In a plant blue-light receptor phototropin, the primary reaction is an adduct formation between the flavin chromophore and a cysteine S-H group. In the case of a LOV2 domain of Adiantum phytochrome3 (phy3-LOV2), we found that Arrhenius plots of the efficiency of the adduct formation provide two kinks at about 190 and 270 K, and a negative activation energy was obtained between the temperatures. They are close to the transition temperatures of protein structural changes observed by the low-temperature difference FTIR spectroscopy. We concluded that the observed temperature dependence of the reaction efficiency in a restricted protein environment originates from protein fluctuation that mainly reflects secondary structure. (c) 2005 Elsevier B.V. All rights reserved.