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.

Electrochromic spectra of monolayers of carotenoids (lutein and β-carotene) in contact with monolayers of chlorophylls and of pheophytin a are measured in thin capacitors. A specific inter­ action of one of the OH-groups of lutein with the Mg-atom of chlorophyll is found. The formation of this oriented complex accounts for the fact that a part of the electrochromic absorption-change of lutein depends linearly on the electric field strength, whereas for lutein alone only a smaller, quadratic electrochromism is found. In the preparation with chlorophyll a, the maximum of this linear electrochromism is located at shorter wavelengths (512 nm) than in the preparation with chlorophyll b (517 nm). The permanent field that has been postulated in photosynthetic membranes (to explain the linear dependence of the field-indicating absorption-changes of the carotenoids) may also be at­ tributed to such a complex formation with chlorophylls. Especially, the field-indicating absorption-change at 520 nm can now be attributed mainly to a lutein-chlorophyll b complex. The absorption-change at 520 nm, calculated according to this model from the present experiments in vitro, is of the same order of magnitude as observed in vivo. Furthermore, this model agrees with the hitherto unexplained observation that in chlorophyll-b-lacking mutants the absorption-change at 520 nm is smaller than in normal plants, and the maximum is located at shorter wavelengths. Besites, it is concluded that lutein is mainly located in the regions of photosystem II. The contributions of the other carotenoids (especially of neoxanthin) to the spectrum of the field-indicating absorption-changes are also discussed. From the above model, some conclusions are drawn on the asymmetrical arrangement of the different pigments in the membrane of photosynthesis: The bulk chlorophyll molecules that serve as complex partners for the carotenoids should be located near to the inner surface of the thylakoid membrane, and the carotenoids attached to these chlorophylls should be located more to the out­ side. The phytol chain of a chlorophyll molecule should form an acute angle with the plane of the porphyrin ring.

No abstract

No abstract