Metal binding sites of oxovanadium(IV) in native and modified soluble collagen

Abstract

Oxovanadium(IV) coordination to native and modified soluble collagen flora calf skin, has been studied by EPR spectroscopy on aqueous and lyophilized samples at physiological pH. At room temperature, the isotropic spectrum g 0=1.967, A 0=95.61×10 −4 cm −1 of a VO(IV)-collagen complex, weakly immobilized in solution has been seen, with vanadyl rotational correlation time τ r≈6×10 −9 s shorter than rotational tumbling time τ≈10 −7 s of the whole protein. This is probing a local environment for the VO(IV) site and vanadyl monocoordination to the protein. Simultaneously, an anisotropic spectrum with axial symmetry has been detected from VO(IV)-native collagen and VO(IV)-derivative collagen complexes in solution at 77 K and, in the solid fibrous state at both r.t. and 77 K. The apparent g 0 and A 0 values calculated for the powder spectra are comparable with those of solution spectra at r.t. of VO(IV)-insulin in the A site and VO(IV)-(N 2O 2) model compounds. However, oxovanadium(IV) seems to interact with two nitrogens and two water molecules on the equatorial plane. The two protein nitrogen ligands could be two imidazole nitrogens, or one imidazole nitrogen and one amino group, or two amino groups. By increasing the vanadyl concentration, the metal-ion interaction with tropocollagen molecules loses its specificity, to give a mixture of VO(IV)-collagen compounds.