Comment

Galactose oxidase is a type-II Cu protein that catalyses the oxidation of primary alcohols to aldehydes with concomitant reduction of molecular oxygen (Whittaker, 1994). Its crystal structure (Ito et al., 1994) reveals a unique mononuclear Cu site with two N donors (from histidine imidazole groups), two O donors (one axial and one equatorial tyrosine group), and an exogenous water or acetate molecule, all arranged in a distorted square-pyramidal coordination. Several different theories have been proposed to explain how galactose oxidase, which contains a single Cu atom, can catalyse a two-electron redox reaction. The currently accepted theory (Whittaker & Whittaker, 2001) suggests that the `inactive' form of galactose oxidase is oxidized by the loss of one electron to produce the `active' form, which contains a tyrosine (tyrosine 272) free radical ion coupled to the Cu^II ion. The active form is then reduced to the Cu^I species and the alcohol oxidized to the corresponding aldehyde.

There has been considerable interest in the study of model compounds of galactose oxidase in recent years (Kruse et al., 2002; Shimazaki et al., 2002; Thomas et al., 2002). One group of compounds that have attracted considerable interest consists of five-coordinate Cu complexes with tridentate Schiff base ligands derived from the condensation of amino acids with substituted salicylaldehydes. In this type of complex, the Cu coordination sphere also contains a monodentate Lewis base. With two exceptions (Plesch et al., 1997; Sivy et al., 1994), X-ray crystallographic studies have shown that these Cu^II compounds contain Cu^II in a distorted square-pyramidal environment and fit into three main types:

(i) monomeric with a water molecule occupying the fifth coordination site (Dawes et al., 1982; Fujimaki et al., 1971; Garcia-Raso et al., 1996; Korhonen & Hamalainen, 1979; Ueki et al., 1969; Warda et al., 1996; Warda, 1997g; Warda, 1998a,d,e,f);

(ii) dimeric with an adjacent phenolic O atom occupying the fifth coordination site (Davies, 1984; Hamalainen et al., 1978; Hill & Warda, 1999; Warda, 1997e; Warda, 1998b,c,g; Warda, 1999; Warda et al., 1998);

(iii) polymeric with the fifth coordination site occupied by an adjacent carboxyl O atom (Ueki et al., 1967; Kettman et al., 1993; Korhonen et al., 1984; Plesch et al., 1998; Warda et al., 1997; Warda, 1997a,b,c,d,f; Sivy et al., 1990).

The tridentate Schiff base ligand derived from the condensation of salicylaldehyde and DL-tyrosine, in the presence of pyridine, forms a square pyramidal five-coordinate Cu complex of type (i) with a water molecule occupying the apical site at a distance of 2.375 (2) Å. Cu is displaced by 0.1861 (8) Å from the basal plane formed by O1, O2, N, and N1P. The Cu-O1, Cu-O2, and Cu-N1 bond distances in the equatorial plane [1.919 (1), 1.989 (1), and 1.939 (2) Å, respectively] do not differ significantly from those of similar dinuclear compounds mentioned above. Both the water molecule and the tyrosine phenolic group participate in strong hydrogen bonding interactions with the C=O groups of adjoining molecules, while the water molecule also forms a hydrogen bond to O1 from an adjoining molecule (see Table 2 and Fig. 2).

Figure 1 View of the title complex, showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 20% probability level. H atoms are represented by circles of arbitrary size.

Figure 2 The molecular packing viewed along the a axis.

Experimental

The title complex was synthesized in two stages. In the first stage, 10 g of DL-tyrosine and an equimolar amount of sodium hydroxide were dissolved in 300 ml of hot water. To this solution was added an equimolar quantity of copper sulfate pentahydrate dissolved in 100 ml of water. The blue-purple compound [Cu(tyr)₂]·nH₂O precipitated on cooling the solution. 6 g of this compound, two mole equivalents of salicylaldehyde, triethylamine (10 ml) and pyridine (10 ml) were refluxed in methanol for one hour. The hot solution was filtered and allowed to stand until the dark-green product precipitated from solution. X-ray quality crystals were grown by slow evaporation from a methanol/acetonitrile solution.

Crystal data

[Cu(C16H13NO4)(C5H5N)(H2O)] Mr = 443.93 Triclinic,  a = 10.0319 (13) Å b = 10.0487 (11) Å c = 10.1973 (14) Å = 81.973 (10)° = 72.624 (9)° = 78.310 (9)° V = 957.3 (2) Å3 Z = 2 Dx = 1.540 Mg m-3 Mo K radiation Cell parameters from 42 reflections = 4.8-17.3° = 1.18 mm-1 T = 293 (2) K Plate, blue 0.58 × 0.46 × 0.15 mm

Data collection

Siemens P4S diffractometer 2/ scans Absorption correction: by integration (Wuensch & Prewett, 1965) Tmin = 0.688, Tmax = 0.883 4594 measured reflections 4347 independent reflections 3782 reflections with I > 2(I) Rint = 0.021 max = 27.5° h = 0 12 k = -12 13 l = -12 13 3 standard reflections every 97 reflections intensity decay: 0.1%

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.031 wR(F2) = 0.082 S = 1.06 4347 reflections 262 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0343P)2 + 0.4004P] where P = (Fo2 + 2Fc2)/3 (/)max = 0.001 max = 0.35 e Å-3 min = -0.27 e Å-3

Table 1 Selected geometric parameters (Å, °) Cu-O1 1.9187 (14) Cu-N1 1.9394 (15) Cu-O2 1.9889 (14) Cu-N1P 2.0086 (16) Cu-O1W 2.3745 (15) O1-Cu-N1 93.61 (6) O1-Cu-O2 164.17 (6) N1-Cu-O2 83.13 (6) O1-Cu-N1P 90.30 (6) N1-Cu-N1P 172.18 (7) O2-Cu-N1P 91.28 (6) O1-Cu-O1W 99.82 (6) N1-Cu-O1W 95.24 (6) O2-Cu-O1W 95.91 (6) N1P-Cu-O1W 90.76 (6)

Table 2 Hydrogen-bonding geometry (Å, °) D-HA D-H HA DA D-HA O1W-H1W1O1i 0.87 1.92 2.793 (2) 175 O1W-H1W2O3ii 0.86 2.07 2.930 (2) 176 O4-H4O3iii 0.82 1.89 2.705 (2) 175 Symmetry codes: (i) -x,-y,-z; (ii) 1-x,-y,-z; (iii) x,y,z-1.

All H atoms were included in calculated positions, with C-H distances ranging from 0.93 to 0.98 Å. The H atoms were then included in the refinement in riding-motion approximation, with U_iso = 1.2U_eq of the carrier atom. The H atoms of the coordinated water molecule were located in a difference Fourier map and were refined with restrained O-H distances but an unrestrained H-O-H angle.

Data collection: XSCANS (Siemens, 1994); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.