Rhodopsins carrying modified chromophores — the ‘making of’, structural modelling and their light-induced reactivity

Abstract

A series of vitamin-A aldehydes (retinals) with modified alkyl group substituents (9-demethyl-, 9-ethyl-, 9-isopropyl-, 10-methyl, 10-methyl-13-demethyl-, and 13-demethyl retinal) was synthesized and their 11-cis isomers were used as chromophores to reconstitute the visual pigment rhodopsin. Structural changes were selectively introduced around the photoisomerizing C11=C12 bond. The effect of these structural changes on rhodopsin formation and bleaching was determined. Global fit of assembly kinetics yielded lifetimes and spectral features of the assembly intermediates. Rhodopsin formation proceeds stepwise with prolonged lifetimes especially for 9-demethyl retinal (longest lifetime τ3 = 7500 s, cf., 3500 s for retinal), and for 10-methyl retinal (τ3 = 7850 s). These slowed-down processes are interpreted as either a loss of fixation (9dm) or an increased steric hindrance (10me) during the conformational adjustment within the protein. Combined quantum mechanics and molecular mechanics (QM/MM) simulations provided structural insight into the retinal analogues-assembled, full-length rhodopsins. Extinction coefficients, quantum yields and kinetics of the bleaching process (μs-to-ms time range) were determined. Global fit analysis yielded lifetimes and spectral features of bleaching intermediates, revealing remarkably altered kinetics: whereas the slowest process of wild-type rhodopsin and of bleached and 11-cis retinal assembled rhodopsin takes place with lifetimes of 7 and 3.8 s, respectively, this process for 10-methyl-13-demethyl retinal was nearly 10 h (34670 s), coming to completion only after ca. 50 h. The structural changes in retinal derivatives clearly identify the precise interactions between chromophore and protein during the light-induced changes that yield the outstanding efficiency of rhodopsin.

Evidence for a vibrational phase-dependent isotope effect on the photochemistry of vision

“…Partial deuteration of the retinal backbone has only a small effect on the torsional mode progression and consequently on the movement of the ground and excited electronic states towards the CI, because the torsion is predominantly localized on the carbon framework. 24,43 Since the surface hopping probability is directly linked to the evolution of this torsion to values close to 90 degrees, no significant kinetic changes are expected upon deuteration in agreement with our experimental and theoretical observation of a weak isotope-dependence on the excited-state lifetime. The main difference between the isotopomers used here is that the C11-H and C12-H wags are coupled for native 11,12-H2 rhodopsin and for 11,12-D2, resulting in the formation of symmetric and anti-symmetric HOOP vibrations where the two hydrogen (deuterium) displacements maintain a specific phase relationship with the carbon torsional coordinate until the molecule decays via the CI.…”

Induced Night Vision by Singlet-Oxygen-Mediated Activation of Rhodopsin

“…RET photoisomerization, following visible-light absorption, is an ultrafast and efficient photoprocess occurring on the subpicosecond time scale, mediated by a conical intersection between excited (S 1 ) and ground (S 0 ) electronic states, as proven by a number of theoretical and experimental studies. − From a chemical standpoint, such a process can be viewed as the breaking of the double-bond character of the isomerizable bond due to direct population of a π–π* electronic state, followed by an accordingly easier rotation. The space-saving isomerization mechanism within the opsin cavity − and the crucial role of the environment in modulating photochemical efficiency and maximum-wavelength absorption , have been also addressed, while biomimetic switches based on RET electronic and chemical structure have been proposed and synthesized. , Even though the most celebrated role of RET isomerization is connected to vision, it should be noted that in different organisms, or cell types, the response to photoisomerization triggers entirely different cascades of processes. For instance, bacteriorhodopsin in bacteria and channelrhodopsins in green algae are able to induce the opening of ion channels in the membrane − and as such are now broadly used in optogenetics .…”

“…4−7 From a chemical standpoint, such a process can be viewed as the breaking of the double-bond character of the isomerizable bond due to direct population of a π−π* electronic state, followed by an accordingly easier rotation. The space-saving isomerization mechanism within the opsin cavity 8−10 and the crucial role of the environment in modulating photochemical efficiency and maximum-wavelength absorption 11,12 have been also addressed, while biomimetic switches based on RET electronic and chemical structure have been proposed and synthesized. 13,14 Even though the most celebrated role of RET isomerization is connected to vision, it should be noted that in different organisms, or cell types, the response to photoisomerization triggers entirely different cascades of processes.…”

Airway smooth muscle photorelaxation via opsin receptor activation

“…Photosensitivity is achieved as 11-cis retinal forms a covalent bond with the previously mentioned OPN receptor lysine residue to create a "Schiff base," a retinylidene protein (also known as the retinal OPN complex). The resultant retinylidene protein exhibits differential activation parameters (excitation wavelength) and differential downstream G protein couplings (i.e., G s vs. G q ), which are thought to be determined by variations in combination between different OPN receptor and chromophore subtypes (12,22,24,30,35). We found that an exogenously added 9-cis retinal pretreatment resulted in a dose-dependent enhancement of blue light-mediated relaxation.…”