Bacteriochlorophylls modified at position C-3: long-range intramolecular interaction with position C-132

Abstract

[3-Vinyl]-bacteriochlorophyll a and related pigments modified at C-3 and/or C-13 z havc been synthesized from bactcriochlorophyll a. The reactivity at C-3 is strongly influenced by the C-132 substituent, and vice versa. Spcctroscopical data and comparison among derivatives modified at the isocyclic ring indicate that this interaction is related to formation of an intcrmcdiatc cnol(atc) structure. The possible role of enol(ate) formation in (bacterio)chlorophylls in nature is discussed. IntroductionThe chemistry of plant chlorophylls (Chl a, b) is a well-studied part of porphyrin chemistry. A large amount of data has been accumulated on the chemical reactivity of side-groups and their physical and spectroscopic properties [1][2][3]. Much of that work is motivated by the impor: 1nee of these chlorophylls in oxygenic photosynthesis. While most of these data originate from in vitro work in organic or micellar solution, much less is known on the structural and functional details in their native protein environment. In the case of bacteriochlorophylls, among which BChl a and b are most prominent, the situation is reversed. Due to the possibility to isolate bacterial light-harvesting complexes [4] and purple bacterial reaction centers [5] for more than 15 years, and to crystallize some of them, high precision structural data in their native protein environment are available [6][7][8][9][10]. However, their chemistry is relatively seldom investigated. In connection with the recently introduced methods for exchanging modified (bacterio)chlorophylls and -pheophytins into bacterial reaction centers [11][12][13][14] To understand the structure-function relationships of bacteriochlorophylls (and chlorophylls) in more detail, it is important to obtain structural links between the different naturally occurring (bacterio)chlorophyll structures. One such link is [3-acetyl]-chlorophyll a [18], bearing the 3-acetyl group characteristic of BChl a and b, and the macrocycle characteristic of the green plant Chl a and b. The complementary link is [3-vinyl]-Bchl a, which differs vice versa from Chl a by the macrocycle, and from BChl a by the presence ~l a C-3 vinyl instead of an acetyl group. Here, we wish to report a procedure to synthesize [3-vinyl]-BChl a and some related pigments, and discuss some physical properties of these pigments in vitro. Attention is given to a pronounced, and hitherto unreported, long-range interaction between substituents at the positions 3 and 13 2 , which was observed during these studies. The reactivity at C-3 is strongly influenced by the nature of the C-13 2 substitucnt, and vice versa. The data suggesl, that this 'connection' is related to f,-.~rmation of enol(ateJ structures at the isocyclic ring. There has been considerable interest before it~ the enolisation and the epimerisation at C-13 2, and their possible involvement in photosynthesis [19][20][21][22][23]. The results are discussed in this context. Material and Methods General conditionsAll chemicals and solvents used were reagent grade. eigl...

Origin of the Two Carbonyl Oxygens of Bacteriochlorophyll a

“…The 13'-oxo group of BChl a, which is less reactive than in Chls a and b, is not reduced under these conditions (compare Struck et al, 1992). The BChl derivative was dried and purified on a DEAE-cellulose column as described by Porra et al (1995a Struck et al, 1992), by heating in vucuu over P20, and was subsequently purified on a DEAEcellulose column as described by Porra et al (1995a).…”

Structural determination of dihydro‐ and tetrahydrogeranylgeranyl groups at the 17‐propionate of bacteriochlorophylls‐a

“…A study on determination of the composition of BChl-a intermediates in isolated pigment-protein complexes is now in progress. Since BChl-a molecules can be easily modified to Chl-type analogs including Chl-a and Chl-d [16][17][18][19], the present Rps. palustris, which accumulates BChl-a intermediates at about 30 % of total BChl-a component, would be useful for the provision of the intermediates to investigate the final stage of Chl biosynthesis in oxygenic phototrophs.…”

Biosynthesis of the 3‐Acetyl and 131‐Oxo Groups of Bacteriochlorophyll a in the Facultative Aerobic Bacterium, Rhodovulum sulfidophilum

“…BChl a was reduced in ethanol with 75 mM NaBH 4 : under these conditions, the 13 1 -oxo group of BChl a is not reduced [16]. The [3 1 hydroxy]-BChl a so formed was purified on DEAE-cellulose columns as previously described [6].…”

“…This dehydration was achieved by heating a dry film of the 3 1 -hydroxy derivative for 30Ϫ40 min at 142°at a reduced pressure using an Edwards Pico-Dry High Vacuum pump operating between 2.5Ϫ1.0 mPa. The progress of the reaction was monitored spectrophotometrically: part of the dry film was dissolved and as dehydration progressed the red Q Y peak moved from 739 nm to 751 nm in toluene or from 728 nm to 745 nm in diethyl ether [16]. Oxidative formation of Chl a by removal of two hydrogen atoms on ring B was minimized by keeping the reaction time between 30 min and 40 min, by reducing the pressure to 2.5 mPa before heating and by cooling to room temperature before releasing the vacuum: this procedure also minimized formation of the pyro-derivative of [3-vinyl]-BChl a by demethoxycarbonylation at C13 2 .…”

“…This program calculates molecular ion patterns for compounds of known structures, based on the natural isotopic composition of all elements. The element file of this program was modified such that sets of oxygen and hydrogen atoms in different isotopic enrichments were replacing the heavy elements, creating e.g a new element '40-O' with a composition of 40% 16 O and 60% 18 O. By this means, it is possible to handle enrichments of individual oxygen atoms to different degrees, like e.g.…”