Photochemistry of Sodium Pyruvate Clusters

Pyruvate is frequently found in aged sea salt aerosols. To test the influence of the salt environment on the photochemistry of pyruvate, small model clusters were generated from sodium pyruvate salt using electrospray ionization. These clusters were studied using Fourier-Transform Ion Cyclotron Resonance mass spectrometry (FT-ICR MS). UV/vis spectra were recorded using a tunable optical parametric oscillator (OPO) system. Additionally, photokinetics as well as sustained off-resonance irradiation (SORI) collision-induced dissociation (CID) experiments help to further understand the mechanisms responsible for the fragmentation of the studied systems. Quantum chemical calculations allow assignment of the transitions responsible for the absorptions. The clusters mainly fragment by losing sodium pyruvate units, Na_mPyr_m, resulting in Na_n-mPyr_n-m-1⁺ fragments. Photodissociation cross sections for nonstoichiometric fragmentation are around 2 orders of magnitude lower. We found that the formation of these fragments starts with the C-C bond photolysis of a pyruvate anion and can lead to several secondary fragments. For the two larger cluster sizes, neutral CH₃COCOO is lost, presumably in the form of a CH₃CO radical, followed by CO₂ elimination. The electron is transferred to a second pyruvate molecule in the cluster, resulting in a radical dianion, CH₃COCOO^2-. For the smallest cluster size, a variety of secondary fragments was observed, including CO₂^- stabilized by two Na⁺ ions in the cluster.