160 Heterocycles in Nature
Niacin (vitamin B 3 ) and nicotinamide adenine
dinucleotide phosphate (NADP)
Nicotinamide adenine dinucleotide phosphate (NADP) is a large complicated co-enzyme, but the signifi cant part,
for its role in oxidation/reduction processes, is the pyridinium ring – think of it as simply an N-alkylpyridinium salt
of nicotinamide. The positively charged nitrogen acts as an electron sink and allows this co-enzyme to accept two
electrons and a hydrogen (proton), that is, effectively, hydride. In line with typical pyridinium reactivity, the hydride
adds at a -position thus producing a 1,4-dihydropyridine (NADPH), the process being facilitated because it is a
stabilised 1,4-dihydropyridine, in which the ring nitrogen is conjugated to the carbonyl of the 3-substitutent, as in
the products of a Hantzsch pyridine synthesis (page 45). In the reverse sense, NADPH is a vital reducing agent in
biosynthesis – it is Nature’s NaBH 4. The rationale for the reverse process is the regaining of the aromaticity in the
co-enzyme product – a pyridinium ion.
Ribofl avin is incorporated into another complex co-enzyme, fl avin adenine dinucleotide (FAD). This is involved in
enzyme-catalysed reductions of carbon–carbon double bonds, and the reverse. After accepting two hydrogens, the
co-enzyme is converted into a dihydro-derivative (FADH 2 ), the driving force being the relief of the unfavoured inter-
action between the polarised, opposed C=N bonds.