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1.
D. Allen Clabo Jr. Hamilton D. Dickson Toby L. Nelson 《Journal of molecular modeling》2000,6(2):341-348
Semiempirical and ab initio theoretical methods have been used to investigate molecular structures of the chalcogen-substituted carboxylic acid isomers RC(=O)XH (chalcogenol acid) and RC(=X)OH (chalcogenon acid). A recent experimental report suggests that the chalcogenon isomers, although less stable at room temperature, predominate at low temperature in polar solvents and that there is only a small barrier to isomerization between the isomers. Theoretical calculations have been used to locate minimum energy structures of chalcogen-substituted carboxylic acid isomers and to calculate energy differences between pairs of isomers. Carboxylic acids are well known to dimerize, especially in the gas phase and in non-polar solvents. We have, therefore, also calculated energies of dimerization of the chalcogen-substituted acids by optimizing the geometries of the symmetric dimers. We note that the PM3 level of theory is only qualitatively correct for sulfur- and selenium-containing species but fails even qualitatively for the tellurium-containing compounds. Ab initio results confirm the experimental observations and provide good estimates of both isomerization and dimerization energies. We conclude that for many functional groups with tautomers RC(=X)YH and RC(=Y)XH, the more acidic tautomer is the one with the acid proton on the smaller, more electronegative atom, although in many cases this may not be the more stable tautomer.Electronic Supplementary Material available. 相似文献
2.
The number of metal atoms contained within a displaceable inorganic component of a metalloprotein was determined by considering X-ray absorption by single crystal samples of holo- and apo-proteins. Since this method is non-destructive, it can be used to determine the number of metal atoms associated with the molecules forming the crystal actually used for X-ray diffraction data collection and subsequent structure solution. The method has been applied to the iron storage protein ferritin, isolated from horse spleen, to give a reliable estimate of the average iron content of the ferritin molecules within the crystal. This value, of around 2000 iron atoms per molecule is consistent with that found for a typical ferritin preparation in solution and suggests non-selectivity of the crystallisation process for ferritin in terms of molecular iron content. 相似文献
3.
Properties of a new crystal form of the complex of concanavalin A with methyl alpha-D-glucopyranoside 总被引:1,自引:0,他引:1
J Yariv A J Kalb M Z Papiz J R Helliwell S J Andrews J Habash 《Journal of molecular biology》1987,195(3):759-760
The complex of concanavalin A with methyl alpha-D-glucopyranoside crystallizes as regular rhombic dodecahedra containing 35% protein by weight. The crystal is of space group I23 with a = 167.8 A (1 A = 0.1 nm) and contains one concanavalin A dimer per asymmetric unit. It diffracts to a resolution of 1.9 A and is suitable for crystallographic investigation of the structure of the saccharide-binding site. 相似文献
4.
5.
To investigate the physical and kinetic properties of sperm carnitine acetyltransferase, the enzyme was purified from bovine spermatozoa and heart muscle. Carnitine acetyltransferase was purified 580-fold from ejaculated bovine spermatozoa to a specific activity of 85 units/mg protein (95% homogeneity). Sperm carnitine acetyltransferase was characterized as a single polypeptide of Mr 62,000 and pI 8.2. Heart carnitine acetyltransferase was purified 650-fold by the same procedure to a final specific activity of 71 units/mg protein. The kinetic properties of purified bovine sperm carnitine acetyltransferase were consistent with the proposed function of this enzyme in acetylcarnitine pool formation. Product inhibition by either acetyl-l-carnitine or CoASH was not sufficient to predict significant in vivo inhibition of acetyl transfer. At high concentrations of l-carnitine, bovine sperm and heart carnitine acetyltransferases were most active with propionyl- and butyryl-CoA substrates, although octanoyl-, iso-butyryl-, and iso-valeryl-CoA were acceptable substrates. Binding of one substrate was enhanced by the presence of the second substrate. Carnitine analogs that have significance in reproduction, such as phosphorylcholine and taurine, did not inhibit carnitine acetyltransferase. Bovine sperm and heart carnitine acetyltransferases were indistinguishable on the basis of purification behavior, pI, pH optima, kinetic properties, acyl-CoA specificity, and sensitivity to sulfhydryl reagents and divalent cations; thus there was no indication that bovine sperm carnitine acetyltransferase is a sperm-specific isozyme. 相似文献
6.
