Fluorescence and quenching of O2(1Δg) and [O2(1Δg)]2 in liquid oxygen

Direct 1O2 optical excitation: A tool for redox biology

Molecular oxygen (O2) displays very interesting properties. Its first excited state, commonly known as singlet oxygen (1O2), is one of the so-called Reactive Oxygen Species (ROS). It has been implicated in many redox processes in biological systems. For many decades its role has been that of a deleterious chemical species, although very positive clinical applications in the Photodynamic Therapy of cancer (PDT) have been reported. More recently, many ROS, and also 1O2, are in the spotlight because of their role in physiological signaling, like cell proliferation or tissue regeneration. However, there are methodological shortcomings to properly assess the role of 1O2 in redox biology with classical generation procedures. In this review the direct optical excitation of O2 to produce 1O2 will be introduced, in order to present its main advantages and drawbacks for biological studies. This photonic approach can provide with many interesting possibilities to understand and put to use ROS in redox signaling and in the biomedical field.

Ab initio potential-energy surfaces of O2(XΣg−3,aΔg1,bΣg+1)+O2(XΣg−3,aΔg1,bΣg+1): Mechanism of quenching of O2(aΔg1)

Ab initio computational studies were carried out in order to explore the possible mechanisms of quenching of O(2)(a (1)Delta(g)) by O(2)(X (3)Sigma(g) (-)): the self-quenching of O(2)(a (1)Delta(g)) and other energy-transfer processes involving two O(2) molecules. All eighteen states arising from two O(2) molecules in the X (3)Sigma(g) (-), a (1)Delta(g), and b (1)Sigma(g) (+) states are considered. After scans at the state-averaged complete active space self-consistent field method to identify possible regions of crossing between states belonging to different asymptotes, complete active state second-order perturbation theory high-symmetry optimization and low-symmetry scans established that four different minima on the seams of crossing (MSXs), arising between the a+a manifold and the X+b manifold and responsible for self-quenching: O(2)(a (1)Delta(g))+O(2)(a (1)Delta(g))-->O(2)(X (3)Sigma(g) (-))+O(2)(b (1)Sigma(g) (+)), have coplanar C(2h) or C(2v) symmetries and are only 0.45 eV barrier relative to the a+a asymptote and energetically easily accessible. The rate constant for this process was estimated based on the Landau-Zener formalism. The MSXs for quenching of O(2)(a (1)Delta(g)) by the ground state O(2)(X (3)Sigma(g) (-)):O(2)(a (1)Delta(g))+O(2)(X (3)Sigma(g) (-))-->O(2)(X (3)Sigma(g) (-))+O(2)(X (3)Sigma(g) (-)) require higher energies and the process is not likely to be important.

Biological inactivation by singlet oxygen: distinguishing O2(1 delta g) and O2(1 sigma g+)

Experiments were performed to determine whether bacterial inactivation in the separated-surface-sensitizer system for singlet oxygen generation is due to O2(1 delta g) or O2(1 sigma g+). The rates of inactivation of Gram-negative Salmonella typhimurium LT-2 and a nonpigmented strain of Gram-positive Sarcina lutea were found to increase linearly with the concentration of 1 delta g. The gas phase lifetime of the inactivating agent was found to be within the range of values expected for the gas phase lifetime of 1 delta g rather than 1 sigma g+. These measurements conclusively demonstrate that bacterial inactivation in this system is due predominantly to 1 delta g. Therefore, studies of bacterial inactivation with this singlet oxygen generating system can be used to assess the role of singlet oxygen in various biological and medically relevant situations.