Conformational analysis of Gly-Ala-NHMe in D(2)O and DMSO solutions: a two-dimensional infrared spectroscopy study

A relevant number of experiments on short peptides has been performed in recent years. One of the major problems rises from the simultaneous presence of slightly different conformers at equilibrium in solution. In the present paper, the conformational characteristics of the Gly-l-Ala-Methyl amide dipeptide in D2O and DMSO solutions are investigated by nonlinear IR spectroscopy. The pump-probe scheme with ultrashort mid-infrared pulses, in the Amide I region, is used to determine the mutual orientation of the two C═O bonds and the dynamics due to solute-solvent interactions. The coupling between Amide I modes is evaluated from both linear and 2D spectra. The interconversion between the different conformations occurs on time scales longer than the vibrational lifetime, and the spectral diffusion observed in 2D spectra is attributed to the solvent dynamics. Quantum mechanical calculations and molecular dynamics simulations are performed to identify the most stable geometries. By comparing the experimental and the theoretical data, we establish the prevalence of β-like polar conformers in both water and DMSO solvents.

Differentiation of Isomers by Wavelength-Tunable Infrared Multiple-Photon Dissociation-Mass Spectrometry: Application to Glucose-Containing Disaccharides

Variation in the wavelength of irradiation in infrared multiple-photon dissociation (IR-MPD) of lithium-tagged glucose-containing disaccharide ions (1-2-, 1-3-, 1-4-, and 1-6-linked isomers of both anomeric configurations) resulted in marked differences in their mass spectral fragmentation patterns. Two-dimensional plots of the fragment yield versus infrared wavelength for each mass spectral product ion were unique for each isomer and can be considered a spectral fingerprint. Individual product ions or diagnostic ratios of key product ions can be optimized at specific IR wavelengths. The technique permits both linkage position and anomeric configuration to be assigned. The ratio of the fragments derived by cleavage at the glycosidic bond (m/z 169/187) is much enhanced for beta-anomers compared to alpha-anomers. Differences in the diagnostic product ions 169 and 187 were largest in the range of 9.0-9.4 microm, where the maximum dissociation yield was observed. Conversely, at 10.6 microm, the wavelength of nontunable CO2 lasers that accompany commercial Fourier transform ion cyclotron resonance mass spectrometers, the dissociation yield was poor and anomeric differentiation was not possible. In contrast to previous studies by collision-induced dissociation, the trends in dissociation behavior between anomers using IR-MPD are significant and allow simple diagnostic rules to be established. By depositing energy into these isobaric ions via narrow-band IR excitation, the resulting internal energy can be finely controlled, thereby obtaining high reproducibility in dissociation patterns. Given the multidimensionality of variable-wavelength IR-MPD of lithiated disaccharides, it is expected that this approach can overcome some of the current limitations in isomer differentiation.

Signatures of β-sheet secondary structures in linear and two-dimensional infrared spectroscopy

Using idealized models for parallel and antiparallel beta sheets, we calculate the linear and two-dimensional infrared spectra of the amide I vibration as a function of size and secondary structure. The model assumes transition-dipole coupling between the amide I oscillators in the sheet and accounts for the anharmonic nature of these oscillators. Using analytical and numerical methods, we show that the nature of the one-quantum vibrational eigenstates, which govern the linear spectrum, is, to a large extent, determined by the symmetry of the system and the relative magnitude of interstrand interactions. We also find that the eigenstates, in particular their trends with system size, depend sensitively on the secondary structure of the sheet. While in practice these differences may be difficult to distinguish in congested linear spectra, we demonstrate that they give rise to promising markers for secondary structure in the two-dimensional spectra. In particular, distinct differences occur between the spectra of parallel and antiparallel beta sheets and between beta hairpins and extended beta sheets.