The Crystal Structure of a Riboflavin-Metal Complex

A crystalline silver perchlorate complex of riboflavin, a possible model for enzymic metal-flavin interactions, has been prepared and characterized by x-ray techniques. The orange crystals are monoclinic, with symmetry C2, and unit cell constants a = 19.464(10) (figures in parentheses are standard deviations in the least significant figures cited), b = 7.886(4), c = 15.459(8)A, β = 107.34(2)°, Z = 4, ρobs = 1.79 g/cm3, and ρcalc = 1.77 g per cm3 for AgClO4· C17H20N4O6(CO)0.45·½H2O. The crystal structure, refined using 2837 counter-measured reflections, gives an R factor of 6.2% and revealed that about 45% of the riboflavin in the crystal used was formylated in the 5′ position by the formic acid solvent. Each riboflavin molecule bonds fairly strongly to two silver ions: one via N(1) (2.304(5)A), O(2) (2.786(5)A), and O(2′) (2.559(6)A), and the other via N(5) (2.295(5)A) and O(4) (2.521(5)A). These results confirm the existence of two separate chelate sites in the N(3)-protonated, quinoid isoalloxazine ring system, and together with earlier crystal structure studies suggest that both these chelate sites will nearly always be occupied by positive ions or dipoles (such as —OH). Two conformationally preferred forms of riboflavin seem both to have the ribityl chain extending above the isoalloxazine ring when viewed with the benzo ring to the right and N(10) at the top. In one, C(1′)—C(4′) and O(4′) are nearly planar and fully extended; in the other, O(2′) and C(2′)—C(5′).