Timeresolved UV/Vis measurements were performed to determine the kinetics of the photocycle intermediates. The decay of the LOV715 triplet state is characterized by a time constant of 1.460.2 ms. This value agrees fairly well with the time constant of 2.8 ms as derived by light-induced transient-grating spectroscopy and is in the range of other LOV domains like phototropin1 LOV1, LOV2 and YtvA. Unfortunately, it was impossible to record the rise kinetics of the LOV390 state due to the triplet state LOV715 absorption in this region that cancels the former changes. Upstream of the LOV domain, aureochrome 1 harbors a bZIP domain. The latter is known to bind DNA after dimerization of the leucine zipper domain with its basic region at the major grove. By formation of a coiled coil of two Piperaquine tetraphosphate tetrahydrate helices, a Y shape structure is created that can bind the target DNA by specific interactions of the C-terminal basic region with the DNA, mostly via hydrogen bonds. The interaction of DNA and the bZIP domain is reflected in the FTIR difference spectrum. We observe a negative band at 1131 cm21 that is assigned to the P-O stretching vibration with single bond character. Formation of a hydrogen bond to one of the phosphate oxygens leads to strengthening of the double bond character of the other P-O bond. As a consequence, the stretching vibration with a double bond character shows up at 1193 cm21. Furthermore, asymmetric and symmetric stretching vibrations assigned to arginine side chains are observed at 1630 and 1670 cm21. At the present stage, we are not able to assign the vibrations to specific arginine residues due to the lack of proper point mutants. Additionally, peaks at 1539 and 1630 cm21 corresponding to the asymmetric and symmetric deformation vibration of the NH3+ group, Chromanol 293B indicate the involvement of lysine side chains. The rise of these bands reflects the formation of hydrogen bonds between the terminal NH3 + groups of lysine residues and the P-O2 groups of the phosphate sugar backbones. The increase in intensities of the bands in the amide I and II region that are overlapping with the C=O stretching region of the nucleotide bases, are indicative for structural changes of the apoprotein as well as changes in the hydrogen bonded network itself. The structural changes probably include the partially unfolding of the Ja-helix which was suggested to be involved in the internal signal transduction as concluded from previous FTIR studies.