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Density Functional Theory Study for Aqueous Bis(L-histidinato)copper(II) Systems: Validation of the B3LYP vs. M06 Results

This study validates the density functional theory calculations obtained previously [1] for aqueous bis(L-histidinato)copper(II) systems using the B3LYP density functional [2, 3]. The validation is aimed at answering if B3LYP can continue to be a standard tool in the study of aqueous copper(II) amino acidato systems, or if it should be substituted by a density functional that accounts better for noncovalent dispersion interactions. Bis(L-histidinato)copper(II) is known as the predominant copper(II)–amino acid complex in human blood serum. L-histidine can coordinate to copper(II) in three ways: glycine-like (G), histamine-like (H), and imidazole–propionic acid-like (I) modes. Since numerous experimental studies of this physiological complex reported several modes to coexist in aqueous solutions, but without providing complete structures, we investigated the relative stability of all possible coordination modes and conformers of isolated bis(L-histidinato)copper(II), and several conformers surrounded with up to 22 water molecules via B3LYP [1]. The conformers studied with explicit water molecules were: trans-GG (the most stable in vacuo [1]), cis-HG (from the X-ray crystal and molecular structure [4]), cis-HH and trans-HH (the HH mode, with four nitrogen atoms placed symmetrically at 1.96 Å and two oxygen atoms axially at 2.28 Å to copper(II), was suggested to be prevailing in aqueous solution at physiological conditions by the extended X-ray absorption fine structure measurements [5]). The B3LYP structural, energetic, and Raman spectra results supported and supplemented the experimental findings, which showed that the HH and HG modes were predominant in aqueous solutions under physiological conditions, with the information on the cis-configuration and complete geometry [1]. The B3LYP results were the basis of our conclusions that the intermolecular interactions greatly influence and stabilize the coordination mode, conformation, and apical position of oxygen atoms, and that local surroundings may cause the variety of modes in aqueous solution by affecting interconversions between them. We validate these conclusions with the results obtained for the same systems using the M06 density functional [6]. References: [1] M. Marković, M. Ramek, J. Sabolović, Eur. J. Inorg. Chem. (2014) 198. [2] A. D. Becke, J. Chem. Phys. 98 (1993) 5648. [2] P. J. Stephens, F. J. Devlin, C. F. Chabalowski, M. J. Frisch, J. Phys. Chem. 98 (1994) 11623. [4] P. Deschamps, P. P. Kulkarni, B. Sarkar, Inorg. Chem. 43 (2004) 3338. [5] Y. Pan, L. Y. Zhang, Y. Z. Liu, Chin. J. Chem. Phys. 24 (2011) 451. [6] Y. Zhao, D. G. Truhlar, Theor. Chem. Acc. 120 (2008) 215.

DFT; B3LYP; M06; density functional; copper; amino acid; L-histidine; Raman

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