The role of vanadium as a biometal is well established1 and encompasses stimulating and regulatory* as well as inhibitory functions, e.g. toward N~, K -A T P~s~.~ Vanadium has also been recognized as an essential element in photosynthesis: as a storage for redox potentials in the "blood" of ascidian^,^ and as an integral part in amavadine, a hydroxamate complex of V occurring in A m a n i t a muscaria.6 Lately, vanadium has adopted t h e status of an catalytically active center in t h e nitrogenase system of a special strain of Azotobacter chroococcuml and in the vanadate(V)-dependent halide peroxidases of several marine brown algae such as Ascophyllum n o d o~u m .~*~ Further, t h e binding of V(1V) and V(V) to serum transferrin has been documented.lO~ll More recently, the excellent NMR properties of the 51V nu-cleus12 have been exploited in the evaluation of the interaction between vanadate(V) and large biomolecules such as ribo-n~clease,'~ transferrin," and peroxidase from A . nodosum,14 the latter being the first 51V NMR detection of an enzyme containing vanadate(V) as the prosthetic group.In order to provide a basis for t h e interpretation of vanadium NMR data of biological material, we report here on a referencing scale for 51V chemical shifts, 6(51V), incorporating t h e presently available shift data on V5+ compounds (including recent work on the interaction between vanadate and biorelevant ligandsI5), (a) Chasteen, N. D. Strucr. Bonding (Berlin) 1983,53, 105. (b) Kustin, K.; McLecd, G. C.; Gilbert, T. A.; Briggs, L. R., 4th. Eady, R. R.; Richardson, T. H.; Miller, R. W.; Hawkins, M.; Postgate, J. R. Nature (London) 1986, 322, 388. (b) Arber, J. M.; Dobson, B. R.; Eady. R. R.; Stevens, P.; Hasnain, S. S.; Garner, C. D.; Smith, B. E. Nature (London) 1987, 325, 372. (a) Vilter, H. Bot. Mar. 1983, 26,451. (b) Vilter, H. Phytochemisfry 19848 23, 1387. de Boer, E.; van Kooyk, Y.; Tromp, M. G. M.; Plat, H.; Wever, R.
