Molecule tetramer

Oligomer (Section 14 15) A molecule composed of too few monomer units for it to be classified as a polymer but more than in a dimer trimer tetramer etc Oligonucleotide (Section 28 6) A polynucleotide containing a relatively small number of bases Oligosaccharide (Section 25 1) A carbohydrate that gives three to ten monosacchandes on hydrolysis Optical activity (Section 7 4) Ability of a substance to rotate the plane of polanzed light To be optically active a sub stance must be chiral and one enantiomer must be present in excess of the other... 

Methyllithium. MethyUithium [917-54 ] CH Li, crystallizes from benzene or hexane solution giving cubic crystals that have a salt-hke constitution (128). Crystalline methyllithium molecules exist as tetrahedral tetramers (129). Solutions of methyllithium are less reactive than those of its higher homologues. Methyllithium is stable for at least six months in diethyl ether at room temperature. A one-molar solution of methyllithium in tetrahydrofuran (14 wt %) and cumene (83 wt %) containing 0.08 M dimethyknagnesium as stabilizer loses only 0.008% of its activity per day at 15°C and is nonpyrophoric (117). 

Hydroxyl Compounds. The aqueous chemistry of zirconium is complex, and in the past its understanding was compHcated by differing interpretations. In a study of zirconium oxide chloride and zirconium oxide bromide, the polymeric cation [Zr4(OH)g (H20)jg was identified (189) the earlier postulated moiety [Zr=0] was discarded. In the tetramer, the zirconium atoms are coimected by double hydroxyl bridges (shown without the coordinating water molecules) ... 

Hemoglobin is a tetramer built up of two copies each of two different polypeptide chains, a- and (5-globin chains in normal adults. Each of the four chains has the globin fold with a heme pocket. Residue 6 in the p chain is on the surface of a helix A, and it is also on the surface of the tetrameric molecule (Figure 3.13). 

Figure 8.22 The lac repressor molecule is a V-shaped tetramer in which each arm is a dimer containing a DNA-hinding site. The helix-tum-helix motifs (red) of each dimer bind in two successive major grooves and the hinge helices (purple) bind adjacent to each other in the minor groove between the two major groove binding sites. The four subunits of the tetramer are held together by the four C-terminal helices (yellow) which form a four helix bundle. The bound DNA fragments are bent. (Adapted from M. Lewis et al., Science 271 1247-1254, 1996.)...

The polypeptide chain of p53 is divided in three domains, each with its own function (Figure 9.16). Like many other transcription factors, p53 has an N-terminal activation domain followed by a DNA-binding domain, while the C-terminal 100 residues form an oligomerization domain involved in the formation of the p53 tetramers. Mutants lacking the C-terminal domain do not form tetramers, but the monomeric mutant molecules retain their sequence-specific DNA-binding properties in vitro. 

It is to be noted that only one condensation reaction is necessary to convert two molecules with values of x = 100 to one molecule with x = 200. A similar reaction between two dimers will produce only tetramers [x = 4). Thus although the concentration of reactive groups may decrease during reaction, individual reactions at later stages of the reaction will have greater effect. 

Crystal structure determination has also been done with -butyllithium. A 4 1 n-BuLi TMEDA complex is a tetramer accommodating two TMEDA molecules, which, rather than chelating a lithium, link the tetrameric units. The 2 2 -BuLi TMEDA complex has a structure similar to that of [PhLi]2 [TMEDA]2. Both 1 1 -BuLi THF and 1 1 -BuLi DME complexes are tetrameric with ether molecules coordinated at each lithium (Fig. 7.2). These and many other organolithium structures have been compared in a review of this topic. ... 

Oligomer (Section 14.15) A molecule composed of too few monomer units for it to be classified as a polymer, but more than in a dimer, trimer, tetramer, etc. 

FIGURE 15.23 The myoglobin and hemoglobin molecules. Myoglobin (sperm whale) one polypeptide chain of 153 aa residues (mass = 17.2 kD) has one heme (mass = 652 D) and binds one O9. Hemoglobin (human) four polypeptide chains, two of 141 aa residues (u) and two of 146 residues (/3) mass = 64.45 kD. Each polypeptide has a heme the Hb tetramer binds four O9. (Irving Geis)... 

As noted, hemoglobin is an tetramer. Each of the four subunits has a conformation virtually identical to that of myoglobin. Two different types of subunits, a and /3, are necessary to achieve cooperative Oa-binding by Hb. The /3-chain at 146 amino acid residues is shorter than the myoglobin chain (153 residues), mainly because its final helical segment (the H helix) is shorter. The a-chain (141 residues) also has a shortened H helix and lacks the D helix as well (Figure 15.28). Max Perutz, who has devoted his life to elucidating the atomic structure of Hb, noted very early in his studies that the molecule was... 

Further protonation of the trimer produces a C9 carhocation which may further react with another propene molecule and eventually produce propylene tetramer. 

Figure 15 shows a stereoscopic view of the crystalline 1 1 complex where R7 = i-CsHn and R8 = (CH2)2Ph 9). The packing mode of the four molecules in the unit cell of this complex corresponds to the association scheme of tetramer 17 (Fig. 8). Of particular interest is that a pair of groups with similar geometrical shape, NMe2 and CHMe2 [part of C6H4NMe2 and (CH2)2CHMe2], are in close contact.

IV II Tetramer (2 ParC 2 ParE subunits) parC/ parE Decatenation of linked daughter DNA molecules... 

This example of step-growth polymerization has two monomers that can react with each other but not with themselves. It has only one dimer that can be self-reactive. This pattern continues indefinitely with two trimers, one self-reactive tetramer, and so on. Molecules with an odd number of mer units will... 

The MoFe proteins are all a2 2 tetramers of 220-240 kDa, the a and (3 subunits being encoded by the nifD and K genes, respectively. The proteins can be described as dimers of a(3 dimers. They contain two unique metallosulfur clusters the MoFeTSg homocitrate, FeMo-cofactors (FeMoco), and the FesSy, P clusters. Neither of these two types of cluster has been observed elsewhere in biology, nor have they been synthesized chemically. Each molecule of fully active MoFe protein contains two of each type of cluster 2-7). 

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