Methaemoglobin
Definition
An oxidised form of haemoglobin in which ferrous iron (Fe2+) is converted to ferric iron (Fe3+), unable to carry oxygen. Its formation in drying stains contributes to the colour shift from red to dark brown, and it retains some peroxidase activity useful for presumptive blood testing.
- Iron oxidation state
- Ferrous (Fe2+) converts to ferric (Fe3+)
- Color in stains
- Dark brown
- Formation timeline
- Begins within minutes to hours after blood leaves the body
- Oxygen binding
- Unable to carry oxygen
Common questions
What is methaemoglobin and how does it form?+
Methaemoglobin is an oxidized form of hemoglobin where the iron atoms shift from ferrous (Fe2+) to ferric (Fe3+) state. It forms when oxyhaemoglobin oxidizes spontaneously, beginning within minutes to hours of blood leaving the body.
Why does a bloodstain turn brown as it ages?+
The color shift from red to dark brown happens because oxyhaemoglobin gradually converts to methaemoglobin, the brown oxidized form. This color change is one of the physical signs that a bloodstain has aged.
Can methaemoglobin carry oxygen?+
No. Once hemoglobin oxidizes to methaemoglobin, the ferric iron state prevents it from binding oxygen. It retains some peroxidase activity, which makes it useful for presumptive blood testing in forensic work.
Related terms
- Haematocrit
- The proportion of blood volume occupied by red blood cells, approximately 45% in adult males and 40% in adult females. Influences the...
- Haemichrome
- A further oxidation product formed when the globin chains of methaemoglobin denature and the haem group becomes internally coordinated. Associated with the...
- Haemoglobin
- The iron-containing protein in red blood cells that carries oxygen. Its peroxidase-like activity is the basis of most presumptive blood tests; its...
- Haemolysis
- The rupture of red blood cells and release of haemoglobin into the surrounding fluid. In casework, haemolysis of aged stains, wet stains,...
- Oxyhaemoglobin
- The form of haemoglobin in freshly shed arterial blood, where iron in the haem group is in the ferrous (Fe²⁺) state and...
- Photodegradation
- Breakdown of biological molecules driven by ultraviolet and visible radiation. In bloodstains it accelerates haemoglobin oxidation and cleaves DNA strands, hastening ageing...
- Plasma
- The liquid fraction of blood (approximately 55% by volume), containing water, proteins (albumin, immunoglobulins, fibrinogen, clotting factors), electrolytes, glucose, hormones, and dissolved...
- Protein denaturation
- Unfolding and cross-linking of protein structure under heat, desiccation, or chemical stress. Affects haemoglobin, albumin, and other blood proteins as a stain...
- Serum
- The fluid that remains after blood has clotted and the clot has been removed. It is plasma minus fibrinogen and other clotting...
- Substrate effect
- The influence of the surface on which a bloodstain lies on the rate and character of chemical change. Porous substrates absorb blood...
Explained in these topics
- Composition and Properties of BloodAn oxidised form of haemoglobin in which ferrous iron (Fe2+) is converted to ferric iron (Fe3+), unable to carry oxygen. Its formation in drying stains contrib...
- Physical and Chemical Changes in Ageing BloodstainsAn oxidised form of haemoglobin where iron is in the ferric (Fe³⁺) state and cannot bind oxygen. Appears brown. Begins forming within minutes to hours of blood...