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Phototropin is a blue light receptor involved in the phototropism (it is a phenomenon which is growth directed by light). This protein is a dimer ( phot1 and phot2) and each subunit contains three domains called LOV1, LOV2 and a kinase domain. LOV2 domain was,here, chosen for study. The structure of a flaving domain,LOV2,from the chimeric phototropin photoreceptor PHY3, has been determined thanks to X-Ray diffraction. This domain is composed by four chains which are identical A,B,C and D. It's a L-polypeptide which weight 50552,99 Da. Each chains has 104 aminoacids and possess four helixs(28residues which represents 26%) and six β sheets (37 residues which represent 35%). The FMN (Flavin MonoNucleotide) is the ligand which is responsible for the light absorption. A single molecule of FMN is bound non convalently in the interior of LOV2 domain. FMN is stabilized thanks to hydrogen bonds, Van der Waals and electrostatic interactions. For example, atoms R983 and R967(alpha C helix) create ionic bond with phosphate group of FMN. Q970,N965,N998,N1008(alpha A helix and beta-strand C,D and E) aminoacid make some bonds which stabilize FMN. Blue light arrives on the only LOV2 cysteine residue(situate 4.2A from atom C(4a)) and induces formation of covalent cysteinyl-C(4a)adduct. (To explain that, refer to the scheme). This complex may interact directly with the kinase and regulate is activity. Kinase will permit the autophosphorylation of phototropin. The rate of phosphorylatide phototropin acts to differential lateral gradients of auxin which is responsible of phototropism phenomenon.