Effect of phenol and acidic side chains on the protonation sites of b2 ions confirmed by IRMPD spectroscopy

Infrared multiple photon dissociation (IRMPD) spectroscopy was used to identify the chemical structures of b2 fragments formed by collision induced dissociation (CID) from the protonated tripeptides YGG, GYG, EGG and GEG. The IR spectra in the hydrogen stretching region (2670–3800 cm−1) were obtained by photodissociating the mass-selected b2 ions with a tunable benchtop optical parametric oscillator (OPO). On weaker bands, the photodissociation yield was boosted by simultaneously irradiating with a fixed wavelength CO2 laser. A comparison of the OH and NH stretching vibrations of these b2 ions to cyclo(GY)H+ and cyclo(GE)H+ reference compounds confirms that “head-to-tail” diketopiperazine structures are not formed. While the comparison of the experimental results to computed linear absorption spectra is more challenging in these systems, the trends in the NH stretching modes still allow a distinction between oxazolone structures protonated at the N-terminal N or the oxazolone ring N. It is shown that proton attachment at the oxazolone ring N is often preferred, but that an aromatic or acidic side chain can stabilize protonation at the N-terminus.

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► IR spectra were recorded in the hydrogen stretching region for the b2 ions of YGG, GYG, EGG and GEG.
► In all cases, exclusive presence of oxazolone structures is confirmed.
► Proton attachment at the N-terminus or oxazolone ring N depends on the sequence order.
► The aromatic or acidic side chain can stabilize protonation at the N-terminus.