The biogeochemistry of tetrapyrrole pigments, emphasizing chlorophyll

Electronic absorption spectra recorded on native geopigments and on in vitro derivatives, obtained with combinations of copper insertion and borohydride reduction reactions, allowed a sensitive 'chromophore identification' scheme to be developed. Quantitation of the Ni and VO geoporphyrins, as well as the benzo-forms of each, was tested and precisions at the level of 100 {dollar}pm{dollar} 5% found.; An overall methodology for obtaining repeatable low-voltage mass spectra on geoporphyrin arrays is presented. Repeatabilities for the various mass spectral derived indices were; %DPEP ({dollar}pm{dollar}2%), X ({dollar}pm{dollar}2%), A.I.({dollar}pm{dollar}5%) and % BENZ ({dollar}pm{dollar}10%). Quantitation of the metallobenzoporphyrins was much better ({dollar}pm{dollar}2%) with UV/VIS techniques.; Tetrapyrrole geochemistry was investigated at all stages of organic evolution. Thus, viable and senescent/dead uni-algal cultures, sediment trap material, surface ({dollar}<{dollar}1m) sediments, deep ocean long cores (DSDP), oil shales and petroleum crudes were investigated.; Results indicate that the chlorophylls can serve as sources for a variety of geoporphyrins. Early in diagenesis two competing reactions dictate further 'fossilization'. First is the loss of the carbomethoxy group. This produces pyropheophorbides which can either lead to the true DPEP series, via a sequence of defunctionalization reactions, or, via intramolecular cyclization (dehydration), to certain 13{dollar}sp2{dollar}, 17{dollar}sp3{dollar}-cyclopheophorbide enols. The latter, following defunctionalization, give rise to DiDPEP and/or DPEP-type pigments with a 7-membered exocyclic ring. Second, chlorophyll nuclei which have undergone oxidative scission of the isocyclic ring can, through purpurins and chlorins, theoretically yield C28-C30 ETIO-series porphyrins. Bacteriochlorophyll-a was found to be a dominant pigment in several anoxic sediments and is suggested as an important and highly specific precursor for certain geoporphyrins (i.e. 3-methyl-desethyl-DPEP).; Heme type compounds (viz. cytochromes) appear to not only yield C-32 ETIO pigments but also, through rhodins derived by the cyclization (dehydration) of propionic acid groups, a series of DPEP-like compounds with 6-membered exocyclic rings.; It was concluded from the studies reported both here and in the literature that, tetrapyrrole pigments evolve in at least five separate assemblages, the first four of which funnel into the last. These are; phytyl esters, steryl esters, carboxylic acids, geopolymers (bound) and alkyl pigments.