Favorite structure

Porphyrin (1), the quintessential pyrrolic macrocycle, is one of the favorite structural motifs of organic chemistry [1], Its biological relevance, combined with a range of useful properties such as the rich electronic absorption spectra and the ability to coordinate metal ions, makes 1 a versatile building block for the synthetic chemist, as well as an important subject for physical investigations. Among the most conspicuous features of the porphyrin macrocycle is its aromatic character, which has a strong influence on the spectroscopic properties and chemical reactivity of 1 and its derivatives. 

A catalyst powder should have a favorite structure in order to be a high active and stable ORR electrocatalyst. Here the catalyst structure contains catalyst s composition, morphology (particle shape), SSA, particle size, porosity, and particle distribution. 

GAMESS stands for general atomic and molecular electronic structure system (we reviewed a version dated Dec. 2, 1998). It is an ah initio and semiempirical program, and has seen the most widespread use for ah initio calculations. The ASCII input hie format is usable but somewhat more lengthy than some other programs. The fact that GAMESS is a free, high-quality software makes it a favorite of many academic researchers. 

Parallam, or laminated strand lumber (LSL) is a beam made by a continuous manufacturing process composed of bigger-size wood needles (very elongated wood particles) reassembled with a structural exterior grade adhesive, the favorite adhesive being isocyanates (pMDI) when heat-curing and PRFs when cold-curing. 

The theory of band structures belongs to the world of solid state physicists, who like to think in terms of collective properties, band dispersions, Brillouin zones and reciprocal space [9,10]. This is not the favorite language of a chemist, who prefers to think in terms of molecular orbitals and bonds. Hoffmann gives an excellent and highly instructive comparison of the physical and chemical pictures of bonding [6], In this appendix we try to use as much as possible the chemical language of molecular orbitals. Before talking about metals we recall a few concepts from molecular orbital theory. 

Obviously, the CCL not only determines the rate of currenf conversion and the major portion of irreversible voltage losses in a PEFC, but also plays a key role for the water balance of the whole cell. Indeed, due to a benign porous structure with a large portion of pores in the nanometer range, the CCL emerges as favorite water exchanger for PEFCs. Once liquid wafer arrives in gas diffusion layers or flow fields, PEFCs are unable to handle if. 

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

From the days of the Egyptians, when emeralds were a particular favorite of kings, beryl has also been a favored gemstone. It was not until the late eighteenth century that Abbe Rene Just Haiiy (1743—1822), the father of crystallography, studied the crystalline structures and densities of emeralds and beryl and determined that they were the same mineral. At about the same time, in 1798, Louis-Nicolas Vauquehn (1763—1829) discovered that both emeralds and beryl were composed of a new element with four protons in its nucleus. The element was named glucina because of its sweet taste. It was not until the nineteenth century that the metal berylhum was extracted from berylhum chloride (BeCy by chemical reactions. Late in the nineteenth century, E Lebeau (dates unknown) separated the metal by the electrolytic process. 

The 778 present lithium organic structures in the CSD contain 3228 Li—C contacts. This means that every lithium atom either forms a multiple contact to the related carbanion or that most of the structures dimerize around the metal or both. On average, every lithium shows four Li—C contacts and certainly it is not just coincidence that this number is identical to the favorite coordination number of lithium in a molecular environment. 

In 1954, Perutz introduced the isomorphous replacement method for determining phases. In this procedure a heavy metal, such as mercury or platinum, is introduced at one or more locations in the protein molecule. A favorite procedure is to use mercury derivatives that combine with SH groups. The resulting heavy metal-containing crystals must be isomorphous with the native, i.e., the molecules must be packed the same and the dimensions of the crystal lattice must be the same. However, the presence of the heavy metal alters the intensities of the spots in the diffraction pattern and from these changes in intensity the phases can be determined. Besides the solution to the phase problem, another development that was absolutely essential was the construction of large and fast computers. It would have been impossible for Perutz to determine the structure of hemoglobin in 1937, even if he had already known how to use heavy metals to determine phases. 

Most, if not all, textbooks in nuclear science have a chapter discussing nuclear structure. Among the favorites of the authors are ... 

The two-stranded a-helical coiled coil is now recognized as one of natures favorite ways of creating a dimerization motif and has been predicted to occur in a diverse group of over 200 proteins.111 This structure consists of two amphipathic, right-handed a-helices that adopt a left-handed supercoil, analogous to a two-stranded rope where the nonpolar face of each a-helix is continually adjacent to that of the other helix. 2 This structure was first postulated by Crick to explain the X-ray diffraction pattern of a-keratin in the absence of sequence information.Pl The coiled-coil dimerization motif is natures way of creating a rod-like molecule that perhaps plays only a structural role in many fibrous proteins, such as the kmef (keratin, myosin, epidermis, fibrinogen) class 3,4 and the intermediate filament proteins)5 6 ... 

Hydrogen atoms in allylic position are favorite sites for hydroperoxidation of chains. So, this mechanism proceeds in the formation of lateral hydroperoxides, and not like for other polymers, in intramolecular peroxides. Rearrangement of chemical structures coming from ozonides are rapidly observed (Scheme 33). 

All the structures of natural products are very beautiful and attractive. Then, I would like only to relate them to my favorite compounds, carbohydrates. In my opinion, carbohydrates are the language of chiral natural products therefore, I have focused on the use of carbohydrates as chiral precursors in organic synthesis. 

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