[PDF]    http://dx.doi.org/10.3952/lithjphys.50404

Open access article / Atviros prieigos straipsnis

Lith. J. Phys. 50, 451–462 (2010)

B.P. Kietis a, M. Macernis b, J. Sulskus b, and L. Valkunas a,b
a Center for Physical Sciences and Technology, Institute of Physics, Savanorių 231, LT-02300 Vilnius, Lithuania
E-mail: mindaugas.macernis@ff.vu.lt
b Department of Theoretical Physics, Faculty of Physics, Vilnius University, Saulėtekio 9, LT-10222 Vilnius, Lithuania

Received 16 November 2010; revised 1 December 2010; accepted 15 December 2010

The static electric dipole moment persisting in bacteriorhodopsin was defined from electro-acoustic measurements of the dried films of purple membranes and compared with the value estimated from quantum chemical calculations. The projection of this value normal to the membrane surface is experimentally estimated to be equal to 40 D and oriented from the cytoplasmic side to the extracellular side of the membrane. This value is almost independent of the environment pH. QM/MM calculations were also performed for the known structures of the ground and intermediate states of bacteriorhodopsin. According to calculations the dipole moment is mainly determined by the cytoplasmic and extracellular coils, while the contribution from the transmembrane helices is smaller and of the opposite direction, and this value corresponding to the active centre is small. The calculated values of the dipole moment of bacteriorhodopsin in the intermediate states K, L, and M provide understanding about the origin of the driving force for the proton pumping. Employing the values of the dipole moments corresponding to the ground and intermediate states of bacteriorhodopsin, defined by means of QM/MM calculations, the experimentally determined photoelectric response of the dried films is explained.
Keywords: dried films of purple membranes, electrostriction, photo-electric response, pH changes, molecular mechanics calculations, quantum chemical calculations
PACS: 87.14.ep, 77.65.-j, 82.80.Yc, 83.10.Rs, 03.65.-w

B.P. Kietis a, M. Mačernis b, J. Šulskus b, L. Valkūnas a,b
a Fizinių ir technologijos mokslų centro Fizikos institutas, Vilnius, Lietuva
b Vilniaus universiteto Fizikos fakultetas, Vilnius, Lietuva

Bakteriorodopsino elektrinis dipolinis momentas įvertintas iš purpurinių membranų sausose plėvelėse elektroakustinių matavimų. Nustatyta dipolinio momento projekcijos į membranos paviršiaus normalę vertė lygi 40 D, kryptis – nuo citoplazminės į išorinę membranos pusę bei nepriklauso (arba labai mažai priklauso) nuo aplinkos pH. Norint atskleisti dipolinio momento susidarymo prigimtį, buvo atlikti žinomų bakteriorodopsino pagrindinės ir tarpinių struktūrų QM/MM skaičiavimai, iš kurių paaiškėjo, kad dipolinis momentas daugiausia priklauso nuo citoplazminės ir išorinės pusės membranos nebaltyminių dalių. Tarpmembraninės spiralės dipolinio turimo momento dalis yra mažesnė ir nukreipta priešingai. Apskaičiuotoji aktyvaus centro dipolinio momento vertė, lyginant su visos struktūros momento verte, yra maža. Gautos bakteriorodopsino dipolinio momento vertės K, L ir M būsenoms leidžia geriau suprasti protonovaros prigimtį. Pagrindinės ir tarpinių būsenų dipolinio momento vertės, apskaičiuotos QM/MM metodu, buvo panaudotos eksperimentiškai gautam sausų plėvelių fotoelektriniam atsakui paaiškinti.

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