Nonlinear Optical Properties of Bacteriorhodopsin: Assignment of the Third-order Polarizability Based on Two-photon Absorption Spectroscopy

The third-order (pi) -electron polarizability, (gamma) (pi), of bacteriorhodopsin in the 0.0 - 1.2 eV optical region is assigned based on an analysis of the experimental two-photon properties of the low-lying singlet state manifold. The following selected values of (gamma) (pi) (units of 10-36 esu) are observed: (gamma) (0;0,0,0) equals 2482 +/- 327; (gamma) (-3(omega) ;(omega) ,(omega) ,(omega) ) equals 2976 +/- 385 ((omega) equals 0.25 eV), 5867 +/- 704 ((omega) = 0.5 eV), 14863 +/- 1614 ((omega) = 0.66 eV), 15817 +/- 2314 ((omega) equals 1.0 eV), 10755 +/- 1733 ((omega) equals 1.17 eV). The third-order polarizability of this protein which contains an all-trans retinyl protonated Schiff base chromophore with six double bonds, is comparable to that observed for much longer chain polyenes (for example, dodecapreno (beta) -carotene, a polyene with 19 double bonds, exhibits a third-order (pi) -electron polarizability at 0.66 eV of 17000 +/- 6000 X 10-36 esu. The authors attribute the enhanced third-order nonlinearity associated with the protein bound chromophore of bacteriorhodopsin to two mutually enhancing origins. First, the chromophore is protonated, and the resultant charge reorganization enhances the polarizability in a fashion that is similar to that known to occur for polaronic and bipolaronic chromophores. It is estimated that protonation generates a five-fold enhancement in (gamma) (pi). Second, the protein bound chromophore exhibits a large change in dipole moment upon excitation into the lowest-lying, strongly-allowed 1B*u+-like state ((Delta) (mu) = 13.5 D). The latter property is responsible for a Type III enhancement of the third-order polarizability, and yields at least a 20-fold increase in (gamma) (pi)

Origins of the Quantum Efficiency Duality In the Primary Photochemical Event of Bacteriorhodopsin

Experimental and theoretical evidence is presented which suggests that two distinct forms of light-adapted bacteriorhodopsin may exist. We propose that these two forms have characteristic photocycles with significantly different primary quantum yields. INDO-PSDCI molecular orbital procedures and semiempirical molecular dynamics simulations predict that one ground state geometry of bR undergoes photochemistry with a primary quantum yield, Φ1, of ~ 0.27, and that a second ground state geometry, with a slightly displaced counterion, yields Φ1 ~ 0.74. This theoretical model is supported by the observation that literature measurements of Φ1 tend to fall into one of two categories- those that observe Φ1 ~ 0.33 or below, and those that observe Φ1 ~ 0.6 or above. The observation that all photostationary state measurements of the primary quantum yield give values near 0.3, and all direct measurements of the quantum yield result in values near 0.6, suggests that photochemical back reactions may select the bacteriorhodopsin conformation with the lower quantum yield. The two photocycles may have developed as a natural biological requirement that the bacterium have the capacity to adjust the efficiency of the photocycle in relation to the intensity of light and/or membrane electrochemical gradient

Nonlinear Optical Properties of Bacteriorhodopsin: Assignment of Second Order Hyperpolarizabilities of Randomly Oriented Systems Using Two-Photon Spectroscopy

We demonstrate that the second order hyperpolarizability of a randomly oriented molecule can be determined directly from two-photon spectroscopic measurements on the low-lying excited state manifold. Equations are derived which allow not only a determination of β, but also a determination of the error associated with the numerical method. We apply our two-photon technique to an analysis of the second order hyperpolarizability of light adapted bacteriorhodopsin. Our analysis of this protein in D2O at ambient temperature yields a value of β (= βxxx + (1/3)[βxyy + 2βyyx + βxzz + 2βzzx]) of (2250 ± 240) × 10−30 cm5/esu for a laser wavelength of 1.06μ (Nd:YAG fundamental). The large second-order nonlinear properties of bacteriorhodopsin are due primarily to the large change in dipole moment associated with excitation into the lowest-lying strongly allowed “1Bu +” π, π* state (Δμ = 13.5 ± 0.8 D). We derive an equation which estimates Ωβδ, the ratio of the number of second harmonic photons generated by the system divided by the number of photons absorbed by the system via two-photon processes. Our analysis indicates that molecular doubters can be optimized by maximizing the oscillator strength of the low-lying charge transfer state (fso), the orientation angle of the transition dipole with the polarization of the laser flux, the sample length and the chromophore concentration. All of the above manipulations will also increase the efficiency of doubling, and thus optimization of these parameters is critical to overall doubling performance

Revised Assignment of Energy Storage in the Primary Photochemical Event in Bacteriorhodopsin

Bactcriorhodopsin is the light-transducing protein in the purple membrane of Halobacterium halobium. Irradiation of the light-adapted form (bR) initiates a photocycle that pumps protons across the membrane. An accurate assignment of the energy storage associated with the primary event of bR is important to an understanding of the molecular mechanism and the stoichiometry of proton pumping. Previous photocalorimetric studies have concluded that ~16 kcal mol-1 is stored in the K photoproduct, an energy sufficient to pump two protons per photocycle

A Spectroscopic, Photocalorimetric, and Theoretical Investigation of the Quantum Efficiency of the Primary Event in Bacteriorhodopsin

The spectroscopic, photochemical, and energetic properties of the primary event of light-adapted bacteriorhodopsin (bR) are investigated with pulsed laser cryogenic photocalorimetry, photostationary-state spectral analysis, INDO-PSDCI molecular orbital theory, and semiempirical molecular dynamics theory. The principal goal is to explore the photophysical origins of the controversy concerning the primary quantum yield. ... Experimental and theoretical evidence is presented which suggests that two distinct forms of light-adapted bacteriorhodopsin may exist. We propose that these two forms have characteristic photocycles with significantly different primary quantum yields