Structure determination major components

It has been shown in several biological assembly systems that, due to the fact that tertiary structure determines quaternary structure, a major component of the system can be assembled on its own to form structures similar to the intact organelle (Caspar and Klug, 1962). Ex-... 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

The tightly bound chromophore could be extracted from the protein with methanol [186], and the major component of the extract was determined to have the enediyne structure 116 (Figure 11.21), related to chromophores of other chromoprotein antitumor agents such as neocarzinostatin. Additional minor components were extracted, variously containing an OH group instead of OMe attached to the enediyne core, with Cl instead of OMe when chloride was present in the buffer salt, or with OEt instead of OMe when ethanol was used for the extraction. Another byproduct was isolated in the form of structure 117, consistent with a facile cy-doaromatization reaction as observed for all other enediyne antibiotics. Surprisingly, 117 also displayed antibiotic and antitumor activity, perhaps due to alkylation of DNA or protein by the aziridine. The interpretation of these results was that 116 and the other enediyne byproducts were merely artifacts of the extraction procedure and that the true structure of the maduropeptin chromophore is the aziridine 118. 

The polyene macrolide filipin was isolated in 1955 from the cell culture filtrates of Sterptomyces filipinensis, and was later shown to be a mixture of four components [36]. Although too toxic for therapeutic use, the filipin complex has found widespread use as a histochemical stain for cholesterol and has even been used to quantitate cholesterol in cell membranes [37]. The flat structure of filipin III, the major component of the filipin complex, was assigned from a series of degradation studies [38]. Rychnovsky completed the structure determination by elucidating the relative and absolute stereochemistry [39]. The total synthesis plan for filipin III relied heavily on the cyanohydrin acetonide methodology discussed above. 

It was not until 1981 that the first brevetoxin structure was determined (13), PbTX-2, the most plentiful of the brevetoxins, was purified to crystallinity and the structure determined by x-ray crystallography. This important first step was the result of a three-group collatoration among Clardy, Lin, and Nakanishi. The yields of the major components from 50 L of culture (ca. 5 x 10 cells) were 0.8 mg of PbTX-1, 5.0 mg of PbTX-2, and 0.4 mg of PbTX-8. The structure of PbTX-2,... 

All this is basic chemical research that has wide importance, determining the molecular structure of a component of living cells. Simply, it tells us the details of how proteins are now made, but more generally it strengthens the picture of how life may have started in a world where RNA was both information molecule and catalyst. It is a major advance in scientific understanding. 

Once a suitable crystal is obtained and the X-ray diffraction data are collected, the calculation of the electron density map from the data has to overcome a hurdle inherent to X-ray analysis. The X-rays scattered by the electrons in the protein crystal are defined by their amplitudes and phases, but only the amplitude can be calculated from the intensity of the diffraction spot. Different methods have been developed in order to obtain the phase information. Two approaches, commonly applied in protein crystallography, should be mentioned here. In case the structure of a homologous protein or of a major component in a protein complex is already known, the phases can be obtained by molecular replacement. The other possibility requires further experimentation, since crystals and diffraction data of heavy atom derivatives of the native crystals are also needed. Heavy atoms may be introduced by covalent attachment to cystein residues of the protein prior to crystallization, by soaking of heavy metal salts into the crystal, or by incorporation of heavy atoms in amino acids (e.g., Se-methionine) prior to bacterial synthesis of the recombinant protein. Determination of the phases corresponding to the strongly scattering heavy atoms allows successive determination of all phases. This method is called isomorphous replacement. 

A major component, or segment, of a project. It is determined by the process known as project breakdown structuring. 

Of the six possible conformers containing chair rings, two tra/j5-fused conformers 18 and 19, with a slight excess of 18, could be identified as major conformers, with one a>-fused conformer as a minor constituent in the C NMR spectrum of l-methylperhydropyrido[l,2-h]pyridazine in acetone-dfi in the temperature range -75 to -89°C (78JA4012). The low intensity of the signal of the c -fused conformer did not allow determination of the exact structure of this component. The results of low-temperature cyclic voltammetry experiments supported the NMR findings. 

The extracts of three species of male North American decorator wasps, Eucerceris rubripes, E. conata and E. tricolor, were analyzed to reveal the presence of one major component in large quantities. This component detected in the head extract of males was identified as (Z )-3-hexenyl-3-hydroxybutanoate. The structure was confirmed by synthesis and the absolute configuration of the chiral center was determined to be R for the three species. In addition, 2- and 3-hexenoic acid and a few other aromatic compounds were also detected in varying quantities in males and females, along with hydrocarbons and fatty acids. 

Urinary casts, often excreted by patients having renal disease, possess414 antigenic determinants in common with the Tamm-Horsfall protein (see p. 414), and, indeed, this glycoprotein is the major component of the casts.414-416 However, the reason for its aggregation is as yet unknown. Other pathological states may well be found to be associated with abnormalities in the structure or metabolism, or both, of glycoproteins. 

Although the answers to questions such as these depend on a complex array of factors ranging from the structure of the relevant molecules to their environment and the chemical activity of the medium containing the molecules, intermolecular (guest-host) interactions play a central role in determining the rate and the efficiency of the ultimate result. A major component of the many possible intermolecular forces is the electrostatic interaction, particularly because of the long-range nature of the Coulombic forces and the inevitable influence of the ionic atmospheres that surround the macromolecules and substrates. 

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