This study describes the sequential alternation of compaction and decompaction in the chromosomes of the Chinese hamster oocyte from diakinesis to metaphase II. A series of micrographs show that the compact metaphase I chromosomes become greatly extended as they enter and pass through anaphase I. Once polarized, the presumptive oocyte chromosomes become exceedingly compact and form a tightly packed mass, each chromosome assuming contours to accomodate dovetailing with its neighbors, while the chromosomes consigned to the polar body remain extended and show signs of the incipient deterioration. Prior to ovulation, the chromosomes of the mass separate and begin to decompact, in part at least, by the previously postulated mechanism of uncoiling. Following ovulation, the chromosomes are greatly extended and, as the metaphase II complement, remain in that state until the advent of fertilization. — Evidence that the compaction patterns are ordered and chromosome specific is presented by observation of the two smallest chromosomes of the complement. At telophase I those chromosomes are markedly different in size and arm ratio; at metaphase II the differences are less pronounced and at mitotic metaphase the two smallest chromosome pairs are so similar in morphology as to be indistinguishable. It is proposed, therefore, that those two chromosomes differ in their fundamental morphology as revealed at the exceedingly compact state of telophase I oocyte chromosomes. Their subsequently established resemblance at mitotic metaphase may be due to allocycly on the part of one or both, resulting in two chromosomes of apparantly similar length and arm ratio.Supported by grants from the Institute of Child Health and Development of the National Institutes of Health, 5 RO1 HDO4846 and the Damon Runyan Foundation, DRG-907.Supported in part by CA-08748 from the Cancer Institute of the National Institutes of Health. 相似文献
7.
Toby A. Ferguson John Vozenilek Christopher M. West 《Development, growth & differentiation》1994,36(6):597-604
As a result of transfecting Dictyostelium discoideum with an actin 6/ lac Z fusion transgene, strain HW80 was created which expresses the β-galactosidase gene product uniformly throughout development. When mixed with an excess of unmarked wild-type cells, however, HW80 cells selectively migrate to the positions of anterior-like cells surrounding the prespore cell mass, and differentiate as if they were anterior-like cells. As the proportion of HW80 cells is increased, they also sort to positions adjacent to anterior-like cells and some differentiate as prespore cells. Thus sorting of HW80 cells toward the opposite ends of the prespore cell zone supersedes how they differentiate, suggesting that position influences whether cells differentiate as anterior-like or prespore cells. 相似文献
8.
Studies indicate that low birthweight (LBW) children display reduced deciduous tooth size but there is little information about permanent tooth size. It has also been shown that dental fluctuating asymmetry (FA) increases in response to various environmental influences, but the relationship between birthweight and FA remains unclear. The aim of this study was to compare tooth size and asymmetry, according to birthweight, in the deciduous and permanent dentitions of a sample of Australian twins. The study sample comprised 436 twins, classified into 2 groups: normal birthweight (NBW > 2500 g) and low birthweight (LBW < or = 2500 g). For each individual it was generally possible to measure maximum mesiodistal crown diameters of both deciduous and permanent central incisors from serial dental models. Correlations were calculated between tooth-size variables and birthweight; subsequently comparisons of tooth size and FA were made between the LBW and NBW samples using Student's t tests. Small positive correlations (around .1) were noted between birthweight and tooth-size variables. There was no evidence of tooth-size reduction in the LBW male sample, but the LBW females displayed tooth-size reduction of approximately 2-3% for both deciduous and permanent incisors, compared to the NBW females. There was no evidence of increased FA in the LBW individuals of either sex. These findings indicate that developing teeth are generally well-protected from developmental disturbances during prenatal and perinatal periods. Further research is needed to clarify the biological basis of an apparently true but weak association between tooth size and birthweight. 相似文献
9.
Abstract. The Channel Tunnel workings on the UK side have yielded nearly 4 million m3 of chalk-marl spoil which now forms a 36 ha landscaped reclamation platform. To establish vegetation of amenity and conservation interest on the spoil, seed mixtures of native wild flowers and grasses were sown with Lolium perenne (perennial rye grass) as a nurse species. Potentially, L. perenne is a suitable nurse species for grassland creation on infertile substrates as it provides rapid initial cover and stability, but it is non-persistent and declines in vigour with time, allowing wild flower species sown alongside to expand their cover and spread in the longer term. On very low fertility substrates like chalk marl, an initial application of fertilizer is needed to encourage plant growth. Results are reported of a fertilizer experiment on Channel Tunnel spoil to determine appropriate levels of fertilizer for establishment of species-rich grassland vegetation. An area hydroseeded with L. perenne and wild flowers in autumn 1992 was subjected to factorial treatment of four levels each of N and P in spring 1993. The results the following summer showed significant positive effects of nitrogen and phosphorus on L. perenne biomass and a negative impact of nitrogen on densities of wild flower species, especially legumes, establishing in the L. perenne sward. In general, low fertilizer applications encouraged low productivity and maximal species richness in the vegetation. Conversely high applications encouraged high productivity and competitive exclusion of sown wild flower species. Fertilizer applications must therefore balance encouragement of the stabilising nurse grass sward, while preventing competitive exclusion of wild flowers by the nurse grass. 相似文献
10.
The 5-upstream region of the pea plastocyanin gene (petE) directed 5–10-fold higher levels of -glucuronidase (GUS) activity than the cauliflower mosaic virus 35S promoter in transgenic tobacco plants, although the levels of GUS mRNA were similar. The sequence (AAAAAUGG) around the translation initiation codon of petE enhanced translation of the GUS mRNA 10-fold compared to translation from the GUS translation initiation codon in transgenic tobacco plants and transfected protoplasts. 相似文献