Three-pulse ESE modulation was used to study ferric P-45O-substrate complexes at g = 2.2 (9.2 GHz), 6-7 K. No 2H modulation was found for the "unnatural" 22(S)-hydroxycholesterol-22-d isomer, suggesting the deuteron is more than 6 Å from the heme. Slightly different modulation patterns were observed for cholesterol-22,22-d2 and 20-azacholesterol-22,22-d2, with analyses ongoing. These experiments demonstrate the potential of electron spin echo spectroscopy to probe structural aspects of substrate binding to paramagnetic enzymes, particularly useful for membrane-bound systems (e.g., P-450) not amenable to crystallography. By examining more deuterated steroids, the relative position of a substrate with respect to the P-450 catalytic site can be approximated under nonperturbing conditions, with studies in progress. Similar deductions for other enzymes use NMR relaxation measurements, which require multiple spin relaxation times and exchange rate constants (done at room temperature), while ESEEM requires no such information but must be carried out at very low temperatures; both methods give interpretable data only when dipolar, Fermi contact, and quadrupolar contributions are properly estimated. The biosynthesis of furanomycin (1) from Streptomyces threomyceticus was investigated. Initially, [U-14C]-L-glutamate and [U-14C]pyruvate (putative precursors due to structural similarity to muscarine) showed low incorporation. In contrast, sodium [1-14C]acetate, [2-14C]acetate, and [1,2-13C]acetate exhibited significant incorporation, with 13C enrichment at C-1/C-3 and C-2/C-4, confirming two intact acetate units. Sodium [1-13C]propionate showed high enrichment at C-5, indicating it as a precursor. Experiments with sodium [2-3H]propionate ruled out keto function formation at C-2 of propionate, suggesting ether linkage formation involves loss of one hydrogen from the prochiral C-2. Thus, furanomycin is derived from two acetate units and one propionate unit. Molecular metal oxides and alkoxides have recently been considered useful models of condensed-phase metal oxides that may provide valuable information about catalytic chemistry on bulk oxide surfaces and the interaction of metal catalysts with such surfaces.