the intrinsic structures of low-energy conformers have not been unambiguously solved. The IR-IR-UV hole-burning spectroscopy technique has been applied to overcome the limitations of the commonly used IR-UV double resonance technique and to measure conformer-specific vibrational spectra of TrpH(+), cooled to T = 10 K. Anharmonic ab initio vibrational spectroscopy simulations unambiguously assign the dominant conformers to the two lowest-energy geometries from benchmark coupled-cluster structure computations. The match between experimental and ab initio spectra provides an unbiased validation of the calculated structures of the two experimentally observed conformers of this benchmark ion. Furthermore, the vibrational spectra provide conformer-specific signatures of the stabilizing interactions, including hydrogen bonding and an intramolecular cation-π interaction.

Initial Steps of Amyloidogenic Peptide Assembly Revealed by Cold‐Ion Spectroscopy

“…An intense and broad absorption within the 3000-3250 cm À1 interval suggests that al arge fraction of NH bonds is involved in strong interactions,which broaden and red-shift the transitions.T his implies acompact structure for the protonated monomer consistent with the results from IM-MS. [8,11] Foro ligomers with n = 3-5 we measured conformer nonspecific IR gain spectra, which reflect vibrational tran- sitions of all abundant conformers of an ion. [24,25] Compared with the spectrum of the monomer (Figure 3a), in the gain spectra of the clusters (Figures 3b-d) the broad intense absorption by strongly coupled NH stretches is largely suppressed, and all NH-stretch absorptions are concentrated now around 3300 cm À1 .T he hydrogen bonds weaken, blueshifting most of the respective peaks.T his suggests as ignificant change in the structure of the monomer units when they form clusters of three or more molecules.T he IR spectra are revealing structural change as the oligomer size increases.The OH stretch (3546 cm À1 )i sp resent in the spectrum of the trimer (Figure 3d)and disappears in the spectra of tetra-and pentamers.W eattribute this difference to the trimers possessing ah igh conformational heterogeneity with respect to the monomer.I nafraction of the spectra of the trimer, some constituent monomers still adopt geometries close to that observed for [M+ +H] + ,c onserving the observed OH stretch. Evidence for this additional high heterogeneity is found in the broadening of the peaks assigned to absorption by the CH-stretch vibrations,w hich become barely distinguishable in the spectrum of the trimer (Figure 3b).…”

“…IR spectroscopy of the protonated monomer and its clusters provides further support to the proposed evolution of structure upon cluster formation. Figure 3a presents aconformer-specific IR spectrum of the [M+ +H] + ion, measured by an IR-UV depletion technique [24,25] with the UV laser tuned to the sharp feature at 37 534 cm À1 in the UV spectrum of the monomer (Figure 2a). Thetwo sharp peaks below 2980 cm À1 and the absorptions in between are typical for CH stretches.…”

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

“…In contrast to TyrH + and PheH + , but similar to TrpH + , the UV spectrum of HisH + in Figure exhibits no vibrationally resolved structure. This prompts us to use, first, IR‐UV gain technique for conformer non‐selective vibrational spectroscopy of HisH + . IR absorption preheats cold ions internally (see Section S3 for details), inducing a broadening and a redshift of their UV absorption (Figure S3).…”

“…In order to assign vibrational transitions in the gain spectrum to individual conformers, we employ the IR‐IR‐UV hole‐burning technique . Figure shows the conformer‐selective spectra of the two revealed conformers, which we name A and B , in the 3 μm spectral region.…”

Desenvolvimento e validação da espectroscopia vibracional de íons em fase gasosa

“…Dispara-se, então, um laser de infravermelho com frequência variada em toda a faixa espectral de interesse. Se um fóton infravermelho for absorvido por ressonância vibracional, este íon terá uma população menor no seu estado fundamental, assim, a transição eletrônica que propiciou a absorção e consequente fragmentação por UV que parte desse nível vibracional fundamental estará menos disponível e a fragmentação induzida pelo laser de UV não será observada enquanto ela ainda estiver excitada vibracionalmente pelo IR 7,52. A espectroscopia utilizando laser de CO 2 serve para casos onde há pouca fragmentação do íon, tal fato pode ter origem na baixa densidade de fótons da fonte de radiação IR, na baixa absortividade ou outros fatores.…”