The Effect of Molecular Polarization on the Electrochromism of Carotenoids * II. Lutein-Chlorophyll Complexes: The Origin of the Field-Indicating Absorption-Change at 520 nm in the Membranes of Photosynthesis

Abstract

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.

Theoretical studies of the electrochromic response of carotenoids in photosynthetic membranes

“…As mentioned above, the semiempirical parameters used in these calculations were chosen so as to produce reasonable absorption maxima for carotenoids. The calculated X.., for lutein is 465 nm, which compares well with the experimental value in benzene of 460 nm (5). For neurosporene, comparison is more difficult because the first two vibrational lines are of approximately equal intensity (13).…”

“…Molecules with permanent dipoles will of course exhibit a linear response. As expected, in vitro experiments on carotenoids do produce a quadratic response (4,5). Surprisingly, however, the electrochromic response of carotenoids in vivo has been found to be linear in all cases that have been studied.…”

“…This result has been explained by assuming that carotenoids in vivo are exposed to a local electric field that is much stronger than the light-induced field (6). The effect of the local field is to induce dipoles in the carotenoid that then give rise to a linear shift in the presence of a weak external field (4,5).…”

Structurally flexible macro-organization of the pigment–protein complexes of the diatom Phaeodactylum tricornutum

“…The strong electrochromic response of these Fx molecules shows that they probably interact with Chl molecules, thereby lending a dipole moment to this molecule. It is also interesting to note that in higher plants the strongest electrochromic response, at around 515 nm, is attributed to lutein/Chl b interactions (Sewe and Reich 1977), suggesting that the long-wavelength absorbing, field-indicating lutein/Chl b pigment pair in the LHCII might be replaced by a similar Fx/Chl c pair in the FCP. (The strong absorption transient of the Fx molecules also indicated that these molecules were oriented in such a way in the thylakoid membrane that they were able to sense the electric field generated by the photosynthetic electron transport.)…”

Cooperative transient trapping of photosystem II protons by the integral membrane portion (CF0) of chloroplast ATP-synthase after mild extraction of the four-subunit catalytic part (CF1)

“…They were based on membrane-bound indicators: the electrochromic absorption changes result from lutein-chlorophyll b complexes (40) that are associated with photosystem II. The pH indicator that we used for the internal pH, neutral red, is highly membrane soluble and it intrinsically responds to the surface pH (28 showed that neutral red responded to pH transients in the inner aqueous bulk phase within <100 Aus (28).…”