Metalloflavoprotein Models

Abstract

Abstract An orange crystalline copper(II) perchlorate complex of riboflavin, a possible model for enzymic metal-flavin interactions, has been prepared and characterized by x-ray techniques. The crystals are triclinic, with symmetry P1; unit cell constants are a = 7.701 (2) A (figures in parentheses following numerical values are standard deviations in the least significant digits cited), b = 11.230 (3) A, c = 17.336 (4) A, α = 101.85 (2)°, β = 96.41 (2)°, γ = 89.90 (2)°, Z = 1, ρobs = 1.70 g per cm3, ρcalc = 1.701 g per cm3 for 2 Cu-(CIO4)2·2C17H20N4O6·12H2O per unit cell. The crystal structure, refined by use of 3636 counter-measured reflections, gives an R factor of 7.9%. There are 2 independent riboflavin molecules and 2 independent Cu(II) ions in each cell. One riboflavin (RFa) is bound to both copper ions, to Cu(1) via O(4) and N(5) at distances of 2.00 (2) and 2.43 (2) A, and to Cu(2) via O(5'), O(4'), and O(2) with distances of 2.30 (2), 1.97 (2), and 2.49 (2) A. The other riboflavin (RFb) is bound to only one copper, Cu(1), via O(4) and N(5) with lengths 2.00 (2) and 2.39 (2) A. A water, H2O(3) is hydrogen-bonded to O(2) of RFb. These results confirm the existence of two separate chelate sites, N(1)-O(2) and O(4)-N(5), in the flavoquinoid system and offer further evidence that both these chelate sites will nearly always be occupied by positive ions or dipoles. Supporting an earlier prediction, both ribityl groups are in an approximate C(2')-C(5') extended planar conformation with O(2') pointing over the secondary chelate site in RFb. However, the copper in the secondary site of RFa alters the relationship of the chain to the ring skeleton, by producing an approximate 180° rotation about the N(10)-C(1') bond. This change permits O(4') to hydrogen bond with N(1), and O(4') and O(5') to coordinate to copper. Water molecules complete distorted octahedral coordination shells about both copper ions.