Conversion of a photon to an electrical signal by sudden polarisation in the N-retinylidene visual chromophore

On the rise time of the R1-component of the "early receptor potential": evidence for a fast light-induced charge separation in rhodopsin

The rising phase of the R1-component of the early receptor potential from isolated cattle retinas was measured with high time resolution. When the measuring capacitance was 133 pF, a latency of about 200 ns was observed. A rise time of about 0.8 mus at 0 degrees C and 1.6 mus at 37 degrees C (extrapolated to ideal measuring conditions) was found. The negative temperature dependence indicates that the rise is not directly related to the production and decay of photolysis products of rhodopsin since the latter have positive temperature coefficients. An increase of the external measuring capacitance caused a slower rise time. The analysis of this effect allowed the determination of the source impedance of the R1-component. The experimental results can be described with a model in which it is assumed that a fast charge separation (ns or ps) takes place in the outer segment of a photoreceptor cell, and spreads passively to the inner segment via the resistance of the interconnecting cilium. The "inner" relaxation could be circumvented by using isolated rod outer segments which lack the passive inner segments, i.e., a rise time of 90 ns could be measured when isolated rod outer segments were attached to Millipore filters. The results suggest that the molecular event leading to the R1-component is an early charge separation which may be as fast as the cis-trans isomerization of the retinal chromophore.

Picosecond chemical and biological events

Picosecond spectroscopy is a relatively new field of science that utilizes ultrashort laser pulses to monitor events taking place in the 10(-12) second regime. The continuing development of picosecond spectroscopy has made possible the detection and measurement of the primary events in many physical and tiological processes. This article describes a currently used picosecond spectroscopy system that is capable of reliably recording picosecond events. Two areas of picosecond research are discussed; one concerns the interaction of electrons in fluids, and the second the primary events in vision.

An observation concerning electronically excited retinal

The observation of Mathies and Stryer of highly polar excited electronic states of all-trans-retinal, its unprotonated Schiff base, a salt of the protonated Schiff base, and 11-cis-retinal leads us to propose a possible consequence of the existence of such species. The highly polar excited molecule may have sufficient potential to oxidize its surroundings, thus achieving a transfer of one electron. Experimental evidence is combined with crude model calculations to support the possible role of these excited states as electron transfer agents.

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

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 (mug), the change in dipole moment on excitation (deltamu), and the direction of mug and deltamu). These experiments have shown that all three species become highly dipolar upon excitation to the first allowed excited singlet state (deltamu = 15.6, 9.9, 12D, respectively). The ground-state and excited-state dipole moments are nearly parallel to the long axis of these molecules. Excitation is accompanied by a shift of negative charge toward the carbonyl or Schiff base terminus, making the ionone end of these molecules positively charged. The large excited state dipole moment of all-trans retinal indicates that the vertically excited state, which is of 1Bu parentage (C2h), has become significantly mixed with even-parity states. On the basis of previous theoretical calculations, this mixing is expected to facilitate isomerization in the singlet manifold. We have also found that 11-cis retinal has a large deltamu (12.7 +/- 1.4 D) on excitation. In the visual pigments, the interaction of the excited-state dipole moment of retinal with a suitably located charged group could control the position of the absorption maximum. Also, the large shift in charge density upon excitation of retinal may lead to new electrostatic interactions between the chromophore and the protein that would act as a driving force for the initial conformational changes in visual excitation.

Bicycle-pedal model for the first step in the vision process

Computer simulation of the molecular dynamics of retinal during its photoisomerisation inside a restrictive active site gives a detailed model for the sequence of events in the first step of the vision process. It is proposed that the prelumirhodopsin intermediate contains a strained all-trans retinal molecule produced directly and rapidly from the 11-cis, 12-s-trans conformation in rhodopsin by a bicycle-pedal isomerisation. The model reproduces the main experimental observations and explains how the protein makes the photoisomerisation path unique.

Theory of photochemical reactions

Although the great number of electronic states available to an excited molecule might seem to preclude a coherent picture of photochemical reaction mechanisms, it is possible to bring out some basic features common to a great many reactions. The electronic states of the primary diradical intermediates, surface crossings, topicity, and avoided surface crossings have been shown to be essential components of the electronic theory of photochemical reactions. Diradicals have four important electronic states. Knowing these states, and making a simple electron count, it is possible to draw state correlation diagrams. Some diagrams show a typical surface crossing of the ground singlet state with the lowest (singlet, triplet) pair of excited states, with clear-cut consequences of quantum yields under various conditions. In other reactions the surfaces stay apart. The critical discriminating feature that determines the type of correlation diagram is the topicity. Photochemical reactions can be classified according to topicity, which is useful in interpreting their mechanisms (53). Avoided surface crossings can also be classified into different types. Figure 7, which illustrates the interplay of a covalent and an ionic surface responsible for photochemical electron transfer, is a typical multidimensional representation of a photochemical reaction. The chemical behavior of the excited zwitterionic states of common intermediates, such as twisted ethylene or diallyl, reflects the quantum mechanical nature of photochemical processes. In these states, for perfectly symmetric systems, charge oscillates back and forth between two symmetry-equivalent sites. Slight geometric perturbations can create a sudden polarization of the excited molecule, with localization of almost a full charge at one end of the molecule. A photon is transformed into an electrical signal thanks to an appropriate molecular distortion. Nature may have used this simple process in the N-retinylidene visual chromophore to trigger an electrical response to vision.