Retinal has a highly dipolar vertically excited singlet state: implications for vision.

Abstract

We have measured the effect of an intense electric field on the absorption spectrum of solutions of all-trans retinal, its unprotonated Schiff base with n-butylamine, and the Cl-salt of this protonated Schiff base. The field-induced change in extinction coefficient as a function of wavelength was analyzed to determine the ground-state dipole moment (), the change in dipole moment on excitation (Z), and the direction of A and AU. These experiments have shown that all three species become highly dipolar upon excitation to the first allowed excited singlet state (1IAA = 15.6,9.9,12 D, respectively). The chromophore in all known vertebrate visual pigments is I1-cis retinal (or dehydroretinal) which is isomerized by light to all-trans retinal (1). In rhodopsin, 11-cis retinal is attached to a specific lysine residue of opsin by a protonated Schiff base linkage (2-6). The known properties of this chromophore have recently been reviewed (7,8). A more detailed knowledge of the excited state properties of retinal should facilitate the elucidation of its function since its absorption spectrum and mechanism of isomerization depend on the character of its excited electronic states. However, relatively few direct measurements have been made on the excited states of retinal because of the difficulty of such experiments.We report here the first determination of the ground and excited state dipole moments of all-trans retinal, its Schiff bases, and 11-cis retinal. This was accomplished by measuring the effect of an intense electric field on the absorption spectrum of solutions of these molecules. The Schiff bases of the all-trans isomer were chosen for this initial study because the interpretation of their electric field spectra is more direct than for those of the 11-cis isomer. The striking finding is that excitation of all-trans retinal and its Schiff bases is accompanied by a large shift in negative charge away from the ionone ring end of the molecule. These observations help to characterize the excited states and isomerization mechanism of retinal. Furthermore, these measurements give insight into the interactions between retinal and opsin during the initial stages of visual excitation.

Resonant optical rectification in bacteriorhodopsin

“…However, experiments with modified bR containing nonisomerizable retinal analogs challenged this model by showing that the initial photo-induced events are not associated with retinal isomerization (8)(9)(10). In fact, in an early alternative hypothesis (11), the essential process was proposed to be dielectric relaxation of the protein as a response to sudden polarization upon retinal excitation (11)(12)(13). Recent molecular dynamics calculations (14) and experiments (15,16) support this view.…”

Mechanism of a proton pump analyzed with computer simulations

“…The dipole moment of retinal changes significantly upon excitation: In Cl -salt, the change in the dipole moment upon excitation of protonated all-trans retinal is 12D [96]; SORCI gives a value of *10D for bacteriorhodopsin [48]. As a result of the dramatic dipole moment change, the protein polarization is different in the ground and excited states.…”

Characterization of the Light-Harvesting Antennas of Photosynthetic Purple Bacteria by Stark Spectroscopy. 2. LH2 Complexes: Influence of the Protein Environment

“…The molecules are embedded in a protein environment which may influence the value of the internal electric field, F int ) f*F ext . 30,32 Since f is a parameter of which it is hard to give a proper estimate, all values for |∆µ| and Tr-(∆r) will be represented in terms of D/f and Å 3 /f 2 , respectively (1 D ) 3.34 × 10 -30 C m, 1 Å 3 ) 1.113 × 10 -40 C m 2 /V). Stark experiments were performed on eight different LH2 complexes.…”