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Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts.

Hoober JK, Eggink LL, Chen M - Photosyn. Res. (2007)

Bottom Line: Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains.The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group.These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

View Article: PubMed Central - PubMed

Affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA. khoober@asu.edu

ABSTRACT
Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

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(A) The structure of the imidazole group of histidine and (B) its electronic charge density, determined by X-ray diffraction at 103 K for the projection in (A) (adapted from Epstein et al. 1982). In (A), R = remainder of the histidine molecule
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Fig2: (A) The structure of the imidazole group of histidine and (B) its electronic charge density, determined by X-ray diffraction at 103 K for the projection in (A) (adapted from Epstein et al. 1982). In (A), R = remainder of the histidine molecule

Mentions: The imidazole sidechain of histidine in the unprotonated form has an unshared pair of electrons on N(3) (designated as Nε2 by Standfuss et al. 2005). H+ binds to the electron pair with a pK value that lies within the range of 5–8, depending upon the environment. Nonpolar environments stabilize the unprotonated form, and thus the electron-rich imidazole group is available for coordination with the Mg of Chl a within a membrane. The dipole moment for imidazole is between 3.66 D (gas phase) and 4.80 D (crystal structure), with the predominant contribution to the dipole provided by the N(1)-H bond (Spackman 1992). When the N(1) hydrogen is replaced with the electron-donating methyl group, the resulting coordination bond at N(3) is stronger (van Gammeren et al. 2004). In aqueous solution, the dipole moment is enhanced to a value of 3.96 D by H-bonding (Table 1). Both N atoms have a small negative charge, and the electron density is distributed nearly symmetrically (Fig. 2). The aromatic character of imidazole allows the dipole to reorganize in response to interaction with another structure.Fig. 2


Chlorophylls, ligands and assembly of light-harvesting complexes in chloroplasts.

Hoober JK, Eggink LL, Chen M - Photosyn. Res. (2007)

(A) The structure of the imidazole group of histidine and (B) its electronic charge density, determined by X-ray diffraction at 103 K for the projection in (A) (adapted from Epstein et al. 1982). In (A), R = remainder of the histidine molecule
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2117338&req=5

Fig2: (A) The structure of the imidazole group of histidine and (B) its electronic charge density, determined by X-ray diffraction at 103 K for the projection in (A) (adapted from Epstein et al. 1982). In (A), R = remainder of the histidine molecule
Mentions: The imidazole sidechain of histidine in the unprotonated form has an unshared pair of electrons on N(3) (designated as Nε2 by Standfuss et al. 2005). H+ binds to the electron pair with a pK value that lies within the range of 5–8, depending upon the environment. Nonpolar environments stabilize the unprotonated form, and thus the electron-rich imidazole group is available for coordination with the Mg of Chl a within a membrane. The dipole moment for imidazole is between 3.66 D (gas phase) and 4.80 D (crystal structure), with the predominant contribution to the dipole provided by the N(1)-H bond (Spackman 1992). When the N(1) hydrogen is replaced with the electron-donating methyl group, the resulting coordination bond at N(3) is stronger (van Gammeren et al. 2004). In aqueous solution, the dipole moment is enhanced to a value of 3.96 D by H-bonding (Table 1). Both N atoms have a small negative charge, and the electron density is distributed nearly symmetrically (Fig. 2). The aromatic character of imidazole allows the dipole to reorganize in response to interaction with another structure.Fig. 2

Bottom Line: Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains.The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group.These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

View Article: PubMed Central - PubMed

Affiliation: School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA. khoober@asu.edu

ABSTRACT
Chlorophyll (Chl) b serves an essential function in accumulation of light-harvesting complexes (LHCs) in plants. In this article, this role of Chl b is explored by considering the properties of Chls and the ligands with which they interact in the complexes. The overall properties of the Chls, not only their spectral features, are altered as consequences of chemical modifications on the periphery of the molecules. Important modifications are introduction of oxygen atoms at specific locations and reduction or desaturation of sidechains. These modifications influence formation of coordination bonds by which the central Mg atom, the Lewis acid, of Chl molecules interacts with amino acid sidechains, as the Lewis base, in proteins. Chl a is a versatile Lewis acid and interacts principally with imidazole groups but also with sidechain amides and water. The 7-formyl group on Chl b withdraws electron density toward the periphery of the molecule and consequently the positive Mg is less shielded by the molecular electron cloud than in Chl a. Chl b thus tends to form electrostatic bonds with Lewis bases with a fixed dipole, such as water and, in particular, peptide backbone carbonyl groups. The coordination bonds are enhanced by H-bonds between the protein and the 7-formyl group. These additional strong interactions with Chl b are necessary to achieve assembly of stable LHCs.

Show MeSH
Related in: MedlinePlus