Aborted double bicycle-pedal isomerization with hydrogen bond breaking is the primary event of bacteriorhodopsin proton pumping.
Quantum mechanics/molecular mechanics calculations based on ab initio multiconfigurational second order perturbation theory are employed to construct a computer model of Bacteriorhodopsin that reproduces the observed static and transient electronic spectra, the dipole moment changes, and the energy stored in the photocycle intermediate K. The computed reaction coordinate indicates that the isomerization of the retinal chromophore occurs via a complex motion accounting for three distinct regimes: (i) production of the excited state intermediate I, (ii) evolution of I toward a conical intersection between the excited state and the ground state, and (iii) formation of K. We show that, during stage ii, a space-saving mechanism dominated by an asynchronous double bicycle-pedal deformation of the C10═C11─C12═C13─C14═N moiety of the chromophore dominates the isomerization. On this same stage a N─H/water hydrogen bond is weakened and initiates a breaking process that is completed during stage iii.
Dipartimento di Chimica G. Ciamician, Università di Bologna, via Selmi 2, Bologna, I-40126 Italy.
Piero
P
Altoè
Alessandro
A
Cembran
Massimo
M
Olivucci
Marco
M
Garavelli
D001436:Bacteriorhodopsins / Q000737:chemistry*; D001665:Binding Sites / Q000235:genetics; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D007536:Isomerism; D008956:Models, Chemical; D008958:Models, Molecular; D015394:Molecular Structure; D011522:Protons; D011789:Quantum Theory; D013057:Spectrum Analysis
D011522:Protons; D001436:Bacteriorhodopsins