Synthesis of Coumarin–Polyamine-Based Molecular Probe for the Detection of Hydroxyl Radicals Generated by Gamma Radiation

A fluorescent turn-on low dose detection of gamma-radiation based on aggregation-induced emission

A new sensitive fluorescence turn-on detection of gamma-radiation is reported by taking advantage of AIE property of the tetraphenylethylene unit and the cascade radiochemical and protonation reactions.

Radiation dose detection by imaging response in biological targets

Imaging was one of the earliest techniques to quantify radiation dose. While films and active fluorescent detectors are still commonly used in physical dosimetry, biological imaging is emerging as a new method to visualize and quantify radiation dose in biological targets. Methods for biological imaging are normally based on molecular fluorescent probes, labeling chromatin-conjugated molecules or specific repair proteins. Examples are chromatin-binding coumarin compounds, which become fluorescent under irradiation, or the H2AX histone, which is rapidly phosphorylated at sites of DNA double-strand breaks and can be visualized by immunostaining. Many other DNA repair proteins can be expressed with fluorescent targets, such as green fluorescent protein, thus becoming visible for dose estimation in vivo. The possibility to visualize radiation damage in living biological targets is particularly important for repair kinetic studies, for estimating individual radiation response, and for remote control of living samples exposed to radiation, for instance in robotic space missions. In vivo dose monitoring in particle therapy exploits the production of positron emitters by nuclear interaction of the incident beam in the patient's body. Positron emission tomography (PET) can then be used to visualize and quantify the particle dose in the patient, and it can in principle also be used for radiotherapy with high-energy X rays. Alternatively, prompt γ rays or scattered secondary particles are under study for in vivo dosimetry of ion beams in therapy.

DNA Binding Hydroxyl Radical Probes

The hydroxyl radical is the primary mediator of DNA damage by the indirect effect of ionizing radiation. It is a powerful oxidizing agent produced by the radiolysis of water and is responsible for a significant fraction of the DNA damage associated with ionizing radiation. There is therefore an interest in the development of sensitive assays for its detection. The hydroxylation of aromatic groups to produce fluorescent products has been used for this purpose. We have examined four different chromophores which produce fluorescent products when hydroxylated. Of these, the coumarin system suffers from the fewest disadvantages. We have therefore examined its behavior when linked to a cationic peptide ligand designed to bind strongly to DNA.

Tuning the substrate specificity by engineering the active site of cytochrome P450cam: a rational approach

Rational design of the active site of cytochrome P450cam has been carried out to catalyse oxygenation of various potentially important chemical reactions. The modeling studies showed that the distal pocket of the heme consisting of the Y96, T101, F87 and L244 residues could be suitably mutated to change the substrate specificity of the enzyme. We found that the mutant enzymes could catalyse oxygenation of indole to produce indigo. While Y96F was found to be several times better as a catalyst for conversion of indole to indigo, the double mutant Y96F/L244A showed the highest NADH oxidation rate as well as yield of indigo. The oxidative catalysis using H(2)O(2) as the oxygen source was found to produce a higher purity of indigo, and lesser or no formation of indirubin was detected. The enzymatic oxygenation of aromatic hydrocarbons such as coumarin and analogues was also found to be enhanced on mutation of Y96 and L244 residues in the enzyme. The studies also showed that mutation of suitable residues can alter the regio-selectivity of hydroxylation of the aromatic hydrocarbons.

Synthesis and application of molecular probe for detection of hydroxyl radicals produced by Na(125)I and gamma-rays in aqueous solution

PURPOSE

To synthesize N-(3-(3-aminopropylamino)propyl)-2-oxo-2H-chromene-3-carboxamide (7), a novel DNA-binding, coumarin-based, fluorescent hydroxylradical ((*)OH) indicator and to assess its quantum efficiency compared with that of coumarin-3-carboxylic acid (1) and N1,N12-bis[2-oxo-2H-chromene-3-carbonyl]- 1,12-diamine-4,9-diazadodecane (9).

MATERIALS AND METHODS

Using computer-generated molecular modeling, 7 and 9 and their respective 7-hydroxylated derivatives 8 and 10 were docked onto DNA dodecamer d(CGCGAATTCGCG)2, the ligand-DNA complexes were energy minimized, and binding free energies and inhibition constants were calculated. Compound 7 was judged an appropriate target molecule and was synthesized. Compounds 1, 7, and 9 were incubated with Na(125)I or irradiated with (137)Cs gamma-rays, and the influence of pH, dose, type of radiation, and the concentration of indicator on fluorescence yield were determined.

RESULTS

Non-fluorescent 7 and 9 are converted to fluorescent, 7-hydroxylated derivatives 8 and 10 after interaction with (*)OH in aqueous solution. For 1, 7, and 9, hydroxylation yield increases linearly with both Na(125)I dose (0-700 x 10(6) decays) and (137)Cs dose (0-11.0 Gy). Fluorescence induction is significantly reduced at acidic pH and the fluorescent quantum yield of 8 is approximately 3 times that of 2 or 10 at pH 7.0. With Na(125)I incubation and gamma-ray irradiation, the fluorescence signal of 7 increases linearly with concentration and saturates at approximately 50 microM.

CONCLUSION

Compound 7 quantifies lower concentrations of (*)OH than do 1 and 9. This detector is therefore likely to be a good reporter of (*)OH produced within a few nanometers of DNA.