Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). The parent porphyrin is porphin, and substituted porphines are called porphyrins. The porphyrin macrocycle has 26 (delocalized) pi electrons in total, therefore by Hückel's rule it is aromatic, possessing 4n+2 π electrons (n=4, for the shortest cyclic path). Thus porphyrin macrocycles are highly conjugated systems and consequently they typically have very intense absorption bands in the visible region and may be deeply colored; the name “porphyrin” comes from the Greek word porphyros, meaning purple.[
Many porphyrins are naturally occurring; one of the best-known porphyrins is heme, the pigment in red blood cells, a cofactor of the protein hemoglobin. (The specific porphyrin in heme B is called protoporphyrin IX and has 4 methyl, two vinyl, and two propionic acid substituents at the indicated positions.)
5-aminolevulinic acid (5-ALA) fluorescence-guided resection is a technique used in neurosurgery, particularly for brain tumor resection, including metastases. This method involves the administration of 5-ALA to the patient before surgery, which is then metabolized by the tumor cells to produce fluorescent porphyrins. The fluorescence emitted by these porphyrins can be visualized using special surgical microscopes equipped with a blue light source, allowing neurosurgeons to distinguish between normal and cancerous tissue during the operation.