Journal of Physical Chemistry A, Vol.117, No.12, 2508-2516, 2013
IR Spectroscopy of b(4) Fragment Ions of Protonated Pentapeptides in the X-H (X = C, N, O) Region
The structure of peptide fragments was studied using "action" IR spectroscopy. We report on room temperature IR spectra of b(4) 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 b(4) 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 b(4) 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 b(4) 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 b(4) 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.