Photochemically-induced reduction and rearrangements of N,N′-bis-(carboxymethyl)-N,N′-dinitroso-1,4-phenylenediamine

When irradiated with ∼10 ns laser pulses of 308 nm light, N,N′-bis-(carboxymethyl)-N,N′-dinitroso-1,4-phenylenediamine (1) fragments in less than a μs to give NO and the denitrosylated radical of 1 (2). Species 1 has great potential as a research tool because the photo-generated NO can subsequently be used to probe fast reactions of biochemical interest. This study focuses on the properties of the reactive intermediate 2, which must be known before 1 can confidently be used in more complex investigations. Experiments in which 1 was irradiated in the presence of myoglobin (Mb) or ferrocytochrome c revealed that 2 can rapidly oxidize either species. For myoglobin, the rate constant kox was measured as (5.5±0.5)×107 M−1 s−1. Experiments in which large amounts of 2 were photo-generated in the presence of free NO revealed that 2 probably recombines with NO to produce metastable isomers of 1, which have lifetimes of ∼20 ms and liberate additional NO as they decay. Previously, 2 was only known to either rapidly recombine with NO to regenerate 1 exclusively, or fragment further to give a second equivalent of NO and the doubly denitrosylated quinoimine derivative of 1 (3). The experiments with myoglobin, which is a chromophoric NO scavenger, also made it easy to quantify the amount of NO generated under various conditions, thus providing indirect information about the reactions of 2. On the basis of these experiments, the value of the rate constant for fragmentation of 2 into 3 and NO (kd) was revised from the previously published 500–2600 s−1. The NO scavenger experiments also suggest that the metastable isomers of 1, generated by recombination of 2 and NO, liberate additional equivalents of NO as they decay.