Diatoms shell

In fact, such biomimetic molecules demonstrate the ability to tailor the growth of silica nanoparticles in a way that is very similar to diatom-extracted species. However, they demonstrate the same limitations in terms of morphological control of nanoparticle assembly. This is because the diatom shell architecture results not only from interactions of silica precursors with templating molecules but also benefits from a cell-driven molding of the vesicular compartment where silicification occurs [29]. Thus, it is very likely that diatom-like synthetic silica will only be achieved when such confinement/molding effects are taken into account in the design of biomimetic experiments [30]. 

The basicity of polyamines depends on their degree of polymerization, their architecture (linear or branched), and their chemical make-up (methylated or non-methylated) [17, 18]. It is then obvious that these properties should also influence the acceleration effect. It is remarkable that nature has not chosen the fastest polyamine-silica system for the biosilicification in diatoms, although a rapid construction of novel shells is needed during cell division. On the other hand, however, the biomineralization process also requires time in order to form the intricate shapes and special morphologies characteristic of diatom shells. 

D analysis of a diatom shell (source C. Hamm/C. Kara, Alfred Wegener Institute for Polar Research, Bremerhaven). 

Hamm, C.E., Merkel, R., Springer, O., Jurkojc, P, Maier, C., Prechtel, K. Smetacek, V, 2003. Architecture and material properties of diatom shells provide efficient mechanical protection. Nature, vol. 421, pp. 841-843. 

The complex morphology of the nanopatterned siUca diatom cell walls has been found to be related to species-specific sets of polycationic peptides, so-called silaffins, which were isolated from diatom cell walls [82], The morphologies of precipitated sihca can be controlled by changing the chain lengths of the polyamines as well as by a synergistic action of long-chain polyamines and silaffins [83,84]. It has been proposed that the delicate pattern formation in diatom shells can be explained by phase separation of silica solutions in the presence of these polyamines [85]. Various linear synthetic analogs of the natural active polyamines in biosilica formation can accelerate the silicic acid condensation even more than the above mentioned... 

Diatoms Shell Si02 f Polyuronic acids Amino acids... 

Wenzl, S., Hett, R., Richthammer, P. and Sumper M. (2008) Silacidins highly acidic phosphopeptides from diatom shells assist in silica precipitation in vitro. Angewandte Chemie International Edition, 47,1729-32. 

One method for measuring the temperature of the sea is to measure this ratio. Of course, if you were to do it now, you would take a thermometer and not a mass spectrometer. But how do you determine the temperature of the sea as it was 10,000 years ago The answer lies with tiny sea creatures called diatoms. These have shells made from calcium carbonate, itself derived from carbon dioxide in sea water. As the diatoms die, they fall to the sea floor and build a sediment of calcium carbonate. If a sample is taken from a layer of sediment 10,000 years old, the carbon dioxide can be released by addition of acid. If this carbon dioxide is put into a suitable mass spectrometer, the ratio of carbon isotopes can be measured accurately. From this value and the graph of solubilities of isotopic forms of carbon dioxide with temperature (Figure 46.5), a temperature can be extrapolated. This is the temperature of the sea during the time the diatoms were alive. To conduct such experiments in a significant manner, it is essential that the isotope abundance ratios be measured very accurately. 

Even molecules such as the short-lived SO and PO molecules can be treated, at the present level of approximation, rather like homonuclear diatomics. The reason is that the outer shell... 

In the molecular orbital description of homonuclear diatomic molecules, we first build all possible molecular orbitals from the available valence-shell atomic orbitals. Then we accommodate the valence electrons in molecular orbitals by using the same procedure we used in the building-up principle for atoms (Section 1.13). That is,... 

Silicic acid (H4Si04) is a necessary nutrient for diatoms, who build their shells from opal (Si02 H20). Whether silicic acid becomes limiting for diatoms in seawater depends on the availability of Si relative to N and P. Estimates of diatom uptake of Si relative to P range from 16 1 to 23 1. Dugdale and Wilkerson (1998) and Dunne et al. (1999) have shown that much of the variability in new production in the equatorial Pacific may be tied to variability in diatom production. Diatom control is most important at times of very high nutrient concentrations and during non-steady-state times, perhaps because more iron is available at those times. 

Laming, G. J., Handy, N. C., Amos, R. D., 1993, Kohn-Sham Calculations on Open-Shell Diatomic Molecules , Mol. Phys., 80, 1121. 

Hydroxyl radical (OH) is a key reactive intermediate in combustion and atmospheric chemistry, and it also serves as a prototypic open-shell diatomic system for investigating photodissociation involving multiple potential energy curves and nonadiabatic interactions. Previous theoretical and experimental studies have focused on electronic structures and spectroscopy of OH, especially the A2T,+-X2n band system and the predissociation of rovibrational levels of the M2S+ state,84-93 while there was no experimental work on the photodissociation dynamics to characterize the atomic products. The M2S+ state [asymptotically correlating with the excited-state products 0(1 D) + H(2S)] crosses with three repulsive states [4>J, 2E-, and 4n, correlating with the ground-state fragments 0(3Pj) + H(2S)[ in... 

Linnett used the concept that an octet of valence shell electrons consists of two sets of four opposite-spin electrons to show that in diatomic and other linear molecules the two tetrahedra are not in general formed into four pairs as we have discussed for F2 and the CC triple bond in C2H2. This idea is the basis of the double-quartet model, which Linnett applied to describe the bonding in a variety of molecules. It is particularly useful for the description of the bonding in radicals, including in particular the oxygen molecule, which has two unpaired electrons and is therefore paramagnetic This unusual property is not explained by the Lewis structure... 

Figure 8. Pauli principle for a diatomic molecule (e.g., HF). In any diatomic molecule, the two tetrahedra (Figs. 7a and 7b) of opposite spin electrons in the valence shell of an atom are brought into coincidence at only one apex, leaving the most probable locations of the remaining six electrons equally distributed in a ring.

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