Ultraviolet–circular dichroism spectra for structural analysis of copper(II) complexes with aliphatic and aromatic ligands in aqueous solution

Abstract

An attempt has been made to identify the characteristic positions, and their eventual displacements with changing co-ordination, of the charge-transfer bands involving donor groups located on molecules of biologicaI interest [and N-benzoylglycyl-L-histidyl-L-leucine) such as carboxylate, alcoholate, amine, deprotonated peptide and imidazole. Information about the species formed in solution was obtained by means of pH-metric readings while ultraviolet-circular dichroism spectra were recorded, at fixed pH values, 298 K and I = 0. I mol dm-3, for the proton-ligand and proton~opper(lr)-ligand systems, in order to evaluate a spectrum for each complex formed in solution. Intraligand and charge-transfer bands were assigned for each spectrum with the aim of relating spectral features to the structure of the species formed in solution.Ultraviolet-circular dichroism (UVjCD) spectra have been employed for the determination of the or-helix content of proteins,' of the influence on ligands of metal ions' and for structural definition in micellar aggregates of a series of chiral surfactants. When metal-ligand interactions are investigated the nature of the donor groups involved in the co-ordination sphere of a given metal ion might be detected. Both visible and ultravioletxircular dichroism spectra can provide very useful structural information for metal complexes in solution. The results obtained by means of the two techniques are complementary; nevertheless from UVjCD spectra it is also possible to study compiex formation (i) with colourless metal ions,4 (ii) with metal ions having low molar absorption in the visible, such as octahedral complexes of nicke1(11), (iii) between chiral anionic ligands (such as anions of carboxylic acids) and cationic ligands (such as protonated amines), and (io) in real samples of biological or environmental interest (by using suitable techniques for chemical separation and for data elaboration).The UVjCD spectra obtained from several systems containing copper(r1) and simple model ligands should allow us to propose a correlation between spectral features and complex structures, as the spectrum ofeach species in solution (including the free and protonated forms of the pro-ligand) can be calculated from experimental UVjCD data recorded on soiutions of different known compositions and from known values of formation constants. It is necessary to distinguish between intraligand (i.1.) bands, which can be modified, in intensity, sign and position, as a consequence of co-ordination and charge-transfer (c.t.) bands, which should be attributed to the donor groups.In the literature there are many articIes in which UVjCD spectra are used for structural definitions in solution but the attributions are directly based on experimental spectra, without taking into account either the superposition of the different metal complexes at a given pH (often only the structures of the predominant species in solution are discussed) or the contribution of the differently protonated forms of the proligand and, moreover, wi...

CuII Ion Coordination to an Unprotected Pentadecapeptide Containing Two His Residues: Competition Between the Terminal Amino and the Side‐Chain Imidazole Nitrogen Donors

“…[16] The two bands at 250Ϫ256 and 350 nm can be ascribed to a charge-transfer transition from imidazole π 2 and π 1 to copper. [17] The positive band at 316Ϫ320 nm is most likely due a charge-transfer transition from a deprotonated amido nitrogen to copper: [17] this band is clearly detected from pH 6.0 on.…”

“…CD spectra at pH 5 clearly show a positive band at 256 nm attributable to a π 2 charge-transfer transition from imidazole to copper. [17] The dϪd band in the electronic spectra has its maximum at 685 nm (at pH 5), exactly the value expected for a CuϪ(N Im ) 2 complex. [20] Subtracting logK for the terminal amino group (7.3, Table 1) from logβ 115 gives 47.9, a value only slightly higher than that reported for the corresponding complex with the protected analogue (47.1).…”

Engineering metal ion coordination to regulate amyloid fibril assembly and toxicity

“…In these cases, Cu 2ϩ can promote peptide nitrogen ionization at pH values as low as 5-6, even though the pK a is 12-13 in solution. The value of ⌬/n for the charge transfer between N Ϫ and Cu 2ϩ at 300 nm is fairly constant, in the range of 0.2-0.3, where n is the number of negatively charged peptide nitrogens involved in coordination (28). Therefore, ⌬ (300 nm) Ϸ 0.23 (Fig.…”

“…The four bands present in the CD spectrum (Fig. 1B), are assigned as a 264-nm transition for a strong NH 2 and/or 2 imidazole-to-Cu 2ϩ charge transfer, 300 nm as charge transfer from N Ϫ to Cu 2ϩ , a weak 340-nm transition as charge transfer from 1 imidazole to Cu 2ϩ , and a 596-nm Cotton effect for a Cu 2ϩ d-d transition (28). Virtually identical Cu 2ϩ ITC, UV, and CD data were obtained for wild-type A␤(13-21) (data not shown), suggesting that Cu 2ϩ adopts the same coordination with both peptides.…”

Structural characterization of copper(II) binding to α-synuclein: Insights into the bioinorganic chemistry of Parkinson's disease

“…The latter can be assigned to a charge transfer transition (c.t.) between the metal center and an imidazole group ( 1 N im ϪCu c.t., 280-345 nm) or a deprotonated peptide nitrogen ( Ϫ NϪCu c.t., 295-315 nm) (44). The CD band at 300 nm saturated with one equivalent of added Cu(II) (Fig.…”