抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Copper is an essential trace element in living systems, present in the parts per million concentration range. It is a key cofactor in a diverse array of biological oxidation-reduction reactions. These involve either outer-sphere electron transfer, as in the blue copper proteins and the Cu{sub A} site of cytochrome oxidase and nitrous oxide redutase, or inner-sphere electron transfer in the binding, activation, and reduction of dioxygen, superoxide, nitrite, and nitrous oxide. Copper sites have historically been divided into three classes based on their spectroscopic features, which reflect the geometric and electronic structure of the active site: type 1 (T1) or blue copper, type 2 (T2) or normal copper, and type 3 (T3) or coupled binuclear copper centers. 428 refs.

Multicopper Oxidases and Oxygenases

Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

For present purposes, a protein-bound metal site consists of one or more metal ions and all protein side chain and exogenous bridging and terminal ligands that define the first coordination sphere of each metal ion. Such sites can be classified into five basic types with the indicated functions: (1) structural -- configuration (in part) of protein tertiary and/or quaternary structure; (2) storage -- uptake, binding, and release of metals in soluble form: (3) electron transfer -- uptake, release, and storage of electrons; (4) dioxygen binding -- metal-O{sub 2} coordination and decoordination; and (5) catalytic -- substrate binding, activation, and turnover. The authors present here a classification and structure/function analysis of native metal sites based on these functions, where 5 is an extensive class subdivided by the type of reaction catalyzed. Within this purview, coverage of the various site types is extensive, but not exhaustive. The purpose of this exposition is to present examples of all types of sites and to relate, insofar as is currently feasible, the structure and function of selected types. The authors largely confine their considerations to the sites themselves, with due recognition that these site features are coupled to protein structure at all levels. In themore » next section, the coordination chemistry of metalloprotein sites and the unique properties of a protein as a ligand are briefly summarized. Structure/function relationships are systematically explored and tabulations of structurally defined sites presented. Finally, future directions in bioinorganic research in the context of metal site chemistry are considered. 620 refs.« less

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Structural and Functional Aspects of Metal Sites in Biology

Polarized single crystal x-ray absorption spectra have been measured for Co(NH,) (ClO,),Cl:KCl. These spectra show pronounced orientation dependence fior several features on the high energy side of the Co K absorption edge. The Co in this salt has a tri onally distorted octahedral structure with N-Co-N angles of 89.5O and 90.5 8 , hence first shell scatterers cannot account for the observed orientation dependence. As we show below, the orientation dependent features can be interpreted in terms of a singlescattering EXAFS model involvin hydrogen-bonded to the Co(NH,), & scattering from a perchlorate which is ion. Co(NH,),(ClO.,),Cl:KCl was prepared by the method of Wilson and Solomon [l] and smgle crystals were oriented by standard diffraction methods. In initial experiments, the Co(NH3)03+ molecules were oriented so that both the propagation (k) and polarization (e) vectors were contained in one of the CoN4 planes. The crystal was rotated about e *k and XANES spectra were measured every 15 degrees for a total of thirteen different orientations of e. In a second experiment, the polarization vector was oriented either parallel or perpendicular to the crystallographic 3-fold axis. In order to compare different orientations it was necessary to develop a procedure for the precise, reproducible normalization of experimental XANES spectra. In brief, this method 121 involves subtraction of a low order polynomial from the data followed by multiplication by a scale factor. The polynomial and scale factor are adjusted to maximize the agreement, both below and above the edge, with tabulated x-ray absorption cross section.

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Polarized XANES of Co(III)(NH3)6 molecular crystals

The Zinc and Cadmium forms of MerR, a metalloregulatory protein involved in bacterial mercury detoxification , have been studied using X-ray absorption spectroscopy. Previous work has shown the Hg-MerR to have a unique three coordinate HgS3 environment. Preliminary analysis of the EXAFS data for the Cd- and Zn-MerR derivatives suggests a higher coordination number in these cases.

Structural characterization of the binding site in the merr metalloregulatory protein

Dimeric in Et2 O and DMS, monomeric in THF-the degree of aggregation of dimethylcuprates depends on the solvent [Eq. (a)]. The Cu-Cu distance of 2.82 A was determined by EXAFS for a dimethylcuprate reagent derived from cuprous halide. In contrast, with cyanide as the counterion, no dimers are formed.

X-Ray Absorption Spectroscopy of Dimethylcuprates: Evidence for Solvent-Dependent Aggregation.

/pdf/xas-investigation-of-the-fe-sites-in-phthalate-dioxygenase-5133jk74c8.pdf

XAS investigation of the Fe sites in phthalate dioxygenase

The spin-polarized electron momentum density along the [100] direction of Eu(Fe0.89Co0.11)2As2 was measured using Magnetic Compton Scattering (MCS). Measurements were performed at different temperatures and applied external magnetic fields. These results provide the first direct insight into the orbital interactions that are responsible for the competing conductivity and ferromagnetism (FM) in this iron pnictide system. The orbital-wise decompositions of the density distribution show that at low applied magnetic field (0.1 T), the spin-polarized average Fe/Co-3d character is enhanced between 15 K and 19 K with a maximum at 17 K, the temperature at which the resistivity shows a local maximum, followed by re-entrant SC (superconducting)-like character. This clearly shows the dominance of the ferromagnetic character over conducting character in this temperature region where the resistivity shows a maximum. At higher applied magnetic field, we do not see this enhancement and subsequent collapse of the Fe/Co-3d character as the temperature decreases, consistent with the disappearance of the re-entrant SC-like feature at higher applied magnetic field. These data support a model in which the spin-polarization of the Fe/Co-3d orbitals is enhanced by the ferromagnetic Eu-ions, leading to the suppression of conductivity at ∼17 K (for 0.1 T), while at lower temperature the conductivity recovers and the Fe/Co-3d spin-polarization collapses.

James E. Penner-Hahn

Papers

Polarized XANES of Co(III)(NH3)6 molecular crystals

Structural characterization of the binding site in the merr metalloregulatory protein

X-Ray Absorption Spectroscopy of Dimethylcuprates: Evidence for Solvent-Dependent Aggregation.

XAS investigation of the Fe sites in phthalate dioxygenase

Competition of 3d/4f orbitals due to competing conductivity and ferromagnetism in Fe/CoAs layers in Eu(Fe0.89Co0.11) 2As2