Structures, Unimolecular Fragmentations, and Reactivities of the Self-Assembled Multimetallic/Peptide Complexes [Mnn (GlyGly-H)2n-1 ](+) and [Mnn+1 (GlyGly-H)2n ](2.).

Abstract

Complexes of Mn(2+) with deprotonated GlyGly are investigated by sustained off-resonance irradiation collision-induced dissociation (SORI-CID), infrared multiple-photon dissociation spectroscopy, ion-molecule reactions, and computational methods. Singly [Mnn (GlyGly-H)2n-1 ](+) and doubly [Mnn+1 (GlyGly-H)2n ](2+) charged clusters are formed from aqueous solutions of MnCl2 and GlyGly by electrospray ionization. The most intense ion produced was the singly charged [M2 (GlyGly-H)3 ](+) cluster. Singly charged clusters show extensive fragmentations of small neutral molecules such as water and carbon dioxide as well as dissociation pathways related to the loss of NH2 CHCO and GlyGly. For the doubly charged clusters, however, loss of GlyGly is observed as the main dissociation pathway. Structure elucidation of [Mn3 (GlyGly-H)4 ](2+) clusters has also been done by IRMPD spectroscopy as well as DFT calculations. It is shown that the lowest energy structure of the [Mn3 (GlyGly-H)4 ](2+) cluster is deprotonated at all carboxylic acid groups and metal ions are coordinated with carbonyl oxygen atoms, and that all amine nitrogen atoms are hydrogen bonded to the amide hydrogen. A comparison of the calculated high-spin (sextet) and low-spin (quartet) state structures of [Mn3 (GlyGly-H)4 ](2+) is provided. IRMPD spectroscopic results are in agreement with the lowest energy high-spin structure computed. Also, the gas-phase reactivity of these complexes towards neutral CO and water was investigated. The parent complexes did not add any water or CO, presumably due to saturation at the metal cation. However, once some of the ligand was removed via CO2 laser IRMPD, water was seen to add to the complex. These results are consistent with high-spin Mn(2+) complexes.