Chlorophylls, transformation

Harris, P.G., Carter, J.F., Head, R.N., Harris, R.P., Eglinton, G., and Maxwell, J.R. (1995) Identification of chlorophyll transformation products in zooplankton faecal pellets and marine sediment extracts by liquid chromatography/mass spectrometry atmospheric pressure chemical ionization. Rapid Cornmun. Mass Spectrom. 9, 1177-1183. 

Chlorophylls are produced by all photosynthetic organisms — and even by some nonphotosynthetic bacteria — and details of their structures depend on their source. Collectively they represent a considerable reserve of organic carbon and nitrogen, although little seems to have been established on their persistence. A wide range of transformation products of chlorophylls has been recovered from the sediments of a freshwater eutrophic lake, and these included the unusual sterol esters of pyrophaeophorbides (Eckardt et al. 1995). It is also presumable that such chlorophyll transformation products produce the pyrroles and indoles that have been described in sediment pyrolysates noted above. 

Eckardt, C.B., B.J. Keely, and J.R. Maxwell. 1995. Identification of chlorophyll transformation products in a lake sediment by combined liquid chromatography-mass spectrometry. /. Chromatogr. 557 271-288. 

Fig. 5. 6 Major early diagenetic (water column and very early sedimentary) pathways for chlorophyll- transformation (after Baker Louda 1986 Keely et al. 1990 Harradine et al. 1996 Louda et al. 1998 Louda et al. 2000).

Thirty-nine chlorophyll transformation products were separated on a C g colunw (. = 400nm) using a 40-min gradient from 90/10 methanol/water to 90/5/5 acetone/methanol/wato- [457]. Positive identification of methylated compounds within fiactions was made by thermospray MS. Pheophoibide a methyl ester, zeaxanthin, pyropheophorbide a methyl ester, isorenieratene, pheophytin a and b and pyiopheoph5 in a and b were confirmed. 

The transformation of the porphyrin intermediate 4 into a chlorin can be achieved after introduction of a C — C double bond into the 15-propanoate side chain of 4 to yield 5. The cyclization of 5 with participation of the 15-acrylic ester side chain under acidic conditions gives the chlorin 6 which is then transformed in a multistep reaction sequence into chlorophyll a. The driving force of chlorin formation from the porphyrin is believed to be the relief of steric strain at the sterically overcrowded porphyrin periphery which gives the desired trans arrangement of the propanoate side chain and the methyl group in the reduced ring. The total... 

Several chemical transformations in the chlorin series were discovered during the course of Woodward s total synthesis of chlorophyll a.3a d An important reaction in the final steps of this total synthesis is the removal of an a-oxo acid ester residue from the 17-position of the chlorin 22, which proceeds very smoothly in the presence of base by a retro-aidol-type fragmentation to yield the chlorin isopurpurin methyl ester (23) which is also available by degradation of chlorophyll a, so that at this point of the synthesis synthetically derived material could be compared with an authentic sample prepared from natural chlorophyll a. 

Transformations which alter the bacteriochlorin chromophore are quite rare. An important reaction in the structural elucidation of the bacteriochlorophylls is the dehydrogenation to chlorophyll derivatives. Thus, bacteriopyromethylpheophorbide a (1) can be smoothly dehydrogenated with 3,4,5,6-tetrachloro-l,2-benzoquinone to the corresponding chlorin 3-acetyl-pyromethylpheophorbide a (2) in high yield.1 la,b... 

Chlorophyll catabolism has been intensively studied in some plants, e.g., rape-seed, barley, spinach, tobacco, Cercidiphyllum japonicum, Lolium temulentum, Liq-quidambar styraciflua and Arabidopsis thaliana, which present all NCC catabolites with similar basic structures. " This suggests a uniform breakdown of chlorophyll in which the oxidative opening of pheophorbide a seems to be a key step. Structural differences among the compounds have been related to at least six basic types of peripheral transformations. Some of them seem to operate either in sequence or in parallel, depending on the plant species, which caused the appearances of different... 

The photosensitized transformation of carotenoids has been studied using several sensitizer molecules, such as chlorophylls, iodine, rose bengal (RB), and methylene blue (MB) and in general terms isomerization is the major pathway of reaction. 

Although studied in great detail, the action of chlorophyll is still not fully understood however, steady progress towards a more complete understanding has taken place over the past several decades. Two photosynthetic systems (photosystems I and II) are present in green plants - each incorporating a different chlorophyll type. When a photon is absorbed by a chlorophyll molecule, its energy is transformed and... 

For this purpose Case 2 water chlorophyll-a concentrations from MERIS were transformed into phytoplankton concentrations using the constant carbon-to-chlorophyll ratio, Rc Chi=60, used in HAMOCC [Maier-Reimer et al (2005)]. 

A famous use of electrochemical reduction of porphyrin systems is that used to convert a chlorophyll a derivative, chlorin-e6 trimethyl ester (64), into a chlorophyll b derivative, rhodin-g7 trimethyl ester (65) (71LA(749)109). Electrolysis of chlorin-e6 trimethyl ester gave the chlorin-phlorin which was photolyzed in dioxane/water to give the trans-d o (Scheme 7) this was simply transformed into rhodin-g7 trimethyl ester (65) to accomplish the first ever interconversion of a chlorophyll a series pigment into one of the companion b series. 

---