A Soret Peak or Soret Band is an intense peak in the blue wavelength region of the visible spectrum. The peak is named after its discoverer Jacques-Louis Soret

Jacques-Louis Soret

Jacques-Louis Soret was a Swiss chemist who in 1878, along with Marc Delafontaine, first observed holmium spectroscopically. Independently, Per Teodor Cleve separated it chemically from thulium and erbium in 1879...

. The term is commonly used in absorption spectroscopy

Absorption spectroscopy

Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating field. The intensity of the absorption varies as a...

, corresponding to a wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 of maximum absorption

Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

. The "soret peak" is used to describe the absorption of vividly-pigmented heme-containing moieties, such as various cytochromes. For example, the cytochrome

Cytochrome

Cytochromes are, in general, membrane-bound hemoproteins that contain heme groups and carry out electron transport.They are found either as monomeric proteins or as subunits of bigger enzymatic complexes that catalyze redox reactions....

s P450, a diverse class of monooxygenase

Monooxygenase

Monooxygenases are enzymes that incorporate one hydroxyl group into substrates in many metabolic pathways. In this reaction, two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation of NADH.-Classification:...

 enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s, exhibit a Soret peak at 450 nm in their reduced form when saturated with carbon monoxide

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

(thus their name). This is called a CO difference spectrum assay: the absorbance spectrum of the solution is subtracted from the spectrum after saturation with carbon monoxide. If the P450 has been denatured, for example due to bad handling, the peak shifts to 420nm. It is possible to see both 420nm and 450nm peaks on the same spectrum if only a portion of the P450 has been denatured.