Graduate Thesis Or Dissertation
 

Formation of N-nitrosodimethylamine and nonvolatile N-nitrosamines from barley malt alkaloids

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  • Amines formed by biosynthesis in germinated barley have been suggested as precursors for N-nitrosodimethylamine (NDMA) in barley malt dried by direct-fired kilning. This hypothesis was verified by germinating raw barley and exposing the resulting malt roots and clean malt to nitrous acid. Quantitation of the NDMA formed indicated that malt roots contained relatively large amounts of NDMA precursor which could only have been formed as a result of germination. The clean malt also contained NDMA precursor, but the amount of precursor in clean malt was only slightly higher than the level of precursor already present in raw barley. The two tertiary amine alkaloids, hordenine and gramine, which are biosynthesized in malt during germination were subjected to nitrosation. Nitrosation of both amines at 65° (pH 4.4 or pH 6.4) resulted in NDMA formation. Gramine was highly susceptible to nitrosation to yield NDMA. At 24° in dilute acetic acid (pH 3.4), the initial rate of nitrosation of gramine to yield NDMA was nearly equal to the initial rate of NDMA formation from dimethylamine. The ratio of initial rates of formation of NDMA from gramine and trimethylamine was 6250:1. At 23°, the ratio of initial rates of formation of NDMA from gramine and hordenine was 5200:1. N-Methyltyramine and N-methyl-S-aminomethylindole, the immediate biosynthetic precursors of hordenine and gramine, were synthesized. Nitrosation of N-methyltyramine yielded p-hydroxy-N-nitroso-N-methyl- 2-phenylethylamine as the major reaction product and p-hydroxy-m-nitro- N-nitroso-N-methyl-2-phenylethylamine as the minor product. Nitrosation of N-methyl-3-aminomethylindole yielded N-nitroso-N-methyl-3- aminomethylindole as the minor reaction product; the major product was a dinitroso compound identified as N¹-nitroso-N-nitroso-N-methyl-3- aminomethylindole. Investigation of the products of hordenine nitrosation at 65° (pH 4.4) indicated the formation of NDMA and p-hydroxy-m-nitro-N-nitroso- N-methyl-2-phenylethyl amine; p-hydroxy-N-nitroso-N-methyl-2- phenylethylamine was not observed as a product of hordenine nitrosation at 65°. NDMA appeared to be the major nitrosamine formed from hordenine. The rapid reaction of gramine with nitrous acid and investigation of the gramine nitrosation reaction products both indicated that gramine did not undergo nitrosation by the expected mechanism of nitrosative dealkylation. A new mechanism is proposed to explain the rapid reaction of gramine with nitrous acid and to account for the fact that NDMA was the only N-nitrosamine formed during the nitrosation of gramine.
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