Acid-base and metal ion-binding properties of flavin mononucleotide (FMN2−). Is a ‘dielectric’ effect responsible for the increased complex stability?
| Autor: | Marcel Felder, Matthias Bastian, Guogang Liang, Bin Song, Helmut Sigel, Ralf Halbach |
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| Rok vydání: | 1995 |
| Předmět: |
Aqueous solution
Stereochemistry Chemistry Metal ions in aqueous solution Flavin mononucleotide Protonation Flavin group Inorganic Chemistry Metal Crystallography chemistry.chemical_compound Deprotonation visual_art Materials Chemistry visual_art.visual_art_medium Titration Physical and Theoretical Chemistry |
| Zdroj: | Inorganica Chimica Acta. 240:313-322 |
| ISSN: | 0020-1693 |
| DOI: | 10.1016/0020-1693(95)04548-1 |
| Popis: | Due to contradictions in the literature we have redetermined the acid-base properties of riboflavin (RiFl; vitamin B2), i.e. 7,8-dimethyl-10-ribityl-isoalloxazine, and of flavin mononucleotide (FMN2−), also known as riboflavin 5′-phosphate, via potentiometric pH titrations (I = 0.1 M, NaNO3; 25 °C). In contrast to various claims, the isoalloxazine ring cannot be protonated at pH > 1, a result in agreement with an early study (pKa = −0.2; L. Michaelis, M.P. Schubert and C.V. Smythe, J. Biol. Chem., 116 (1936) 587–607); deprotonation of the ring system occurs in both compounds with p K a ⋍ 10 . The pKa value of ∼ 0.7 determined for the deprotonation of H2(FMN) must be attributed to the release of the first proton from the fully protonated phosphate group; its second proton is released with pKa = 6.18 in agreement with the acidity constants of various other monoprotonated monophosphate esters. The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+ (M2+) and FMN2− were determined by potentiometric pH titrations in aqueous solution (I = 0.1 M, NaNO3; 25 °C). The log stability constants of all these M(FMN) complexes are about 0.2 log units higher than expected from the basicity of the phosphate group. This slight stability increase cannot be attributed to the formation of a seven-membered chelate involving the ribit-hydroxy group at C-4′ as the stability constants for the M2+ 1:1 complexes of glycerol 1-phosphate (G1P2−) demonstrate: G1P2− contains the same structural unit which would also allow in this case the formation of the mentioned seven-membered chelate; however, the stability of the M(G1P) complexes is solely determined by the basicity of the phosphate group. Hence, in agreement with earlier conclusions (J. Bidwell, J. Thomas and J. Stuehr, J. Am. Chem. Soc., 108 (1986) 820–825) regarding Ni(FMN) one must conclude that the slight stability increase of the M(FMN) complexes has to be attributed to the isoalloxazine ring. The equality of the stability increase of the complexes for all the mentioned ten metal ions precludes its attribution to an interaction with an N site and makes a specific interaction with an O site also somewhat unlikely. In addition, carbonyl oxygens appear as not very favorable for the formation of macrochelates by a further interaction with already phosphate-coordinated metal ions. Therefore, we propose that the slight but significant stability increase originates from M(FMN) species (with a formation degree of about 30%) in which the hydrophobic flavin residue is close to the metal ion, thereby lowering the ‘effective’ dielectric constant in the microenvironment of the metal ion and thus indirectly promoting the −PO32−/M2+ interaction. |
| Databáze: | OpenAIRE |
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