Which of the following molecules are associated with copper and magnesium
respectively?
(a) Plastocyanin and DNA polymerase
(b) Plastoquinone and chlorophyll
(c) Cytochrome and DNA polymerase
(d)
PSI reaction centre protein and chlorophyll
Answers
Explanation:
Introduction
Photosynthetic pigments represent a kind of "lifeblood" of all photoautotrophic organisms. There are basically three classes of natural photosynthetic pigments: chlorophylls, carothenoids and phycobilins.1 In nature, there are approximately 10 types of chlorophylls, isolated from the green part of the plants. In some organisms only chlorophyll is detected, while in other species, the main chlorophyll is accompanied by other green pigments. This principal component is the chlorophyll α (Figure 1), which is found together with the chlorophylls b, c, d and the protochlorophyll.2 It is important to register that the version of chlorophyll α without metallic center, where Mg2+ is substituted by two hydrogen atoms, is known as pheophytin α, which has been previously evaluated.3 This derivative can be obtained by the acid hydrolysis of chlorophyll α (Figure 2).4
Chlorophyll α has a ubiquitous role as electron donor in photochemical reactions of oxygenic photosynthesis, in which two kinds of photosystem, namely photosystem I (PSI) and photosystem II (PSII), cooperatively drive photosynthetic electron flow from water to NADP+. The reduced cofactor, NADPH, is used for CO2 fixation.5 Indeed, PS-I and PS-II have been intensively studied with different aims in the literature.6-8 It is a porphyrin derivative classified as chlorin, being metallated with Mg2+ (Figure 1). Differently from porphyrins, the molar absorptivity of the Q bands of chlorin (assigned to the S0→S1 π-π* electronic transition) is considerably intense,9,10 occurring into the "Phototherapeutic Window".10 Besides, this class of compounds tends to present considerably long-lived triplet excited states, being promising phototherapeutic agents in Photodynamic Therapy (PDT).10-17
