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Why Does the Coordination Mode of Physiological Bis( L ‐histidinato)copper(II) Differ in the Gas Phase, Crystal Lattice, and Aqueous Solutions? A Quantum Chemical Study
Author(s) -
Marković Marijana,
Ramek Michael,
Sabolović Jasmina
Publication year - 2014
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201300746
Subject(s) - chemistry , conformational isomerism , copper , crystallography , intramolecular force , crystal structure , molecule , imidazole , aqueous solution , stereochemistry , organic chemistry
In bis( L ‐histidinato)copper(II), the amino acid L ‐histidine can bind to copper(II) in glycine‐like (G), histamine‐like (H), and imidazole–propionic acid like (I) coordination modes. This complex is known as the predominant copper(II)–amino acid complex in human blood serum. Numerous experimental studies of this physiological complex reported several coordination modes to coexist in aqueous solutions, but without providing complete structures. This paper is the first to investigate the relative stability of all possible copper(II) coordination modes and conformations of isolated bis( L ‐histidinato)copper(II), and several conformers surrounded with up to 22 water molecules by DFT/B3LYP calculations. The vibration wavenumbers of four bis( L ‐histidinato)copper(II) · 20H 2 O structures were calculated and assigned for IR and Raman spectra. Among 83 isolated conformers obtained, 37 are in trans configuration, 45 in cis configuration, and one exhibits a trigonal‐bipyramidal structure. The most stable isolated conformer has a trans ‐GG coordination. A comparison between the known X‐ray crystal and B3LYP vacuum molecular structures of bis( L ‐histidinato)copper(II) dihydrate showed that the X‐ray cis ‐HG mode with an intramolecular apical Cu–O carboxylato bond is unstable under vacuum and thus is greatly affected by crystal‐lattice effects. In the systems with 20 water molecules, the lowest energy was estimated for the conformer with a cis ‐HH coordination and two axial Cu–O carboxylato bonds. This structural finding complements previous experimental studies, which reported an HH coordination mode as the prevailing in aqueous solutions under physiological conditions. The axial Cu–O carboxylato bond, unformed in any of the 83 isolated conformers, is stabilized by intermolecular interactions. The arrangement of water molecules around the complex might affect the coordination mode formation and stability.