Counterion-controlled spectral tuning of the protonated Schiff-base retinal

Abstract

Color vision is based on the ability of different Opsin proteins to tune the absorption band of their chromophore: the retinal protonated Schiff base (RPSB). Two main mechanisms proposed for this tunability are geometric and electrostatic. Here we probe the latter effect experimentally and by a quantum chemical calculation of the absorption by an isolated complex containing the retinal chromophore and molecules with a strong dipole moment. Betaine complexation causes an anomalously large blue shift. The shift provides direct evidence that the strong charge-transfer character of the electronic transition is the cause of the opsin shift, and shows that the electric field of the counterion is responsible for the color tuning which allows absorption of light in the blue region of the visible spectrum by opsin proteins.

Intrinsic photoisomerization dynamics of protonated Schiff-base retinal

“…4e, f ). The mechanism of internal conversion is thus defined solely by the interplay between the ground and first excited states in RPSB, where the latter is dominated by a charge-transfer character, as also demonstrated recently for RPSB in the gas phase 43 .…”

Gas-phase studies of the retinal protonated Schiff base chromophore

“…b PISA measurements of the photon absorption band of different RPSB isomers [29]. c ZITA measurement of the photoabsorption band of a complex containing N.N.N.trimehtyl glycine betain, non-covalently bound to the dimethylated RPSB derivative [30]…”

“…Taken together, the studies of RPSB derivatives and the PISA studies show that geometric effects can tune the RPSB absorption within the 500−650 nm absorption band, but cannot alone explain the existence of blue absorbing opsins. In a recent study, zwitterion tag action spectroscopy (ZITA) was used to show that electrostatic effects, and specifically the interaction with the protein's negative counterion, are the dominant spectral tuning mechanism, allowing opsin proteins to absorb light in the blue regions of the visible spectrum [30].…”

“…The technique was established by studying a range of chromophores, showing that in chromophores where the photon-absorption has chargetransfer character, betaine complexation causes a blueshift, which is not seen for symmetric chromophores for which photo-absorption does not involve charge transfer [37,38]. Indeed, a ZITA study of the RPSB has shown an anomalously large blue shift of the chromophores photo-absorption band [30] (significantly larger than any previously measured blue shift by the ZITA technique [37,38]). One complication of this study was that fragmentation of the RPSB-betaine complex resulted in two charged fragments corresponding to either the bare RPSB or to protonated betaine.…”

Intrinsic Photophysics of Light‐harvesting Charge‐tagged Chlorophyll a and b Pigments

“…To obtain the intrinsic properties, the molecule has to be studied isolated in the gas phase. After the absorption spectrum of the bare Chl is established, perturbations can be included by complexation of the pigment with various adducts to study the effect of individual interactions …”