Electronic homologs

One final comparison can be made by looking at the oxides formed by Dy and Cf, which are electronic homologs. The oxide system of Dy is essentially that of Dy203, while with Cf, in addition to its sesquioxide, there are Cf70j2 and Cf02 (Cf02 is difficult to form, as is Tb02). Comparisons between other pairs of lanthanide and actinide oxides are also informative. 

Assuming that the lanthanide and actinide dioxides are truly stoichiometric, a self-consistent and relative set of quadrivalent radii can be calculated from their lattice parameters. These radii are given in table 30. The actinide dioxides are known to have larger lattice parameters than their lanthanide electronic homologs. Thus, PuOj has a parameter similar to that for Ce02, which is four elements to the left of samarium, the vertical 4f homolog of Pu. This is a greater shift than that observed with the sesquioxides (see below). Structurally, the dioxides of the lanthanides and actinides are isostructural, and crystallize in the fluorite-type, cubic (fee) structure. 

In fig. 23 plots are shown of the lattice parameters for the cubic sesquioxides and mononitrides of the lanthanides and actinides, and the atomic volumes for the metals of both series (Haire and Gibson 1992). The purpose of these plots is to point out that relationships (such as bonding, volumes of electronic homologs, etc.) between these series change considerably with the material being considered i.e., the bonding involved in the oxides differs from that in the nitrides, which also varies from that in the... 

Although californium is the electronic homolog of dysproaum, its divalent-trivalent behavior in solution may be more similar to that of samarium, and its trivalent-tetravalent behavior may be comparable to that of terbium. 

BL Sibanda, TL Blundell, JM Thornton. Conformation of (I-hairpms m protein stractures A systematic classification with applications to modelling by homology, electron density fitting and protein engineering. J Mol Biol 206 759-777, 1989. 

The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

Asymmetnc tandem cycloaddicion of a chiral carbohydrate tutroalkene with ethyl vmyl ether in the ptesence of electron-withdrawmgalkenes produces a facile assembly of bicychc systems, which can further be seletnvely cleaved to give homologated carbohydrates fEq 8 110 ... 

Where FCl is the solute gas-liquid partition coefficient, r is the tendency of the solvent to interact through k- and n-electron pairs (Lewis basicity), s the contribution from dipole-dipole and dipole-induced dipole interactions (in molecular solvents), a is the hydrogen bond basicity of the solvent, b is its hydrogen bond acidity and I is how well the solvent will separate members of a homologous series, with contributions from solvent cavity formation and dispersion interactions. 

The cyclization of the homologous epoxide 36 under acidic conditions was also investigated (Table 9.5) [110]. As would be expected, compound 36a reacted by a 6-exo cyclization to give tetrahydropyran 38a (Entry 1). The a, 3-unsaturated hydroxy epoxide 36b gave a 1 3.5 mixture of oxepane 37b and tetrahydropyran 38b (Entry 2). Subjection of 36c and 36d, which both contain more electron-rich 71-systems, to the reaction conditions resulted in preferential 7-endo cyclization to give 37c and 37d, thus confirming the powerful regiodirecting effect of the vinyl moiety (Entries 3 and 4). 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

The Homer - Emmons reagent (52) is effective in the one carbon homologation of ketones possessing acidic a-hydrogen atoms <96SL875> and electron-deficient alkenes add to 2-phenylseleno-l,3-dithiane in a photo-initiated heteroatom stabilised radical atom transfer process, giving products of considerable synthetic potential <96TL2743>. 

The prototype of this class of soluble ferredoxins was initially obtained from spinach chloroplasts and subsequently been shown to play a role in physiological electron shuttling between PSl and a number of redox proteins, most prominently ferredoxin-NADP-reduc-tase 13). Homologous proteins were purified from several cyanobac-... 

This study of such of an electron transfer chain is most timely, since the 3D structures of all the components involved are known (and related components can easily be obtained by homology molecular modeling). Proposals of structural models for the complexes formed between D. gigas AOR and flavodoxin, based on the available X-ray... 

Molecular replacement is where the phases of a known structure are used to determine the structure of a protein that may be identical but crystallized in a different space group or may adopt essentially the same structure (e.g., a homologous protein). Essentially, the calculations find the rotation and translation of the molecule that work with the phases to produce an interpretable electron density map. 

Recently, this concept could be transferred to the homologous silylium ions 3-Si (Si in cx-position to the ferrocene core) [18, 19], which were found to be potent catalysts for Diels-Alder reactions at low temperature [19]. The electron-rich ferrocene core buffers the Lewis acidity, thus avoiding the irreversible formation of Lewis pairs. 

---