NEAR UV CIRCULAR-DICHROISM FROM BIOMIMETIC MODEL COMPOUNDS DEFINE THECOORDINATION GEOMETRY OF VANADATE CENTERS IN MEVI-RABBIT AND MEADPVI-RABBIT MYOSIN SUBFRAGMENT-1 COMPLEXES IN SOLUTION

The circular dichroism (CD) spectrum was measured from vanadate (Vi) c yclic esters of chiral vicinal diols, hydroxycarboxylates, and cyclode xtrines as a function of Vi concentration ([Vi]) and at the lowest ene rgy transitions of the vanadium. At low [Vi] and in the presence of ex cess vicinal diols, hydroxycarboxylates, or cyclodextrines the CD sign al intensity scales linearly with [Vi] indicating the predominance of a monomeric cyclic eater. 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 eater. Vanadium-51 NMR (V-51-NMR) indicates the coordination geometry of several of these model Vi centers in solution and identifies the CD signals characteri stic to Vi trigonal bipyramidal (tbp) and octahedral (Oh) coordination geometries from monomeric and dimeric species. The CD spectra from mo nomeric and dimeric forms of the tbp-coordinated model compounds have two apparent transitions with amplitudes of opposite sign at wavelengt hs greater than or equal to 240 nm. Spectra from the monomeric and dim eric Oh coordinated species are distinct from the tbp-type spectra ove r the same wavelength domain because of the presence of two additional transitions with opposite sign amplitudes. These model spectra were c ompared to the vanadate CD spectra from Vi bound to rabbit myosin subf ragment 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 p roduce a CD signal resembling that from the tbp model. The trapped ATP ase transition state analog MeADPVi produces a different CD signal res embling that from the Oh model. (C) 1998 Elsevier Science B.V. All rig hts reserved.