Stabilization of diborane(4) by transition metal fragments and a novel metal to pi Dewar-Chatt-Duncanson model of back donation

The feasibility of using transition metal fragments to stabilize B2H4 in planar configuration by donating 2 electrons to the boron moiety is investigated. Building upon the existing theoretical and experimental data and aided by the isolobal analogy, the model transition metal complexes Cr(CO)(4)B2H4 (6), Mn(CO)-CpB2H4 (7), Fe(CO)(3)B2H4 (8) and CoCpB2H4 (9) are chosen to illustrate this unique bonding feature bond strengthening with pi-back donation. Other possible types of complexes with B2H4 and the metal fragment are also explored and the energies are compared. One of the low energy isomers wherein the planar B2H4 interacts with the metal fragment in an in-plane fashion represents a unique case study for the Dewar-Chatt-Duncanson model. In this complex the back-donation from the metal fills the p bonding orbital between the two boron atoms thus forming a B=B double bond.

The generalized anomeric effect in the 1,3-thiazolidines: Evidence forboth sulphur and nitrogen as electron donors. Crystal structures of various N-acylthiazolidines including mercury(II) complexes. Possible relevance to penicillin action

Evidence for the generalized anomeric effect (GAE) in the N-acyl-1,3-thiazolidines, an important structural motif in the penicillins, was sought in the crystal structures of N-(4-nitrobenzoyl)-1,3-thiazolidine and its (2:1) complex with mercuric chloride, N-acetyl-2-phenyl-1,3-thiazolidine, and the (2:1) complex of N-benzoyl-1,3-thiazolidine with mercuric bromide. An inverse relationship was generally observed between the. C-2-N and C-2-S bond lengths of the thiazolidine ring, supporting the existence of the GAE. (Maximal bond length changes were similar to 0.04 angstrom for C-2-N-3, S-1-C-2, and similar to 0.08 angstrom for N-3-C-6.) Comparison with N-acylpyrrolidines and tetrahydrothiophenes indicates that both the nitrogen-to-sulphur and sulphur-to-nitrogen GAE's operate simultaneously in the 1,3-thiazolidines, the former being dominant. (This is analogous to the normal and exo-anomeric effects in pyranoses, and also leads to an interesting application of Baldwin's rules.) The nitrogen-to-sulphur GAE is generally enhanced in the mercury(II) complexes (presumably via coordination at the sulphur); a 'competition' between the GAE and the amide resonance of the N-acyl moiety is apparent. There is evidence for a 'push-pull' charge transfer between the thiazolidine moieties in the mercury(II) complexes, and for a 'back-donation' of charge from the bromine atoms to the thiazolidine moieties in the HgBr2 complex. (The sulphur atom appears to be sp(2) hybridised in the mercury(II) complexes, possibly for stereoelectronic reasons.) These results are apparently relevant to the mode of action of the penicillins. (c) 2006 Elsevier B.V. All rights reserved.

The evolution of genomic imprinting

We explore three possible pathways for the evolution of genomic imprinting. (1) Imprinting may be advantageous in itself when imprinted and unimprinted alleles of a locus confer different phenotypes. If a segment of DNA is imprinted in the gametes of one sex but not in those of the other, it might lead to effects correlated with sexual dimorphism. More fundamentally, in certain organisms, sex determination might have evolved because of imprinting. When imprinting leads to chromosome elimination or inactivation and occurs in some embryos but not in others, two classes of embryos, differing in the number of functional gene copies, would result. A model for sex determination based on inequality in the actual or effective copy-number of particular noncoding, regulatory sequences of DNA has been proposed (Chandra, Proc. natn. Acad. Sci. U.S.A. 82. 1165–1169 and 6947–6949, 1985). Maternal control of offspring sex is another possible consequence of imprinting; this would indicate a potential role for imprinting in sex ratio evolution. (2) Genes responsible for imprinting may have pleiotropic effects and they may have been selected for reasons other than their imprinting ability. Lack of evidence precludes further consideration of this possibility. (3) Imprinting could have co-evolved with other traits. For instance, gamete-specific imprinting could lead to a lowered fitness of androgenetic or gynogenetic diploids relative to the fitness of ‘normal’ diploids. This in turn would reinforce the evolution of anisogamy. The reversibility of imprinting raises the possibility of occasional incomplete or improper erasure. If the site of imprinting is the egg – as appears to be the case with the human X (Chandra and Brown, Nature 253. 165–168, 1975) – either improper imprinting or improper erasure could lead to unusual patterns of inheritance (as in the fragile-X syndrome) or fitness effects skipping generations.

