Proton Magnetic Resonance Studies of Flavoquinone-Metal Complexes

Abstract

Flavoquinone-metal complexesA detailed magnetic resonance investigation of the metal ion interaction with flavoquinones has been carried out with the purpose of obtaining the stoichiometry, stability and (sub) molecular structure of the complexes. The isotropic shifts of the paramagnetic chelates are analysed in terms of contact and pseudocontact contribution. It is shown that the flavoquinone-metal binding is mainly of o-type regardless of the nature of the ion.Flavoquinone-metal complexes with mono-and bivalent d-metal ions are unstable in dilute aqueous solution, but occur stoichiometrically in solvents of low dielectric constant 2 . Proton magnetic resonance at 60 MHz was performed in acetone-d6 in order to investigate stoichiometry, stability and submolecular structure of these complexes. With few exceptions, the sample temperature was in the range where ligand exchange is fast enough for averaging out chemical shifts of protons in free and complexed flavin. Fig. 1 shows the proton NMR trace for the Fe (II) complex compared to the spectrum of the free flavin. In Fig. 2 the observed (diamagnetic plus paramagnetic) shifts for flavin protons are plotted against the metal/ligand ratio. Under these conditions, the magnitude of the shift of flavin protons is at first linear with metal ion concentration, then reaches a stationary position corresponding to complete complexation. The analysis of these titration curves indicates that 3/1 octahedral complexes (o) and 2/1 tetrahedral (t) or square planar (s) complexes are obtained in the presence of an excess Nt3HH «fl-OjC-CH^

IRMPD spectroscopy of metalated flavins: structure and bonding of Mq+–lumichrome complexes (Mq+ = Li+–Cs+, Ag+, Mg2+)

“…Various properties of metal-flavin complexes were extensively investigated in solution. 8,[10][11][12][13][14][15] Here, the complexation with metals has an important influence on the electronic and redox properties of the flavin, which is signalled by large shifts in the absorption spectra. 11 However, the considerable influence of the solvent and counter ions in these studies is not well characterized.…”

Kinetics of flavine oxidation-reduction. II. Metal ion interactions

“…Functionally, the two-step oxidation of the flavohydroquinone by ferric ions showed that flavine dismutation may occur after formation of the flavosemiquinone, providing an indirect pathway for the second oxidation step. We already proposed (Lauterwein et al, 1972) a scheme for electron transfer in biological chains, providing a link between twoelectron carriers such as NAD+ or NADP and one-electron carriers such as the heme or iron-sulfur. Following this scheme, flavohydroquinone should be the electron donor and the flavine dismutation, which can be controlled by sterical factors and acidic-basic reactions should then provide the pathway for complete oxidation.…”

sHigh resolution 1H nuclear magnetic resonance studies of a flavine and its product with MoCl4

“…The protons in positions 6 through 10 were again assigned for lumiflavin and its 9-bromo-and 9-deutero-and 6,9-dideuteroderivatives (4,5 between isoalloxyazine rings of neighboring FAD molecules (6,7). In 1972, a brief account of the 'H nrnr spectra was published of tetraacetylriboflavine (R, = H ; R, = R, = CH,; R,, = CH2(CHOCOCH,),CH20COCH,) and its complexes with Fez+, Co2+, Ni2+, and Cu2+ in acetone-d, (8). A full account of that work including additional information on the 'H nrnr of complexes of Cu+, Agf , Zn2 +, Cd2 +, Mn2 +, Mg2+, and Fe3+ has now been published (9,10).…”