Molecular chains tetramerization

Practical realizations of regular antiferromagnetic chains are provided either by regular chains of monomers, with one electron per monomer or p = 1, or by regular chain of dimers, with one electron (one hole) per dimer or p = 3(2). Then the 2kv distortion results in a dimerization of the molecular chains in the first case, and in a tetramerization, or, more correctly, a dimerization of dimers [19] in the second case (see also Fig. 5). 

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)... 

Schoenmakers et al. [72] analyzed two representative commercial rubbers by gas chromatography-mass spectrometry (GC-MS) and detected more than 100 different compounds. The rubbers, mixtures of isobutylene and isoprene, were analyzed after being cryogenically grinded and submitted to two different extraction procedures a Sohxlet extraction with a series of solvents and a static-headspace extraction, which entailed placing the sample in a 20-mL sealed vial in an oven at 110°C for 5,20, or 50 min. Although these are not the conditions to which pharmaceutical products are submitted, the results may give an idea of which compounds could be expected from these materials. Residual monomers, isobutylene in the dimeric or tetrameric form, and compounds derived from the scission of the polymeric chain were found in the extracts. Table 32 presents an overview of the nature of the compounds identified in the headspace and Soxhlet extracts of the polymers. While the liquid-phase extraction was able to extract less volatile compounds, the headspace technique was able to show the presence of compounds with low molecular mass... 

The discussion in the previous sections should suffice to indicate the importance of the transformations of molecular compounds to materials. Studies of such transformations are only making a beginning, and there appears to be a great future for such investigations, Thus, the recent synthesis of a sodalite-related structure from a molecular zinc phosphate is noteworthy, besides the transformation of the centrosymmetric, tetrameric, molecular alumino-phosphate to open-framework structures. Preliminary investigations53 have shown that interesting transformations also occur in metal carboxylates. Thus, molecular zinc oxalate monomers and dimers are found to transform to chain or three-dimensional structures on heating with piperazine in an aqueous medium, The chain structure transforms to a layer structure, We believe that it would... 

Crystallographic data [21—23] indicate that for TEA(TCNQ)2, there are two molecular formulas per unit cell, or Z = 2. In the crystal structure, segregated parallel TCNQ chains are built up from identical A-B dimer units according to the tetramerized sequence... 

Flavocytochrome 62 is composed of a single polypeptide chain with a molecular weight, as estimated by SDS-polyacrylamide gel electrophoresis, of 57,500 in the case of the S. cerevisiae enzyme (28) and 58,000 for the enzyme from H. anomala (29). The native enzymes behave as oligomers with molecular weights around 230,000 (29-31), suggesting that each is a homotetramer. This has been confirmed for S. cerevisiae flavocytochrome 62 by X-ray crystallography, which shows the subunits to be related by a four-fold axis of symmetry (Fig. 3) (23-25). The tetrameric structure is important for activity because dissociation of the H. anomala enzyme at low ionic strength yields inactive monomers that can be reassociated with recovery of activity (32). 

Cytochrome b2 (EC 1.1.2.3) is a tetrameric protein in which each subunit contains one molecule of flavin mononucleotide and one molecule of heme. The molecular weight is 238 000. The enzyme is contained in Saccharomyces cerevisiae and Hansenula anomala, where it transfers electrons in the respiratory chain from lactate to cytochrome c. With the artificial electron acceptor hexacyanoferrateflll) the respective Km values are 0.4 and 1.3 mmol/1. The pH optimum is between 6.5 and 8. 

Serum levels of lactate dehydrogenase (LDH) were formerly used to diagnose an acute Ml. LDH is present in cells as a tetramer of four identical, or nearly identical, subunits. Each subunit is a separate polypeptide chain with a molecular weight of 35 kD (approximately 35,000 g/mole). These subunits are present as two isoforms, the H isoform (for heart) and the M isoform (for skeletal muscle). Although the heart produces principally the H4 form (four H subunits combined into a tetramer) and skeletal muscles produce principally the M4 isoform, the heart, skeletal muscle, and other tissues produce several intermediate combinations (e.g., H3M, HjMj). These tetrameric isoforms all have similar activities, but the individual monomeric subunits are inactive. Measurements of LDH isozymes in the serum are no longer used for diagnosis of a recent Ml because the enzyme is large, released slowly, and the isozyme pattern is not as specific for the heart as is CK. 

Different polypeptide chains may interact to form more complex multi-polypeptide proteins, wherein each individual polypeptide is known as a subunit. Subunit interactions and interrelationships are illustrated for the tetrameric protein haemoglobin (Figure 1.32). In the case of globular proteins, polypeptide chain association allows functions of individual polypeptide elements to be coordinated or indeed supplemented to give the whole molecule the opportunity to perform multiple biological functions. In the case of hbrous proteins, quaternary structure formation enhances overall molecular strength. 

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