Abstract
Quadratic electro-optic effects (dc or low frequency Kerr effect) of bacteriorhodopsin dispersed in dried gelatin thin films are examined in the near resonance region at three wavelengths: 633, 647, and 676 nm. The films show relatively large quadratic electro-optic effects compared to other molecular dispersed systems. The purple membrane is fixed within the polymerized gelatin matrix, and we show that the electronic contribution to γ dominates over possible orientational contributions. At 676 nm, the quadratic electro-optic coefficient s1133(−ω;0,0,ω) is 6.7×10−20 m2/V2 and the third order nonlinear susceptibility χ1133(3)(−ω;0,0,ω) is 7.0×10−13 cm4 statCoulomb−2, with both values obtained for a protein concentration of 6.9×1018 cm−3. The orientationally averaged second molecular hyperpolarizability 〈γ(−ω;0,0,ω)〉 determined from the quadratic electro-optic coefficients at 676 nm assuming an Onsager ellipsoidal local field factor is (10.8±5.1)×10−32 cm7 statCoulomb−2 [(1.34±0.63)×10−56 F3 m4 C−2]. The 〈γ(−ω;0,0,ω)〉 value increases roughly tenfold when the probe wavelength is decreased to 633 nm. The behavior of γ(−ω;0,0,ω), when fit to a two-state model, predicts that γ(−ω;0,0,ω) is strongly enhanced via type III processes. Thus, the magnitude of γ(−ω;0,0,ω) is dominated by a term (Δμ102×μ102)/(ω10−ω)3, where Δμ10 is the change in dipole moment, μ10 is the transition moment, and ω10 is the transition energy of the lowest-lying allowed 1Bu*+-like π,π* state. We calculate that Δμ10 is 12.8±1.2 D, in good agreement with previous Stark and two-photon experimental values. Time-dependent Hartree–Fock methods based on the MNDO Hamiltonian yield reasonable agreement with experiment, underestimating γ(−ω;0,0,ω) by factors of only 2–4, with the error increasing as the frequency approaches resonance.
Highlights
BacteriorhodopsinBR, M.W.Ϸ26 000͒ is the light transducing protein found in the purple membrane formed in the cell wall of the bacterium, Halobacterium salinarium.[1,2,3,4,5,6,7,8,9,10,11]
We report the direct observation of ␥ of bR dispersed in dried gelatin thin films in the near resonance region
The electromagnetic field associated with the laser light is modulated with a change in the phase difference between s- and p-polarized component caused via linear electro-optic effects and/or quadratic electrooptic effectalso referred to as dc or low frequency Kerr effects
Summary
Quadratic electro-optic effects in bacteriorhodopsin: Measurement of ␥„؊;0,0,... in dried gelatin thin films. Quadratic electro-optic effects in bacteriorhodopsin: Measurement of ␥„؊;0,0,... Mikio Yamazaki,a) Jerry Goodisman, and Robert R. M. Keck Center for Molecular Electronics, Syracuse University, Syracuse, New York 13244-4100
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