Tetramer crystal structure

Figure 15. Stereoview of the melittin tetramer crystal structure (186, 187) with ribbon overlay that shows the bend in each helix and the hydrophobic contacts...

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. ... 

Fig. 7.3. Crystal structures of some lithium etiolates of ketones. (A) Unsolvated hexameric enolate of methyl t-butyl ketone (B) tetrahydrofuran solvate of tetramer of enolate of methyl r-butyl ketone (C) tetrahydrofuran solvate of tetramer of enolate of cyclopentanone (D) dimeric enolate of 3,3-dimethyl-4-(r-butyldimethylsiloxy)-2-pentanone. (Structural diagrams are reproduced from Refs. 66-69.) by permission of the American Chemical Society and Verlag Helvetica Chimica Acta AG.

Interestingly, the 28-hehcal fold identified by NMR analysis of /9-peptide 109 compares well with the model of a /9 -peptide consisting of 1-aminomethylcyclo-propanecarboxylic acid residues (Fig. 2.24). This model was generated using ideal torsion angle values ( = + 120°, 9i=-72°, ii/=0°, and < =180°) derived from crystal structures of dimer 110, trimer 111 and tetramer 112 [163] (Fig. 2.25). 

Fig. 2.25 Conformational preferences and eight-membered turn motif of jS -peptides with 1 -aminomethylcyclopropanecarboxylic acid residues. X-ray crystal structures of dimer no, trimer m and tetramer 112 together with a model constructed using...

The crystal structures of both the cis and the trans isomers of 2,8-dihydroxy-2,4,4,6,6,8,10,10,12,12-decamethylcyclohexasiloxane have been determined. In this case (unlike the cyclotetrasiloxane in Fig. 26), the cis isomer does contain an intramolecular hydrogen bond, and intermolecular hydrogen bonds link the molecules into cyclic pairs 57. The trans isomer cannot form intramolecular hydrogen bonds, but forms cyclic tetramers which are further hydrogen-bonded to form infinite sheets 58 (280). 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

Remarkably, although there is little sequence homology between the members of the GFP super family (DsRed and avGFP share less then 30% sequence homology), their crystal structures are highly similar [25-28]. The /l-barrel structure is a feature common to all members of the GFP super family for which the crystal structure has been solved. However, whereas avGFP is present mainly as a monomer, many other VFPs form obligate di- or tetramers. 

The crystal structure of the extracellular domain of P0 has also been determined [41]. The arrangement of molecules in the crystal indicates that P0 may exist on the membrane surface as a tetramer (Fig. 7-7) that can link to other tetramers from the opposing membrane to form an adhesive lattice, like a molecular Velcro . The structure also suggests that P0 mediates adhesion through the direct interaction of apically directed tryptophan side chains with the opposing membrane [42], in addition to homo-philic protein-protein interaction. 

Figure 11 X-ray crystal structure of tetramer [(tacn)Co(6-put-N7,S6 N9)]4 (28A). (Reprinted with permission from 98IC5611, Copyright 1998, American Chemical Society.)...

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