Substituent-controlled photoisomerization in retinal chromophore models: Fluorinated and methoxy-substituted protonated Schiff bases

The effect of substitution on the intrinsic (i.e. in vacuo) photoisomerization ability of retinal chromophore models has been explored using CASPT2//CASSCF minimum energy path computations to map the S₁ photoisomerization paths of two substituted minimal models of the retinal chromophore: the 2-cis-penta-2,4-dieniminium and the all-trans-epta-2,4,6-trieniminium cations, which have been modified using fluorine or methoxyl substituents as representative examples of electron-withdrawing and electron-releasing groups, respectively. A systematic analysis has been performed involving substitutions in all the possible positions along the chain. It is shown that the photochemical reactivity and photoisomerization efficiency of these systems may be tuned or indeed changed, although this effect strongly depends on the position of the substituent. In particular, we have shown that (i) most of the systems preserves qualitatively the reactivity of the parent (i.e. unsubstituted) compound; (ii) substitution at positions C4 or C6 leads to a different relaxed excited state structure of the chromophore and in general to a very flat photoisomerization path (or to a tiny S₁ energy barrier in some cases); (iii) the nature of the TICT state (i.e. the S₁ → S₀ decay funnel) may be turned from a conical intersection into an excited state minimum; (iv) for the C4 methoxy-substituted system the isomerization path as well as the S₁/S₀ decay funnel involve an unusual torsional angle. Thus, substitution turns out to be a good tool not only to tune the optical properties (i.e. the absorption and emission features) of the chromophore (as we have already shown in a previous work: I. Conti, F. Bernardi, G. Orlandi, M. Garavelli, Mol. Phys. 104 (2006) 915–924), but it may also play a crucial role in tuning and controlling photoisomerization selectivity and efficiency, affecting excited state lifetime and reaction rate. A rationale for these effects is presented, which provides a basis for understanding reactivity properties and the intrinsic photochemical behavior of substituted retinal chromophores.     

Complete-active-space self-consistent-field/Amber parameterization of the Lys296–retinal–Glu113 rhodopsin chromophore-counterion system

A special hybrid quantum mechanics/molecular mechanics forcefield is defined, parameterized and validated for studying the photoisomerization path of the retinal chromophore in the rhodopsin protein. It couples a multireference ab initio Hamiltonian (CASSCF and second-order multireference many-body perturbation theory using a CASSCF reference) to describe the chromophore while the rest of the protein is approximated with the Amber forcefield. The frontier has been carefully parameterized in order to reproduce full quantum mechanics torsional energy profiles, for both the ground state and the first excited state. It is also shown that replacing the chromophore counterion with point charges is a valid approximation. This result is interpreted in terms of a cancellation effect for which a possible explanation is given.     

All-trans-N-retinylidenetryptamine Schiff base in surfactant solubilized water pools in heptane—a fluorescence study

All-trns-N-retinylidenetryptamine Schiff base was incorporated into aerosol-OT (AOT, sodium bis(2-ethylhexyl)sulphosuccinate)/heptane reverse micelles. This micellar system was used as a model to study the retinal-tryptophan interactions in retinal proteins. The retinylidene Schiff base remains stable in the presence of reverse micelle-solubilized water pools. Partition coefficient and microviscosity measurements show that the Schiff base is located in the micellar interphase. The results are discussed in terms of the interaction between the retinylidene chromophore and the active site environment of rhodopsin and bacteriorhodopsin. In the present model, the quencher and emitting units are covalently attached, and are separated by two carbon spacer units. The fluorescence emission data obtained for the micelle-intercalated Schiff base chromophore are compared with the fluorescence of the native protein and intermediates in the photochemical cycle of bacteriofhodopsin. A comparison of the data obtained for tryptamine and the Schiff base with the results available for bacteriorhodopsin and bacterioopsin reveals that there is a large degree of quenching on intercalation of the retinylidene chromophore in the vicinity of the fluorophore. Evidence provided by this model suggests that energy transfer to retinal can occur from tryptophan residues located in the retinal pocket in the native protein. Thus the retinylidene unit can act as a quencher of the energy of tryptophan, the nature and extent of which may depend on the conformation and relative orientation of the protein-bound fluorophore.     

Single-run analysis of retinal isomers, retinol and photooxidation products by high-performance liquid chromatography

A high-performance liquid chromatographic (HPLC) procedure was developed to allow the rapid separation, in a single run, of a mixture of the main retinal isomers (all-trans, 13-cis, 9-cis), all-trans-retinol, and of the two major photooxygenated photoproducts (5,8-peroxyretinal and 5,6-epoxyretinal). The mixture was separated by HPLC on an octadecyl (ODS) column with 16% (v/v) diethyl ether in hexane as mobile phase and anthracene as the internal standard. A commercial type cosmetic formulation containing 0.05% all-trans-retinal was analyzed successfully for this analyte.     

Field-induced motion of ferrofluids through immiscible viscous media: Testbed for restorative treatment of retinal detachment

Biocompatible, hydrophobic ferrofluids comprised of magnetite nanoparticles dispersed in polydimethylsiloxane show promise as materials for the treatment of retinal detachment. This paper focuses on the motion of hydrophobic ferrofluid droplets traveling through viscous aqueous media, whereby the movement is induced by gradients in external fields generated by small permanent magnets. A numerical method was utilized to predict the force on a spherical droplet, and then the calculated force was used to estimate the time required for the droplet to reach the permanent magnet. The calculated forces and travel times were verified experimentally.     

