Configurational, conformational, and solvent effects on the reduction of a Schiff base by reduced pyridine nucleotide...

The rate of reduction of the Schiff base, delta 1-pyrroline-2-carboxylic acid, by beta-NADH has previously been shown to be 6.8 times smaller than that calculated from the substituent effects in 1-substituted 1,4-dihydronicotinamides. The factors governing this anomalous rate have been studied by measuring the reduction rate with a number of reduced pyridine nucleotides in water and water-methanol mixtures. The beta-configuration for the nicotinamide-ribosidic linkage was found to be responsible for 75% of the anomaly; the same reduced coenzyme with alpha-linkage, however, behaves normally. It was also shown that the low reactivities of the reduced coenzymes with beta-configuration are entirely the result of their abnormal thermodynamic stabilities. Conformational effects (the folding of beta-NADH) account for only 20% of the reduction rate anomaly. Addition of organic solvents causes only a modest decrease in the overall rate. These solvent effects are interpreted in terms of the opposing effects of solvents on the basicity of the Schiff base and on the reduction step itself. Specific solvation effects appear important in controlling the reduction rates.