The Effect of Molecular Polarization on the Electrochromism of Carotenoids—i. The Influence of a Carboxylic Group

Abstract

Abstract— Electrochromism of oriented all‐trans‐β‐apo‐8′‐carotenoic acid is studied in thin capacitors. The linear electrochromism is very strong, in contrast to that of symmetrical carotenoids. It is proportional to the first derivative of the absorption spectrum. The quadratic electrochromism can be described as a superposition of fractions proportional to the first and second derivatives of the absorption spectrum. The permanent dipole moment difference between the ground state and the excited state of the carotenoic acid molecule is Δμ= 3.6 × 10‐29 C·m (±20%) (10.7 Debyes). The polarizability difference parallel to the long axis of the molecule is Δα|| = 1.17 × 10‐37 C·m2·V‐1 (±20%) (1050 Å3). Furthermore, the relative permittivity of the solid carotenoic ethyl ester is r= 3.5 ± 0.2. Δμ is due to the polarizing force of the carboxylic group. This force is equivalent to a mean local electric field of Ft≅3 × 106V/cm. Such a “local field” may also be exerted on a symmetrical carotenoid in the membrane of photosynthesis, e.g. by asymmetrical complex formation with a polarizing molecule. To obtain an effective permanent field of Fp≅ 2 × 106V/cm across the membrane, as postulated in photosynthesis, a local field of Fl≅ 5.5 × 105 V/cm would be sufficient. Fp is shown to be directed from inside to outside of the thylakoid. To realize this, e.g. a positive polar (i.e. electron‐attracting) complex partner of the carotenoid, located more to the inside of the thylakoid, can be postulated.

Influence of the chlorophylls on the electrochromism of carotenoids in the membranes of photosynthesis

“…(1) *The complexes must have a preferential orientation, so that the effective, local molecular fields, exerted on the distinguished carotenoids, have a mean component parallel to the light-induced field [ 3] In the case shown here, the observed absorption-change is zero. However, by a further flash, when the left flank of the shifting absorption band reaches the wavelength of observation, the absorption-change will be negative, i.e., de transmission-change will be positive.…”

“…This effect is equivalent to the action of a local electric field [3] , which shifts the absorption spectrum of the carotenoid to longer wavelengths. When the BChl molecule is oxidized, the local field, and hence the absorption shift, are increased.…”

“…If a polarizing complex partner is attached to one end of a symmetrical carotenoid**, the polarizing force can be formally described as a local molecular field F, parallel to the long axis of the molecule [3] . If the polarizability component in this direction is denoted by (~11, the wavelength shift due to the complex formation is given by the second term of eq.1 as…”

Activation of Zeaxanthin Is an Obligatory Event in the Regulation of Photosynthetic Light Harvesting

“…A shift to from 503 nm to 525 nm would require a refractive index of about 1.8. Such a highly polarizing environment could also arise from a strong electric field of a dipole or charge in the zeaxanthin vicinity or binding site (25,26). The signature of the specific binding of this red-shifted zeaxanthin can clearly be seen as a pronounced and characteristic enhancement of the 4 region of qE-associated RR spectrum (Fig.…”

Electrochromic absorption changes of a chloroplast suspension induced by an external electric field

“…The quadratic electrochromism of these carotenoids having a dipole moment difference induced by the chloropllylls and a polarizability difference must be described by a superposition of parts proportional to the first and second derivative of their absorption spectrum (terms 2,3 of eq. (2)) [29]. A third contributions is expected by the quadratic electrochromism of chl a and chl b.…”