Three-dimensional aromaticity

Point of view of the chemistry lignin is a natural amorphous cross-linked resin that has an aromatic three-dimensional polymer structure containing a number of functional groups such as phenolic, hydroxyl, carboxyl, benzyl alcohol, methoxyl, and aldehyde as shown in Figure 5.1, which make lignin potentially useful as an adsorbent material for removal of heavy metal ions from water [52]. 

The methodology for preparation of hydrocarbon-soluble, dilithium initiators is generally based on the reaction of an aromatic divinyl precursor with two moles of butyUithium. Unfortunately, because of the tendency of organ olithium chain ends in hydrocarbon solution to associate and form electron-deficient dimeric, tetrameric, or hexameric aggregates (see Table 2) (33,38,44,67), attempts to prepare dilithium initiators in hydrocarbon media have generally resulted in the formation of insoluble, three-dimensionally associated species (34,66,68—72). These precipitates are not effective initiators because of their heterogeneous initiation reactions with monomers which tend to result in broader molecular weight distributions > 1.1)... 

This is nicely illustrated by members of the chymotrypsin superfamily the enzymes chymotrypsin, trypsin, and elastase have very similar three-dimensional structures but different specificity. They preferentially cleave adjacent to bulky aromatic side chains, positively charged side chains, and small uncharged side chains, respectively. Three residues, numbers 189, 216, and 226, are responsible for these preferences (Figure 11.11). Residues 216... 

Clearly a wide range of phenols and amines are possible. Where appropriate multi-ringed phenols are used such as 4,4-dihydroxybenzophenone or bisphenol A then three-dimensional cross-linked polymers will be obtained. Amines may be aliphatic such as methylamine or aromatic such as aniline. 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arac/tno-boranes tend to be more reactive (and less stable to thermal decomposition) than niiio-boranes and reactivity also diminishes with increasing mol wt. C/oio-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

Lignin is a high-molecular, mainly three-dimensional aromatic substance, that, in contrast to polysaccharides, can be hydrolyzed only in small partitions [16] ... 

The second step is the condensation reaction between the methylolphe-nols with the elimination of water and the formation of the polymer. Crosslinking occurs hy a reaction between the methylol groups and results in the formation of ether bridges. It occurs also by the reaction of the methylol groups and the aromatic ring, which forms methylene bridges. The formed polymer is a three-dimensional network thermoset ... 

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

Fig. 15. A view, perpendicular to the aromatic ring, of the three-dimensional hydrogen-bonded network formed by p-(HOMe2Si)2C6H4, with hydrogen atoms omitted for clarity. Drawn using coordinates taken from the Cambridge Crystallographic Database.

The connection of the three-dimensional carboranes and classical aromatic compounds, the two-dimensional benzenes, has been a fertile area for exploration. 

Fig. 11.2. Schematic representation of the primary structure of secreted AChE B of N. brasiliensis in comparison with that of Torpedo californica, for which the three-dimensional structure has been resolved. The residues in the catalytic triad (Ser-His-Glu) are depicted with an asterisk, and the position of cysteine residues and the predicted intramolecular disulphide bonding pattern common to cholinesterases is indicated. An insertion of 17 amino acids relative to the Torpedo sequence, which would predict a novel loop at the molecular surface, is marked with a black box. The 14 aromatic residues lining the active-site gorge of the Torpedo enzyme are illustrated. Identical residues in the nematode enzyme are indicated in plain text, conservative substitutions are boxed, and non-conservative substitutions are circled. The amino acid sequence of AChE C is 90% identical to AChE B, and differs only in the features illustrated in that Thr-70 is substituted by Ser.

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