Conformations and vibrational spectroscopy of metal-ion/polylalanine complexes

The thermochemistry and structures of complexes of dialanine and trialanine with a series of singly and doubly charged metal ions have been examined by spectroscopic and computational approaches. Complexes with Li+, K+, Cs+, Ca2+, Sr2+ and Ba2+ were formed by electrospray ionization, and studied by infrared multiple photon dissociation spectroscopy (IRMPD) in a Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer using irradiation in the 1000–1900 cm−1 infrared (IR) region from the FELIX free electron laser. Spectra were correlated with computed thermochemistry and simulated IR spectra from density functional (DFT) calculations of likely candidate conformations to make conformational assignments and characterize the variation of characteristic normal mode frequencies as a function of metal-ion identity. All trialanine complexes were found to have the charge-solvated (CS) form of the ligand with all three carbonyl oxygens chelating the metal. Dialanine was more varied: the Ba complex had largely the salt-bridge (SB) form, the Sr and Ca complexes showed a mixture of CS and SB, the Cs complex was CS with (OO) chelation, and the K, Na and Li complexes were CS in an undetermined mixture of (OO) and (NOO) chelation. Binding of the metal ion to the carboxyl carbonyl and to the amide carbonyl(s) (Amide I band) gave a red shift of the CO stretches, and a blue shift to the Amide II band. The frequency shifts were greater for more strongly interacting metal ions (smaller size, larger charge), and were found to be linearly proportional to the metal–ligand binding energy.

Graphical abstractMetal-ion complexes of dialanine and dialanine are reported for alkali and alkaline earth ions. Metal/ligand interaction effects on conformations and normal mode frequencies are determined using IRMPD spectroscopy combined with computation.Figure optionsDownload full-size imageDownload high-quality image (152 K)Download as PowerPoint slideResearch highlights▶ IRMPD spectra show conformations of AlaAla and AlaAlaAla metal ion complexes. ▶ Mixed charge solvated and zwitterion populations for dialanine with Sr2+ and Ca2+. ▶ Dominant zwitterion population for dialanine with Ba2+. ▶ Charge solvated conformations for alkali ion complexes. ▶ Vibrational mode frequencies correlate linearly with metal ion binding strength.