Establishing the Binding Affinity of Organic Carboxylates to 15-Metallacrown-5 Complexes

Abstract

The reaction of L-phenylalanine hydroxamic acid (H(2)L-pheHA) with copper(II) and lanthanide(III) salts yields 15-Metallacrown-5 structures of the general composition Ln(X)n[Cu(II)(L-pheHA)](5)((3-n)+) where X can represent a wide variety of anions. With five copper ions and one central lanthanide ion, the Ln[15-MC-5] complexes have multiple positions where these anionic guests may bind to the metallacrown host. In addition, these metallacrowns are amphiphilic, containing one face that is primarily hydrophobic (due to the five benzyl side chains which are oriented upon the same face of the molecule) and a face that is hydrophilic which has no impediment to solvent access. While it has been known that aromatic carboxylates bind preferentially to the hydrophobic face and short chain aliphatic carboxylates bind preferentially to the hydrophilic face, there have been no quantitative assessments of the stability of these host-guest complexes. Using Isothermal Calorimetry (ITC) we have determined the binding constants for several carboxylate anions to a variety of metallacrown complexes. The affinities of anions that coordinate to the lanthanide ion on the hydrophobic face are related to the hydrophobicities of the guests, with higher binding strength observed for the more hydrophobic carboxylates. Central metal such as La(III) or Nd(III) which are nine coordinate are able to accommodate two guests on the hydrophobic side; however, central metals such as Gd(III) or Dy(III) which are eight coordinate are limited to encapsulating one guest into the hydrophobic pocket. A second guest, bound to the hydrophilic face is often observed with these 8-coordinate lanthanides. The significantly weaker second binding constant between benzoate and Gd(III)[15-MC-5] supports the model that the second benzoate binds to the central metal through the hydrophilic side. Unlike the Gd(III)[15-MC-5], the higher binding constant of the second benzoate with La(III)[15-MC-5] is consistent with the crystallographic model which shows that the second guest binds to the hydrophobic side.

Facile One‐Pot Route toward Water‐Soluble Lanthanide–Copper–Glycinehydrox­imate 15‐Metallacrown‐5 Complexes

“…[3] A large amount of systematic structural studies of Ln[15MC CuRha -5] complexes derived from α-aminohydroxamic acids with various organic anions have been performed, [4][5][6][7][8][9][10][11][12][13][14] and the affinity of the Ln III ion Scheme 1. It has been demonstrated that the anion affinity for the metallacrown is dependent on the R-substituted hydroximate ligands (Rha).…”

The peculiar behavior of Picha in the formation of metallacrown complexes with Cu(ii), Ni(ii) and Zn(ii) in aqueous solution

“…These metals often possess a series of axial coordi-nation sites available for interactions with solvent molecules or co-ligands. [10][11][12] For these reasons MCs have been studied as recognition agents for anions and cations, [13][14][15][16][17][18][19][20] as single molecule magnets, [21][22][23] as building blocks for mesoporous solids, 24 and as luminescent materials for the purpose of in vivo imaging. 25,26 Despite a number of structural and functional studies, numerous aspects which regulate the stability of MCs in solution and the correlation between the species isolated in the solid state and those present in the solution phase are far from being fully elucidated.…”

Survey of the year 2009: applications of isothermal titration calorimetry

“…In 2009 we identified more than 400 articles after searching Web of Science, PubMed, SciDir and OVID databases using ‘isothermal AND titration AND calorimetry’ or ITC or ‘Isothermal Titration Calorimetry’ search terms. These have been classified into the following categories: Pre‐2009 references cited in the text and review articles 1–43 Protein‐protein and protein‐peptide interactions 44–124 Protein/peptide‐small molecule interactions 125–218 Protein/peptide‐metal ion interactions 219–253 Protein/peptide‐nucleic acid interactions 254–273 Protein/peptide‐lipid interactions 274–292 Protein/peptide‐polysaccharide interactions 293–316 Nucleic acid‐small molecule interactions 317–348 Small molecule interactions …”