Covalently linked dimers

It should be noted that the existence of different centers Is also found in covalently-linked dimer 11. Moreover, the analysis of all data obtained for dimers points clearly towards the efficient transfer of the excited singlet state energy from two centers of compound 1 to two acceptor centers of compound 2 in dimers (14,30). Increase In the porphyrin concentration by 300-700 times (say, for compounds 1 and 2) does not cause additional changes In electronic spectra as against diluted solutions. If the results obtained from temperature experiments (Fig. 2b) and measurements of fluorescence lifetimes In different bands are taken Into account, one may conclude that the additional centers observed In... 

Asparagine and glutamine are the amides of two other amino acids also found in proteins, aspartate and glutamate, respectively, to which asparagine and glutamine are easily hydrolyzed by acid or base. Cysteine is readily oxidized to form a covalently linked dimeric amino acid called cystine, in which two cysteine molecules or residues are joined by a disulfide bond (Fig. 3-7). The disulfide-linked residues are strongly hydrophobic (nonpolar). Disulfide bonds play a special role in the structures of many proteins by forming covalent links between parts of a protein molecule or between two different polypeptide chains. 

Dubowchik, G. and Hamilton, A.D. 1985. Controlled conformational changes in covalently-linked dimeric porphyrins, J. Chem. Soc., Chem Commun., 904-906. 

Most of the IL-lOs, as well as IL-19, IL-20, IL-22, and IL-24, have an even number of cysteine residues that all participate in disulfide bonds. However, human, simian, and baboon cmvIL-10 as well as IL-26 contain one additional unpaired cysteine residue (Eig. 6). The crystal structure of human cmvIL-10 revealed the unpaired cysteine forms an inter-chain disulfide bond resulting in a covalently linked dimer structure that differs from the non covalent dimers of ebv and cellular IL-10 and IEN-7 (Jones et aL, 2002b). The unpaired cysteine is located in the BG loop of cmvIL-10. However, the position of the unpaired cysteine in simian and baboon... 

Lunn, C. A., Davies, L., Dalgarno, D., Narula, S. K., Zavodny, P. J., and Lundell, D. (1992a). An active covalently linked dimer of human interferon-gamma. Subunit orientation in the native protein. J. Biol. Chem. 267, 17920-17924. 

MPO is a covalently linked dimeric glycoprotein of Mr 140 kDa comprised of 745 amino acids. MPO consists of three different isoenzyme forms termed a, b, and c. The natme of the chemical differences between these isoforms is not fiilly understood, and only isoform c has been crystallized. Catalytically active recombinant human MPO has been expressed in Chinese hamster ovary cells. However, incomplete posttranslational processing of the recombinant enzyme yields a monomeric form of the enzyme consisting of a single polypeptide chain of Mr 84 kDa with altered carbohydrate content. ... 

MPO is a covalently linked dimer which is ellipsoidal in shape with overall dimensions of 110 x 60 x 50 A. The dimer can be cleaved by reduction of a disulfide bond into two identical halves. Each half of the dimer termed hemi-MPO has the same optical and catalytic properties of the dimer. Hemi-MPO consists of two polypeptides of466 and 108 amino acid residues, and a heme prosthetic group covalently bound to the large polypeptide. Like CcP and LIP, MPO is largely a helical bundle protein with very little -sheet stmcture. The bulk of the large polypeptide folds into five separate domains and one... 

The crystal structure of canine MPO has recently been determined to 3-A resolution (7, 14). This mammalian enzyme is a covalently linked dimer of molecular weight 140 kDa that can be cleaved into two identical halves by reduction of a single disulfide bond. Each half of the dimer, termed hemi-MPO, has the same optical properties and catalytic activity as the parent. Hemi-MPO consists of two polypeptides of 466 and 108 amino acid residues, and a heme-type prosthetic group is covalently bound to the larger polypeptide in a crevice 15 A below the protein surface. Like CCP and LIP, the secondary structure of MPO is dominated by a-helices with relatively little /3-sheet structure (Fig. 

In bacterial RCs, the light-driven reactions are initiated by a non-covalently linked dimeric primary electron donor, D, also called a special pair . The existence of such a dimer was first postulated from spin-resonance experiments (Norris et al., 1971). This special pair was identified as a homodimeric bacteriochlorophyll a (Rb. sphaeroides) or b (Rp. viridis) complex. Its position is close to the periplasmic side at the L and M polypeptide interface. In the Rp. viridis RC the dimer is located close to heme 3 of the cytochrome-c subunit. The helices C, D, E, and cd of both, L- and M-subunits, stabilize the... 

The entropy of dimerization was overcome by covalently linking two molecules of pyrochlorophyll a together. Later two chlorophyll a macrocycles were also successfully joined. The covalently linked dimers of chlorophyll a or pyrochlorophyll a exist in one of three forms (Fig. 11). In the presence of nucleophiles the two molecules appear to be independent of one another. The absorbance and fluorescence spectra are identical to monomeric chlorophyll a. The only difference between monomeric chlorophyll a and the dimer in the presence of nucleophiles is a much smaller cross-section for stimulated emission. Under nucleophile-free conditions in... 

Covalently linked dimers of both chlorophyll a and pyrochlorophyll a have been prepared, which mimic the spectroscopic and redox properties of P700. The two chlorophylls are joined in each case at their propionic acid side chains via an ethylene glycol diester linkage. The orientation of the chlorophyll macrocycles with respect to one another (Figs. 11 and 12) and consequently their electronic properties depend strongly on the solvent. The structure of most interest is the folded one (Fig. 11) because of its similarity to the photoactive dimer in photosystem I. 

Kinetic studies on the mechanism of asymmetric (salen)Cr(ni) catalysed ring opening of epoxides by TMSN3 provide strong support for a mechanism involving catalyst activation of both nucleophile and electrophile by two different catalyst molecules. This observation led Jacobsen et al. to construct covalently linked dimeric salen complexes... 

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