Abstract
High resolution (k = 18 Å(-1) or k = 17 Å(-1)) copper K-edge EXAFS and MXAN (Minuit X-ray Absorption Near Edge) analyses have been used to investigate the structure of dissolved [Cu(aq)](2+) in 1,3-propanediol (1,3-P) or 1,5-pentanediol (1,5-P) aqueous frozen glasses. EXAFS analysis invariably found a single axially asymmetric 6-coordinate (CN6) site, with 4×O(eq) = 1.97 Å, O(ax1) = 2.22 Å, and O(ax2) = 2.34 Å, plus a second-shell of 4×O(water) = 3.6 Å. However, MXAN analysis revealed that [Cu(aq)](2+) occupies both square pyramidal (CN5) and axially asymmetric CN6 structures. The square pyramid included 4×H(2)O = 1.95 Å and 1×H(2)O = 2.23 Å. The CN6 sites included either a capped, near perfect, square pyramid with 5×H(2)O = 1.94 ± 0.04 Å and H(2)O(ax) = 2.22 Å (in 1,3-P) or a split axial configuration with 4×H(2)O = 1.94, H(2)O(ax1) = 2.14 Å, and H(2)O(ax2) = 2.28 Å (in 1,5-P). The CN6 sites also included an 8-H(2)O second-shell near 3.7 Å, which was undetectable about the strictly pyramidal sites. Equatorial angles averaging 94° ± 5° indicated significant departures from tetragonal planarity. MXAN assessment of the solution structure of [Cu(aq)](2+) in 1,5-P prior to freezing revealed the same structures as previously found in aqueous 1M HClO(4), which have become axially compressed in the frozen glasses. [Cu(aq)](2+) in liquid and frozen solutions is dominated by a 5-coordinate square pyramid, but with split axial CN6 appearing in the frozen glasses. Among these phases, the Cu-O axial distances vary across 1 Å, and the equatorial angles depart significantly from the square plane. Although all these structures remove the d(x(2)-y(2)) , d(z(2)) degeneracy, no structure can be described as a Jahn-Teller (JT) axially elongated octahedron. The JT-octahedral description for dissolved [Cu(aq)](2+) should thus be abandoned in favor of square pyramidal [Cu(H(2)O)(5)](2+). The revised ligand environments have bearing on questions of the Cu(i)/Cu(ii) self-exchange rate and on the mechanism for ligand exchange with bulk water. The plasticity of dissolved Cu(ii) complex ions falsifies the foundational assumption of the rack-induced bonding theory of blue copper proteins and obviates any need for a thermodynamically implausible protein constraint.