title: Energetics and role of the hydrophobic interaction during photoreaction of the BLUF domain of AppA.
abstract: A recently developed method for time-resolved thermodynamic measurements was used to study the photochemical reaction(s) of the BLUF domain of AppA (AppA-BLUF), which has a dimeric form in the ground state, in terms of the energetics and heat capacity changes (DeltaC(p)) in different time domains. The enthalpy change (DeltaH) of the first intermediate that forms within 1 ns after photoexcitation was 38 (+/-8) kJ mol(-1) at 298 K. The heat capacity change (DeltaC(p)) upon formation of this intermediate was positive [1.4 (+/-0.3) kJ mol(-1) K(-1)]. This positive DeltaC(p) suggests that the hydrophobic surface area of AppA-BLUF exposed to the bulk solvent increased. After this initial transition, a dimerization reaction with another ground-state dimer (i.e., tetramer formation) takes place. Upon this reaction, the energy was stabilized to 26 (+/-6) kJ mol(-1) at 298 K. Interestingly, the dimer formation was accompanied by a larger but negative DeltaC(p) [-6.0 (+/-1) kJ mol(-1) K(-1)]. This negative DeltaC(p) might indicate buried hydrophobic residues at the interface of the dimer and/or the existence of trapped water at the interface. We suggest that hydrophobic interactions are the main driving force for the formation of the dimer upon photoactivation of AppA-BLUF.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Partha
authors.initials: P
authors.lastname: Hazra
authors.forename: Keiichi
authors.initials: K
authors.lastname: Inoue
authors.forename: Wouter
authors.initials: W
authors.lastname: Laan
authors.forename: Klaas J
authors.initials: KJ
authors.lastname: Hellingwerf
authors.forename: Masahide
authors.initials: M
authors.lastname: Terazima
mesh_terms: D000465:Algorithms; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D015226:Dinucleoside Phosphates / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004735:Energy Transfer; D010777:Photochemistry; D012242:Rhodobacter sphaeroides / Q000737:chemistry / Q000235:genetics; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D015226:Dinucleoside Phosphates; C012276:diadenosine pyrophosphate