-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2 -deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2 -deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed.

Ad B. 13C NMR Spectroscopy.-In comparison with the corresponding ketones, the 13C signals of the carbonyl groups in acyl silanes are dramatically shifted downfield (Table 2).30-31" Carbonyl groups in acyl silanes have chemical shifts differing by between ca. 25 and 100 ppm from those of the analogous ketones; '* the effect is approximately additive (Table 2, entry 16).13C studies also expose some interesting resonance features of acyl silanes. The carbonyl group of an alkyl phenyl ketone (e.g. Ph-CO-But) displays a 13C chemical shift close to its aliphatic analogue (i.e. Me-CO-But); however, the difference between the two corresponding silicon species (e.g. Ph-CO-SiMe3 and CH3-CO-SiMe3) is a little more marked. Benzoyltrimethylsilane (2) exhibits a carbonyl shift ca. 11-14 ppm upfield of that displayed by acetyltrimethylsilane (1) (Table 2, entries 2 and 5). This could be ascribed to participation of resonance structures such as (5) which, by reducing the amount of positive charge on the carbonyl carbon atom, increase its shielding, thus displacing the chemical shift upfield relative to the aliphatic analogue where such an effect cannot operate.

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