Photosensitization

The reactions of a molecule that does not absorb directly can be made to occur if another absorbing molecule is present, because the latter may be able to transfer its energy to the former during a collision. An example of this photosensitization is the reaction used to generate excited state O₂ in a type of treatment known as photo dynamic therapy (Impact I23.3). Another example is the reaction used to generate atomic hydrogen, the irradiation of hydrogen gas containing a trace of mercury vapour using radiation of wavelength 254 nm from a mercury discharge lamp. The Hg atoms are excited (to Hg*) by resonant absorption of the radiation, and then collide with H2 molecules. Two reactions then take place:

Hg*+H2→Hg+H·+H·

Hg*+H2→HgH+H·

The latter reaction is the initiation step for other mercury photosensitized reactions, such as the synthesis of formaldehyde from carbon monoxide and hydrogen:

H· +CO→HCO ·

HCO · + H₂→HCHO+H·

HCO · + HCO · →HCHO+CO

Note that the last step is termination by disproportionation rather than by combination.

In photodynamic therapy (PDT), laser radiation, which is usually delivered to dis eased tissue through a fibre optic cable, is absorbed by a drug which, in its first excited triplet state 3P, photosensitizes the formation of an excited singlet state of O₂, ¹O₂. The ¹O₂ molecules are very reactive and destroy cellular components and it is thought that cell membranes are the primary cellular targets. Hence, the photochemical cycle below leads to the shrinkage (and sometimes total destruction) of diseased tissue.

Absorption:  P + hν →P*

Intersystem crossing:   P* →3P

Oxidation reactions:   ¹O₂ +reactants → products

The photosensitizer is hence a ‘photocatalyst’ for the production of ¹O₂. It is common practice to use a porphyrin photosensitizer, such as compounds derived from hematoporphyrin (11). However, much effort is being expended to develop better drugs with enhanced photochemical properties. A potential PDT drug must meet many criteria. From the point of view of pharma cological effectiveness, the drug must be soluble in tissue fluids, so it can be trans ported to the diseased organ through blood and secreted from the body through

urine. The therapy should also result in very few side effects. The drug must also have unique photochemical properties. It must be activated photochemically at wave lengths that are not absorbed by blood and skin. In practice, this means that the drug should have a strong absorption band at λ > 650 nm. Drugs based on haematopor phyrin do not meet this criterion very well, so novel porphyrin and related macro cycles with more desirable electronic properties are being synthesized and tested. At the same time, the quantum yield of triplet formation and of ¹O₂ formation must be high, so many drug molecules can be activated and many oxidation reactions can occur during a short period of laser irradiation. Photodynamic therapy has been used successfully in the treatment of macular degeneration, a disease of the retina that leads to blindness, and in a number of cancers, including those of the lung, bladder, skin, and oesophagus.

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مواضيع ذات صلة

Experimental probes of reactive collisions

Rate laws of complex photochemical reactions

The overall quantum yield of a photochemical reaction

Electron transfer reactions

Resonance energy transfer

The primary quantum yield

Timescales of photophysical processes

The catalytic efficiency of enzymes

The Michaelis–Menten mechanism of enzyme catalysis

Enzymes

Application of the techniques

Monitoring the progress of a reaction