Photodegradation
Definition
Breakdown of biological molecules driven by ultraviolet and visible radiation. In bloodstains it accelerates haemoglobin oxidation and cleaves DNA strands, hastening ageing of both colour and DNA yield.
- Cause
- Ultraviolet and visible radiation exposure
- Effect on bloodstains
- Accelerates haemoglobin oxidation
- Effect on DNA
- Cleaves DNA strands
- Overall impact
- Hastens both colour change and DNA yield loss
Common questions
Why does a sun-exposed bloodstain look different in colour from a shaded one?+
UV and visible light accelerate haemoglobin's oxidation to methemoglobin and further breakdown products, shifting the stain's colour faster than it would change in the dark, which examiners can factor into rough age estimates.
Does photodegradation affect the ability to get a DNA profile from an old bloodstain?+
Yes, light-driven strand cleavage adds to the general degradation a bloodstain experiences over time, so a stain that sat in direct light typically yields lower-quality or lower-quantity DNA than a comparable stain kept shaded.
Can photodegradation alone be used to estimate how long a bloodstain has been present?+
Not reliably on its own, since exposure intensity, temperature, and humidity all vary independently, so colour and DNA-yield changes from light exposure are considered alongside other ageing indicators rather than as a standalone clock.
Related terms
- Haemichrome
- A further oxidation product formed when the globin chains of methaemoglobin denature and the haem group becomes internally coordinated. Associated with the...
- Methaemoglobin
- An oxidised form of haemoglobin in which ferrous iron (Fe2+) is converted to ferric iron (Fe3+), unable to carry oxygen. Its formation...
- Oxyhaemoglobin
- The form of haemoglobin in freshly shed arterial blood, where iron in the haem group is in the ferrous (Fe²⁺) state and...
- Protein Denaturation
- The loss of a protein's three-dimensional structure without breaking peptide bonds, caused by heat, pH change, UV radiation, or chemical agents such...
- Substrate Effect
- The influence of the surface on which a bloodstain lies on the rate and character of chemical change. Porous substrates absorb blood...