Search

The nature of the chromophore binding site of light-adapted bacteriorhodopsin is analyzed by using all-valence electron MNDO and MNDO-PSDCI molecular orbital theory to interpret previously reported linear and nonlinear optical spectroscopic measurements. A total of 45 binding site models are investigated. The binding site is simulated by including the chromophore, the lysine residue (LYS216), the following nearby amino acids (ARG82, ASP85, ASP115, ASP212, THR90, TRP86, TRP138, TRP182, TYR57, TYR83, and TYR185) and zero, one, or two divalent cations. We conclude that the unique two-photon properties of the chromophore are due in part to the electrostatic field associated with a Ca2+ ion near to the chromophore. Four amino acids and three water molecules contribute significantly to the assigned chromophore adjacent calcium binding site (ASP85, ASP212, TYR57 and TYR185), and two conformational minima are predicted. The higher energy conformation has the calcium ion stabilized primarily by ASP85 and the chromophore imine proton by ASP212. The lower energy conformation has the calcium ion stabilized primarily by ASP212 and the imine proton by ASP85. The latter configuration is more stable due to strong hydrogen bonding between TYR185 and ASP212 coupled with electrostatic stabilization of the divalent cation by TYR57. Although both tyrosine residues are predicted to exhibit some “unprotonated” character, models involving full deprotonation of either TYR57 or TYR185 do not fit the spectroscopic data. We conclude that the cation binding site identified in this study is the second high affinity binding site for calcium, and that the chromophore binding site is, to a first approximation, positively charged. The chromophore “1B” and “1A” states, despite extensive mixing, exhibit significantly different configurational character. The lowest-lying “1B” state is dominated by single excitations (> 80% for all models studied) whereas the second-excited “1A” state is dominated by double excitations (> 70% for all models studied with extensive participation by spin-coupled triplet-triplet excitations). © 1995 John Wiley & Sons, Inc.

Semiempirical molecular orbital theory and semiclassical solvent effect theory are used to analyze the conformational and electronic properties of the 12-s-cis and 12-s-trans conformers of 11-cis retinal. The goal is to examine the influence of solvent environment on the equilibrium geometries of these conformers as well as to provide a perspective on the electronic transitions that contribute to the four band systems that are observed in the 200-500 nm region of the optical spectrum. We conclude that the 12-s-cis isomer is more stable in vacuum, but that the 12-s-trans conformer is preferentially stabilized in both polar and nonpolar solvent environment due to dispersive as well as electrostatic interactions. This observation is in substantial agreement with previous literature results. In contrast, our analysis of the excited state manifold indicates that the spectral features observed in the absorption spectrum are associated with a complex set of overlapping transitions. A total of 18 pi*

Time-resolved circular dichroism (TRCD) spectral studies of the Pr→Pfr phototransformation in native oat phytochrome (124 kDa) were performed in the 500-800 nm spectral region from 500 ns to 500 ms after photolysis at 10 o C. The first intermediate observed in our measurements, lumi-R, had an absorption maximum at 695 nm, but its difference TRCD spectrum showed a CD maximum near 660 nm. This result differed from results of previous steady-state CD measurements where the intermediates of the photoconversion were trapped at low temperatures

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 electronic and dipolar properties of the all‐trans retinyl polyene in light‐adapted bacteriorhodopsin are examined by using two‐photon double resonance spectroscopy to assign the Franck–Condon maxima, the absolute two‐photon absorptivities and the change in dipole moments upon excitation of the low‐lying ‘‘forbidden’’ 1A*−g ‐like and ‘‘allowed’’ 1B*+u ‐like π, π* excited singlet states. The second‐order hyperpolarizability is also determined. The two‐photon double resonance spectrum, collected with laser excitation from 820–1200 nm in 10 nm steps, displays two maxima, an intense band at ∼18 000 cm−1 assigned to the 1B*+u ‐like π, π* excited singlet state and a weaker shoulder at ∼21 000 cm−1 assigned to the 1A*−g ‐like π, π* excited singlet state. Thus, the 1A*−g ‐like state is 3500±500 cm−1 above the 1B*+u ‐like state, which is indicative of a protonated Schiff base chromophore.A log‐normal fit of the two‐photon spectrum indicates that the maximum two‐photon absorptivity of the 1B*+u ‐like state is 2...

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).

This contribution reports a new approach to the synthesis of thin-film secondorder nonlinear optical (NLO) materials in which self-assembled chromophorecontaining multilayer structures are built up in the repeating reaction sequence: I3Si(CH2)2C6H4CH21 (Cp) , 4- [N,N-bis(3-hydroxypropyl)aminojstyryl-4' -pyridine (Ch), Cl3SiOSiCl2OSiCl3 (Si) , polyvinylalcohol (PVA) . The resulting multilayer structures have been characterized by optical and XPS spectroscopy, advancing contact angle measurements, null ellipsometry, and NLO properties. The latter mepirements reveal multilayer superlaces with good structural uniformity and that = 2 x l0 esu at A = 1064 nfl' (Xzz 6 x l0 esu for a single CpCh monolayer of 22 A estimated thickness).

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

During the past five years, investigators have shown considerable interest in the potential use of light transducing proteins to perform optical switching functions.1,2 There are significant advantages inherent in the use of biological molecules, either in their native form, or modified via organic synthesis or protein engineering, as active components in molecular electronic devices. These advantages derive in large part from the natural selection process, and the fact that nature has solved through trial and error problems of a similar nature to those encountered in harnessing organic molecules to carry out logic, switching and energy transducing functions. One example of this type of serendipity to be explored in the present paper is the use of the light harvesting protein, bacteriorhodopsin, as the photoactive element in an optically coupled random access memory.