Short-Wavelength Absorbing Complexes of Chlorophyll a in Micellar Solution of Cationic Detergents

Abstract

Micellar aggregates have been prepared from chloro­phyll a. Whereas most detergents including non-ionic, zwitterionic and anionic ones, produce aggregates with long-wavelength shifted and enhanced QY absorption, the cationic detergents hexadecyl-trimethylammonium-bromide and hexadecyl-pyridiniumchloride produce complexes with opposite traits: The QY absorption is shifted to shorter wavelengths, and it is hypochromic. The optical activity is increased as in the long-wave­ length shifted complexes, but the cd bands are non-conservative and coincide with the absorption maxima. The complexes are most likely small aggregates of chlorophyll with a changed geometry as compared to the “standard” ones.

Bacteriochlorophyll aggregates in positively charged micelles

“…BPhe a forms a complex with a considerably broadened Qy band, which extends towards longer wavelengths, but is generally only poorly structured (not shown). A similar type of spectrum has been obtained with Chl a in TX-100 [12]. Another distinct feature is the rather large intensity of the Qx band, which is reminescent of the hypochromism of this band (and the concomitant pronounced hyperchromism of the Qy band), in complexes of Chl a with cationic detergents [12].…”

“…Many of the spectral properties of photosynthetic systems have been related to interactions among closely spaced chlorophylls [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. The aggregation of these pigments has, therefore, received much attention in the past.…”

“…They interact either directly or -if present -via multifunctional ligands like water, dioxan, etc. A new type of chlorophyll aggregation has recently been proposed from studies on chlorophyll in micelles [4,5,[9][10][11][12][13][14][15][16]. This micellar aggregation is due mainly to interaction of the 7r-systems producing aggregates in which the tetrapyrroles are nearly parallel, and generally occurs irrespective of the absence and presence of the central magnesium.…”

“…In a systematic study of the self-organization of chlorophyllous pigments in micellar environment, we have now screened a variety of detergents. Previous results with chlorophyll a (Chl a) indicated that complexes like the ones in Triton X-100 (TX-100) are formed in all detergents except those with positively charged head-groups [11,12]. With the latter, blueshifted complexes were formed instead.…”

How does the electron get to the other side?

“…We have used Triton X-100 at concentrations of 0.05-0.1% to study the reaction of detergent-solubilized chlorophylls with NaBH4. Under these conditions, Chl a is present in two forms (Schmidt, 1987): the monomeric form which absorbs at 668 and 434 nm, and an oligomeric form absorbing at 740 and 443 nm ( Fig. 8 spectrum a).…”