Near UV circular dichroism from biomimetic model compounds define the coordination geometry of vanadate centers in MeVi- and MeADPVi-rabbit myosin subfragment 1 complexes in solution

The circular dichroism (CD) spectrum was measured from vanadate (Vi) cyclic esters of chiral vicinal diols, hydroxycarboxylates, and cyclodextrines as a function of Vi concentration ([Vi]) and at the lowest energy transitions of the vanadium. At low [Vi] and in the presence of excess vicinal diols, hydroxycarboxylates, or cyclodextrines the CD signal intensity scales linearly with [Vi] indicating the predominance of a monomeric cyclic ester. At higher [Vi], the signal intensity in the presence of the vicinal diols and hydroxycarboxylates become nonlinear in [Vi], indicating formation of a dimeric cyclic ester. Vanadium-51 NMR (⁵¹V-NMR) indicates the coordination geometry of several of these model Vi centers in solution and identifies the CD signals characteristic to Vi trigonal bipyramidal (tbp) and octahedral (Oh) coordination geometries from monomeric and dimeric species. The CD spectra from monomeric and dimeric forms of the tbp-coordinated model compounds have two apparent transitions with amplitudes of opposite sign at wavelengths ≤ 240 nm. Spectra from the monomeric and dimeric Oh coordinated species are distinct from the tbp-type spectra over the same wavelength domain because of the presence of two additional transitions with opposite sign amplitudes. These model spectra were compared to the vanadate CD spectra from Vi bound to rabbit myosin subfragment 1 (S1) in solution, in the presence of divalent metal cations (MeVi-S1) or trapped with MeADP (MeADPVi- S1). Polymeric MeVi binds to the active site of S1 and the vanadate centers in MnVi-S1 or CoVi-S1 produce a CD signal resembling that from the tbp model. The trapped ATPase transition state analog MeADPVi produces a different CD signal resembling that from the Oh model.