Characterization of protonated AcAlaDab and AcDabAla by IRMPD spectroscopy and molecular modeling

Abstract

Alanine (Ala) based oligopeptides containing a residue of 2,4-diaminobutyric acid (Dab) serve as a unique model to examine the effect of the side-chain ionization on the conformational changes of the peptide backbone structure. In this work, two isomeric dipeptide ions, AcAlaDabH+ and AcDabAlaH+, are studied by infrared multiple photon dissociation (IRMPD) spectroscopy and quantum chemical modeling. The IRMPD spectra were recorded in a Fourier transform-ion cyclotron resonance mass spectrometer coupled to an infrared free electron laser. For each peptide ion, seven theoretical infrared (IR) spectra were obtained from a set of unique low-energy conformations calculated at the ωB97X-D/6-311 + G(d,p) level of theory. A manually weighted IR spectrum containing two conformations gives the best prediction for each IRMPD spectrum. An interesting spectral feature is that the frequency of the amide I band of AcAlaDabH+ is about 30 cm−1 lower compared to that of AcDabAlaH+. This redshift of the amide I band implies a stronger hydrogen-bonding interaction between the charged NH3+ group on the side-chain of Dab and the CO group of the backbone in the former ion. The stronger hydrogen-bonding interaction is likely due to a combination of a shorter H⋯O distance and a more favorable alignment of the N–H–O moiety in a unique conformation adopted by AcAlaDabH+, which results in an increased stability of the ion. The results explain an earlier observation that AcAlaDab has a higher proton affinity than AcDabAla.