Origin of the two carbonyl oxygens of bacteriochlorophyll a. Demonstration of two different pathways for the formation of ring E in Rhodobacter sphaeroides and Roseobacter denitrificans, and a common hydratase mechanism for 3-acetyl group formation.

A respiring culture of Rhodobacter sphaeroides, grown in the dark under defined aerobic conditions, produced cells capable of immediately commencing adaptation to photosynthetic growth on exposure to light and further reduction of oxygen tension. Adaptation was complete after 12 h and the bacteriochlorophyll a content increased 10-20-fold. This adaptation was performed in the presence of either H2(18)O or 18O2.

The derivation of the oxygen atoms of the 13(1)-oxo and 3-acetyl groups of bacteriochlorophyll a from water in Rhodobacter sphaeroides cells adapting from respiratory to photosynthetic conditions: evidence for an anaerobic pathway for the formation of isocyclic ring E.

Using mass spectrometry, we have demonstrated 18O-labelling of both the 13(1)-oxo and 3-acetyl groups of newly-formed bacteriochlorophyll a synthesized by Rhodobacter sphaeroides cells during adaptation from respiratory to photosynthetic conditions in the presence of H218O. This derivation of the 13(1)-oxo group of bacteriochlorophyll a from water provides a stark contrast with that of chlorophylls in higher plants where ring E formation is an aerobic process in which the 13(1)-oxo group arises from molecular oxygen via an oxygenase activity. The formation of the 3-acetyl group of bacteriochlorophyll a, however, is consistent with the enzymic hydration of the 3-vinyl group of a derivative of chlorophyll a.

The derivation of the formyl-group oxygen of chlorophyll b in higher plants from molecular oxygen. Achievement of high enrichment of the 7-formyl-group oxygen from 18O2 in greening maize leaves.

The mechanism of formation of the formyl group of chlorophyll b has long been obscure but, in this paper, the origin of the 7-formyl-group oxygen of chlorophyll b in higher plants was determined by greening etiolated maize leaves, excised from dark-grown plants, by illumination under white light in the presence of either H2(18)O or 18O2 and examining the newly synthesized chlorophylls by mass spectroscopy. To minimize the possible loss of 18O label from the 7-formyl substituent by reversible formation of chlorophyll b-7(1)-gem-diol (hydrate) with unlabelled water in the cell, the formyl group was reduced to a hydroxymethyl group during extraction with methanol containing NaBH4: chlorophyll a remained unchanged during this rapid reductive extraction process. Mass spectra of chlorophyll a and [7-hydroxymethyl]-chlorophyll b extracted from leaves greened in the presence of either H2(18)O or 18O2 revealed that 18O was incorporated only from molecular oxygen but into both chlorophylls: the mass spectra were consistent with molecular oxygen providing an oxygen atom not only for incorporation into the 7-formyl group of chlorophyll b but also for the well-documented incorporation into the 13(1)-oxo group of both chlorophylls a and b [see Walker, C.

Carotenoid triplet state formation in Rhodobacter sphaeroides R-26 reaction centers exchanged with modified bacteriochlorophyll pigments and reconstituted with spheroidene.

Triplet state electron paramagnetic resonance (EPR) experiments have been carried out at X-band on Rb. sphaeroides R-26 reaction centers that have been reconstituted with the carotenoid, spheroidene, and exchanged with 13(2)-OH-Zn-bacteriochlorophyll a and [3-vinyl]-13(2)-OH-bacteriochlorophyll a at the monomeric, 'accessory' bacteriochlorophyll sites BA,B or with pheophytin a at the bacteriopheophytin sites HA,B. The primary donor and carotenoid triplet state EPR signals in the temperature range 95-150 K are compared and contrasted with those from native Rb.

Derivation of the formyl-group oxygen of chlorophyll b from molecular oxygen in greening leaves of a higher plant (Zea mays).

Using mass spectroscopy, we demonstrate as much as 93% enrichment of the 7-formyl group oxygen of chlorophyll b when dark-grown, etiolated maize leaves are greened under white light in the presence of 18O2. This suggests that a mono-oxygenase is involved in the oxidation of its methyl group precursor. The concomitant enrichment of about 75% of the 13(1)-oxygen confirms the well-documented finding that this oxo group, in both chlorophyll a and b, also arises from O2.