A probable function of the sulfur in d-biotin could be to coordinate in a specific way with the appropriate metal ions and thereby form an active enzyme-metal ionsubstrate complex. Hence, the stability constants of the binary 1:1 complexes of Mn(II), Cu(II), and Zn(II) with d-biotin and its ring-altered derivatives, viz., dethio, sulfoxide, and sulfone were measured by potentiometric titrations. Stabilities of the complexes in 50% aqueous dioxane (I = 0.1, T = 25°) are in the range: log KMnMnL ~ 2.0, log KCuCuL ~ 3.4, and log KZnZnL ~ 2.4. For d-biotin and water as solvent, the values are log KCuCuL = 1.63 and log KZnZnL = 0.82. The values found (in 50% aqueous dioxane) for the complexes of chain-shortened biotin derivatives are of about the same order as for the other ligands in the same solvent. The investigaron of the formation of ternary, i.e., mixed, complexes between these ligands and the 1:1 complexes of Cu(II) -2,2′-bipyridyl and Zn(II)-2,2′-bipyridyl was also included in this study. The stability constants found for all these complexes are of the size expected, if basicity of the carboxylic acid group determines the complex stability. However, that the sulfur in biotin can also complex with metal ions, is demonstrated by the complexing between tetrahydrothiophene and Cu(II). Furthermore, nuclear magnetic resonance spectra of d-biotin with increasing amounts of Mn(II) or Cu(II) show, besides line broadening of the signal due to the methylene group neighbored to the carboxyl function, broadening of the quartet that is due to only one of the protons in the methylene group next to the sulfur. This suggests that these Me(II) complexes are formed in a stereospecific manner, i.e., the orientation of these metal ions in the complex with d-biotin is analogous to that of oxygen in d-biotin d-sulfoxide. This structure-specific behavior is confirmed by comparison of the stability of the Cu(II)-chelate (1:1) of tetrahydrothiophene-2-carboxylic acid (log KCuCuL = 4.31) with that of the Cu(II)-d-tetranorbiotin complex (1:1) (log KCuCuL = 2.89); this difference in stability cannot be explained by the small differences of the acidity constants. In the lastmentioned complex, Cu(II) coordinated at the carboxylic acid group could only form a chelate by binding in an l-sulfoxide-like way; however, due to the steric hinderance of the ureido ring this seems not to occur, i.e., at least not in a significant stability-increasing way. © 1969, American Chemical Society. All rights reserved.