Syntheses of C2-labelled 11Z-retinals

To enable solid-state NMR investigations of the rhodopsin chromophore and its photointermediates, a series of 11Z-retinal isotopomers have been synthesised containing pairs of adjacent ¹³C labels at C9/C10, C10/C11 or C11/C12, respectively. The C9 labelled carbon atom was introduced through the Heck reaction of a ¹³C-labelled Weinreb acrylamide derivative, and the label at the C12 position derived from a ¹³C-containing ethoxy Bestmann-Ohira reagent. The ¹³C labels at C10 and C11 were introduced through the reaction of β-ionone with labelled triethyl phosphonoacetate.     

Novel organelles in primate retinal epithelium

We are investigating age-related changes in organelles in monkey retinal epithelium using transmission and analytic electron microscopy. We previously described a circular organelle in retinal epithelium with a diameter of about 0.5 μm. The organelle is unique in containing a single, round vacuole within an otherwise electron dense interior. We suggested that the organelle might be a melanosome with lysosomal properties. We now find that there are two similar organelles with such a single vacuole but which differ in their chemical composition, electron density, cell location and according to age.Epon embedded sections from the macular epithelium of seven monkeys, ranging from 1 to 35 years of age, were examined by transmission electron microscopy. A seven year old monkey was processed for analytic electron microscopy to determine the chemical composition of the organelles. The number and location of the organelles in the retinal epithelium were determined.The chemical composition of these two organelles was different. One of the organelles contained high mole fractions of oxygen and nitrogen and little phosphorous characteristic of melanin; the other had little oxygen and nitrogen and higher mole fractions of phosphorous uncharacteristic of melanin, but more common with lysosomal organelles. The latter had an electron dense rim around the vacuole, a less electron dense interior than the melanin containing organelle and also contained iron. The melanin containing organelle was more common in young monkeys and in the middle third of the cell. The organelle without melanin was more common in old monkeys and localized in the basal third of the cell.Two similarly vacuolated organelles, not identified before in retinal epithelium, differ in their chemical composition. One contains melanin; the other does not. The former is more common in young and the latter more common in old monkeys. This suggests reorganization and or degradation of melanin-containing organelles with age. These changes show how analytic electron microscopy can distinguish major ultra-structural differences in organelles when mere observation fails to do so easily.     

不同观察者锥细胞响应的同色异谱性能差异测试分析

为了研究不同年龄色觉正常观察者在不同观察条件下的锥细胞光谱响应差异,在CIELAB颜色空间选择了均匀分布的9个不同色调的颜色,组织27～35名观察者开展了两种不同光源照度和两种不同观察视场的四种观察条件下,基于显示色-打印色的颜色匹配实验。通过配色实验采集到不同观察者匹配颜色的光谱数据,与9个目标色的光谱数据计算、比较,以匹配的9个颜色最小平均CIEDE2000色差值出现的概率大小作为评价标准,检验CIE1931,CIE1964,CIE2006,Sarkar(S1～S8)共11种颜色匹配函数的表现,研究不同观察者锥细胞响应的同色异谱性。结果表明在四种不同实验条件下,表现最好的配色函数依次是CIE2006,S6,表现最差的是S1和S2。不同观察者的锥细胞响应同色异谱性差异较大,年龄因素对观察者的锥细胞响应的影响也较为明显。(λ)通道的光谱响应差异是造成不同观察者存在同色异谱现象的主要原因,大多数观察者的差异性主要集中在(λ)通道的峰值波长响应,峰值波长的位移对颜色匹配的结果影响不大。实验结果可为进一步划分不同观察者的颜色匹配函数,研究不同观察者的同色异谱性提供数据支持和理论依据。     

人眼视网膜成像液晶自适应光学系统的优化设计

针对已有的人眼视网膜成像液晶自适应光学系统的不足,提出了新的优化设计方案.新设计的系统能对不同视度下的人眼进行高分辨率成像.新系统还采用了瞳孔监控装置和成像区域快速精确定位装置,并且采用了改进的消杂散光方法,能够使探测准确度和定位准确度得到保证.研究证明,该系统新的设计方案操作方便、灵活,便于推广使用.     

About the intrinsic photochemical properties of the 11-cis retinal chromophore: computational clues for a trap state and a lever effect in Rhodopsin catalysis

CASPT2//CASSCF/6-31G* computations are used on the singlet S ₁ and S ₂ states to map the photoisomerization process of the 11-cis retinal protonated Schiff base in vacuo and to characterize its optical properties. It is shown that the spectroscopic observations recorded in Rhodopsin are reproduced quite well, calling for a substantially neutral effect of the protein. Furthermore, a rationale is proposed for the unreactive population recently observed in Rhodopsin, which is here addressed to the accessible S ₂ state, behaving as a trap. The experimental transient absorption and (absorption-wavelength dependent) emission are discussed and interpreted under the light of this novel model. Finally, a planarization of the β-ionone ring is observed on S ₁, which may cause a steric lever effect into the protein pocket, thus assisting photoisomerization catalysis. The reported results constitute a solid reference for further studies aimed to rationalize the effect of the environment on the photochemical reactivity of retinal chromophores. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.