Molecular structure of electron donor-acceptor complexes of metallotetraphenylporphyrins with trinitrobenzene

The metallotetraphenylporphyrins, MTPPs, where M=Co(II), Cu(II) and Ag(II) form one to one molecular complexes in solution with 1,3,5-trinitrobenzene (TNB). The crystal structure of CoTPP.TNB.2CH3OH revealed that the mean separation between the porphyrin and TNB planes is 3·27 Å and the centre of the aryl ring of TNB is displaced by 0·90 Å from the centre of the porphyrin plane. Extended Huckel molecular orbital calculations suggest that acceptor orbitals are predominantly nitro-group based rather than an aryl π framework. The contribution of the metal orbitals in the donation is also seen as predicted by the structural data. Electrochemical redox measurements in solution provide evidence for charge-transfer stabilisation in these complexes.

The Indian-Institute-Of-Science - 8 Decades Of An Interdisciplinary Center

The Indian Institute of Science at Bangalore is probably the oldest institute in India, established with a donation from the great industrial visionary Jamsetji Tata over eight decades ago. It has gradually become a key centre of scientific and engineering research and higher education. From its very inception the institute had an interdisciplinary approach to research and teaching.

Parasites exert conflicting selection pressures to affect reproductive asynchrony of their host plant in an obligate pollination mutualism

1. Plant reproductive phenology is generally viewed as an individual's strategy to maximize gamete exchange and propagule dispersal and is often considered largely dependent on patterns of floral initiation. Reproductive phenology, however, can be affected by proximate responses to pollinators, parasites and herbivores which could influence floral longevity or fruit development time. 2. We examined the influence of insect interactants on within-plant reproductive phenology in the fig-fig wasp nursery pollination mutualism in Ficus racemosa (Moraceae). Most figs support a wasp community comprised of a mutualistic pollinator, with several host-plant-specific non-pollinating herbivorous gallers and parasitoids. These wasps reproduce within enclosed inflorescences called syconia, which develop into fruit after pollination. While different wasp species oviposit into syconia at varying times during its ontogeny, all wasp progeny are constrained to exit syconia simultaneously just prior to fruit ripening. Developing larvae of early-ovipositing wasps may hasten syconium ontogeny through formation of earlier and larger nutrient sinks, whereas larvae of late-arriving parasites may lengthen syconium ontogeny to complete their development successfully. Seeds are also important nutrient sinks. The number of seeds and the type and number of developing wasps may therefore be expected to influence syconium development times, thereby affecting the reproductive synchrony of syconia on a plant. 3. Observations on naturally pollinated and parasitized syconia indicated that their seed and wasp content affected syconium development time. Experimental manipulations of syconia to produce only seeds or various combinations of wasps confirmed this finding. Early-ovipositing galler progeny reduced syconium development times, while gallers ovipositing concurrently with pollinators had no effect on syconium development. Late-ovipositing parasitoid progeny, the presence of only seeds within the syconium, or delayed pollination increased syconium development time. The differential development of syconia, which was influenced by mutualistic or parasitic progeny, accordingly contributed to within-tree reproductive asynchrony. 4. Synthesis. Individual reproductive units in fig trees called syconia, which also function as brood sites for pollinating and parasitic fig wasps, have plastic development durations dependent on pollination timing and species of wasps developing within them. Syconium development times are a likely compromise between conflicting demands from developing seeds and different wasp species.

Negative hyperconjugation and red-, blue- or zero-shift in X-Z center dot center dot center dot Y complexes

A generalized explanation is provided for the existence of the red-and blue-shifting nature of X-Z bonds (Z = H, halogens, chalcogens, pnicogens, etc.) in X-Z center dot center dot center dot Y complexes based on computational studies on a selected set of weakly bonded complexes and analysis of existing literature data. The additional electrons and orbitals available on Z in comparison to H make for dramatic differences between the H-bond and the rest of the Z-bonds. The nature of the X-group and its influence on the X-Z bond length in the parent X-Z molecule largely controls the change in the X-Z bond length on X-Z center dot center dot center dot Y bond formation; the Y-group usually influences only the magnitude of the effects controlled by X. The major factors which control the X-Z bond length change are: (a) negative hyperconjugative donation of electron density from X-group to X-Z sigma* antibonding molecular orbital (ABMO) in the parent X-Z, (b) induced negative hyperconjugation from the lone pair of electrons on Z to the antibonding orbitals of the X-group, and (c) charge transfer (CT) from the Y-group to the X-Z sigma* orbital. The exchange repulsion from the Y-group that shifts partial electron density at the X-Z sigma* ABMO back to X leads to blue-shifting and the CT from the Y-group to the sigma* ABMO of X-Z leads to red-shifting. The balance between these two opposing forces decides red-, zero- or blue-shifting. A continuum of behaviour of X-Z bond length variation is inevitable in X-Z center dot center dot center dot Y complexes.