Linkage bonds

P212121 Z = 4 D, = 1.413 R = 0.041 for 3,165 intensities. Thepyran-oside conformation is 4C1, with Q = 58 pm, 6 = 5°. The primary alcohol group is gauche-trans. The linkage-bond torsion-angles are O-5-C-l-0-1 -C-7 = — 87°, C-1-0-1-C-7-0-7 = — 67°. The two rings of the monoterpene moiety are inclined to each other, with C-C-C-C torsion-angles, about the common bond, of 149 and —97°. 

The chemical nature of the main-chain linkages of step-growth polymers makes this class of polymers particularly reactive to a wide variety of chemical species. Solvolysis reactions break the C-X bond at the polymer linkage bonds. These types of reactions are often pH-dependent, so the stability of the polymer is highly dependent on the acidity or basicity of the prodegradant. 

With three linkage bonds as in the molecular mechanics study of the 1-6 disaccharide gentiobiose (12), the lowest 24 minima within 7 kcal mole which were reported did not include that observed in the crystal structure (20). Had the crystal structure conformation been one of the sets of starting parameters, it would have been included, but it is reasonable to assume that the conformational population in solution was also not fully represented. 

Figure 2. Two maltotetraose models constructed from residues of glucose having different D. They have identical linkage bond and torsion angles.

In analyzing the conformational properties of carbohydrates, it has often been the practice to consider individual monomer rings to be rigid units, with the only molecular flexibility lying in torsional rotations about the linkage bonds. 

Phenolic compounds can be condensed forming aryl-aryl and aryl-oxygen-aryl (ether linkages) bonds to yield diaryl and diaryl ether polymers (59). These are in many ways similar to natural humic acids, confirming earlier research by others (60-62) that humic acids are formed from the copolymerization of phenolic compounds with amino acids, peptides, and amino sugars. 

Linkers. Linkers mainly affect the attachment and detachment steps. Figure 7.7 gives data for the Stille reaction 18 with various chemical linkers. The characteristic carbonyl bands of the squarate moiety at 1780 and 1750 cm, and those of the ester or amide linkage bonds, allow facile monitoring... 

Linkage bonds are defined with reference to the atoms having the highest Klyne-Prelog priority,21 i.e., 0-C-0-C-C(CH0H)2C02. ... 

P2j Z = 4 Dx = 1.40 R = 0.044 for 3,430 intensities. There are two symmetry-independent molecules in the crystal structure. The orientation of their linkage bonds is very similar, but the orientation of the phenyl group and the primary alcohol groups is different The linkage torsion-angles are 0-5-C-1-0-1-C-4, +108, +110° C-l-O-l-C-4 -... 

When the donors and acceptors lack spherical symmetry, there will also be an orientation dependence. In cases such as those to be discussed below, where the donor and acceptor moieties are linked by covalent bonds, there is considerable evidence that in certain situations the electron transfer occurs through the linkage bonds [22]. Although such linkages are not present in photosynthetic reaction centers, it has been proposed that the accessory Bchl or other intervening material may still take part in electron transfer through a superexchange mechanism [8, 26]. The distance dependence of photoinitiated electron transfer has recently been reviewed [13]. 

The Q-P-P-Q tetrad 43 has been synthesized by Sessler and coworkers [69, 98]. As expected, it demonstrates strong quenching of the porphyrin fluorescence, and this presumably occurs by electron transfer. It is probable that selective metallation of such a tetrad would produce a species which would behave as a triad in a manner analogous to 17 and 18, but it is not obvious that such a molecule would have any significant advantages over these systems. As is the case with a number of the other complex systems, 43 may in principle demonstrate stereoisomerism when rotations about the linkage bonds are slow on the time scales of electron and energy transfer. 

The ATP molecule contains pyrophosphate linkages (bonds formed when two phosphate units are combined together) that release energy when needed. ATP can be hydrolyzed in two ways the removal of terminal phosphate to form adenosine diphosphate (ADP) and inorganic phosphate, or the removal of a terminal diphosphate to yield adenosine monophosphate (AMP) and pyrophosphate. The latter is usually cleaved further to yield two phosphates. This results in biosynthesis reactions, which do not occur alone, that can be driven in the direction of synthesis when the phosphate bonds are hydrolyzed. 

Cellulose has beta-1,4 linkages, bonds which human enzymes cannot break, but the bacterial enzymes in cows can. Therefore cows eat grass, but we don t. 

Bonds formed by cysteinyl residues can be made susceptible to tryptic action when the thiol group of the cysteine side chains is reacted with i8-bromoethylamine (Bindley, 1956). The resulting ( -( -aminoethyl)-cysteinyl side chains provide the specificity requirements necessary for tryptic action. One methylene carbon of the lysyl side chain is replaced by a sulfur atom in thioether linkage. Bonds formed by the carboxyl group of this derivative are susceptible to tryptic action. Reduction of... 

To further investigate the role of motion in triplet energy transfer, three carotenoporphyrins, 32,33, and 34, were synthesized [73]. In these molecules, the static coupling along the linkage bonds was nearly constant and relatively weak, but the constraints on intramolecular motion differed considerably. From detailed NMR studies in solution the time average conformations were determined. The para-linked isomer (32) was found to be roughly linear, the meta-linked one (33) approximately L shaped, and the ortho-linked one (34) partially folded. 

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