Differential Ion Mobility Spectrometry (DIMS) provides orthogonal separation to mass spectrometry, and DIMS combined with the high sensitivity of a quadrupole ion-trap is shown to be useful for the separation and identification of saccharides. A comprehensive analysis of the separation of anomers (α- and β-methylated glucose) and epimers (α-methylated glucose and mannose) ionized with Li(+), Na(+), and K(+) is performed. DIMS separation is found to be better for saccharides cationized with the two latter species. The corresponding resolving power for the two glucose anomers with Na(+) is found to be very close to the corresponding drift-tube IMS value. The lithiated complexes are investigated further using a combination of infrared spectroscopy integrated to ion-trap mass spectrometry and quantum chemical calculations. Together with DIMS, consistent results are obtained. It is found that two competing structural motifs might be at play, depending on the subtle balance between the maximization of the coordination of the metal cation and the intrinsic conformational energetics of the saccharide, which is for a large part driven by hydrogen bonding. The comparison of simulated and observed spectra clearly shows that a band at ∼3400 cm(-1) is specific to a structural motif found in the lithiated glucose complexes, which could explain the trends observed in the DIMS spectra of the saccharide complexes. It is shown that DIMS-MS/MS using wavelength specific IR activation would provide a new orthogonal dimension to mass spectrometry.

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Infrared spectroscopy of nucleotides in the gas phase 2. The protonated cyclic 39,59-adenosine monophosphate3

The structure of peptide fragments was studied using "action" IR spectroscopy. We report on room temperature IR spectra of b4 fragments of protonated GGGGG, AAAAA, and YGGFL in the X-H (X = C, N, O) stretching region. Experiments were performed with a tandem mass spectrometer combined with a table top tunable laser, and the multiple photon absorption process was assisted using an auxiliary high-power CO2 laser. These experiments provided well-resolved spectra with relatively narrow peaks in the X-H (X = C, N, O) stretching region for the b4 fragments of protonated GGGGG, AAAAA, and YGGFL. The 3200-3700 cm(-1) range of the first two of these spectra are rather similar, and the corresponding peaks can be assigned on the basis of the classical b ion structure that has a linear backbone terminated by the oxazolone ring at the C-terminus and ionizing proton residing on the oxazolone ring nitrogen. The spectrum of the b4 of YGGFL, on the other hand, is different from the two others and is characterized by a band observed near 3238 cm(-1). Similar band positions have recently been reported for one of the four isomers of the b4 of YGGFL studied using double resonance IR/UV technique. As proposed in this study, the IR spectrum of this ion at room temperature can also be assigned to a linear N-terminal amine protonated oxazolone structure. However, an alternative assignment could be proposed because our room temperature IR spectrum of the b4 of YGGFL nicely matches with the predicted IR absorption spectrum of a macrocyclic structure. Because not all experimental IR features are unambiguously assigned on the basis of the available literature structures, further theoretical studies will be required to fully exploit the benefits offered by IR spectroscopy in the X-H (X = C, N, O) stretching region.

No abstract

Using quantum chemical calculations and infrared multiphoton dissociation (IRMPD) spectroscopy in the fingerprint and X-H stretching regions, we demonstrate here that the all-Ala (b6) fragment ion features a macrocyclic structure with C(2) symmetry. For this structure, the ionizing proton is equally shared by the Ala(1) and Ala(4) amide oxygens in a Zundel-type symmetric (X…H(+)…X) H-bond.

Characterization of ε-N-acetylated lysine containing peptides, one of the most prominent post-translational modifications of proteins, is an important goal for tandem mass spectrometry experiments. A systematic study for the fragmentation reactions of b ions derived from ε-N-acetyllysine containing model octapeptides (K Ac YAGFLVG and YAK Ac GFLVG) has been examined in detail. Collision-induced dissociation (CID) mass spectra of b n (n = 4-7) fragments of ε-N-acetylated lysine containing peptides are compared with those of N-terminal acetylated and doubly acetylated (both ε-N and N-terminal) peptides, as well as acetyl-free peptides. Both direct and nondirect fragments are observed for acetyl-free and singly acetylated (ε-N or N-terminal) peptides. In the case of ε-N-acetylated lysine containing peptides, however, specific fragment ions (m/z 309, 456, 569 and 668) are observed in CID mass spectra of b n (n = 4-7) ions. The CID mass spectra of these four ions are shown to be identical to those of selected protonated C-terminal amidated peptides. On this basis, a new type of rearrangement chemistry is proposed to account for the formation of these fragment ions, which are specific for ε-N-acetylated lysine containing peptides. Consistent with the observation of nondirect fragments, it is proposed that the b ions undergo head-to-tail macrocyclization followed by ring opening. The proposed reaction pathway assumes that b n (n = 4-7) of ε-N-acetylated lysine containing peptides has a tendency to place the K Ac residue at the C-terminal position after macrocyclization/reopening mechanism. Then, following the loss of CO, it is proposed that the marker ions are the result of the loss of an acetyllysine imine as a neutral fragment.