Its electron arrangement is different from abundant normal diradical triplet oxygen. Singlet oxygen can be formed from triplet oxygen with the assist of photosensitizers, like Rose Bengal. Such compounds use light to convert triplet into singlet oxygen. Due to low activation energy and its electron configuration singlet oxygen is a highly reactive molecule and can readily form bonds with a number of organic molecules. It is less stable than the normal form of oxygen. However, depending on the environment singlet oxygen has the possibility to exist for more than one hour at room temperature. (Schweitzer, et al., 2003) One of the reactions that singlet oxygen can take part in, is the oxidation of amino acid residues of proteins. Cysteine and methionine residues are particularly susceptible to such forms of oxidation. In contrast to oxidation of other amino acids, this type of oxidation is reversible. Both R- and S-stereoisomers of methionine sulfoxide form as a consequence of methionine residue oxidation. The reaction can be reversed with the aid of stereospecific methionine sulfoxide reductases which can be found in most cells. The enzyme catalyses the formation of methionine residues from methionine sulfoxide (Carey, et al., 1984). …
(Stadtman, et al., 2005) This work, for example, uses methionine oxidation by singlet oxygen, generated using Rose Bengal and laser as a radiation source. This report is initially focused on describing the key literature associated with oxidation of methionine by singlet oxygen, highlighting relevant for the project issues and arguments, then moving on to presenting the research that has been done so far, making accent on the methods used and results achieved, before finally identifying the existing gaps in the study and setting up a plan for future work. Singlet and triplet oxygen Triplet oxygen is the most stable and abundant form of oxygen. The difference between two forms of oxygen can be seen by comparing the molecular orbitals of both forms (Figure 1). Figure 1: Triplet (left) and singlet (right) form of oxygen. (Min and Lee, 1999) (Frimer, et al., 1985) To define spin states of molecules spin multiplicity is used. It can be described as 2S+1. Here S is the spin quantum number, which is 1 for triplet oxygen. Therefore, the spin multiplicity for triplet oxygen is 3 and it is paramagnetic. Triplet oxygen easily reacts with radicals, but peptides in general and methionine in particular are in the singlet state. Singlet oxygen, in contrast, has different electron configuration in the ?-antibonding orbital. It is a non-radical, electrophilic molecule. The spin quantum number is 0 and multiplicity is 1, these parameters characterise the form of oxygen presented on the Figure 1 as a singlet. It does not follow the Hund’s rule, extremely reactive and 22.5 Kcal/mole higher in energy then triplet oxygen. Five excited states are produced due to electron repulsion. The 1? represents the singlet form which is responsible for the reaction with

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