双缓血氨二乙三胺五乙酸酰胺钆,镱螯合物的合成及其弛豫性能

缓血氨（Ｔｒｉｓ，三羟甲基氨基甲烷）修饰多氨多羧酸（二乙三胺五乙酸，ＤＴＰＡ和乙二胺四乙酸，ＥＤＴＡ）得到两种新的双酰胺型氨羧衍生物配体ＤＴＰＡВＴｒｉｓ和ＥＤＴＡＢＴｒｉｓ，并合成了它们的Ｇｄ３＋，Ｙｂ３＋，Ｍｎ２＋，Ｆｅ３＋等顺磁性金属螯合物，研究了Ｇｄ３＋，Ｍｎ２＋和Ｆｅ３＋螯合物作为磁共振成像造影剂的主要性能。结果表明，ＧｄＤＴＰＡＢＴｒｉｓ，ＭｎＥＤＴＡＢＴｒｉｓ和ＦｅＥＤＴＡＢＴｒｉｓ在体外水溶液中对水质子的纵向弛豫率Ｒ１分别为５４，３．１和１．９Ｌ·ｍｍｏｌ－１·ｓ－１，均高于其相应母体螯合物在相同条件下的弛豫率。此外，这些螯合物还具有医用磁共振成像造影剂所要求的高水溶性和化学稳定性。     

甲醇燃料车尾气净化催化剂的研究Ⅲ甲醇深度氧化用Ag-Pd/γ-Al2O3催化剂中Ag与Pd的协同作用

以银氨络离子和氯钯酸铵为前驱体制备了５％Ａｇ－０．１％Ｐｄ／γ－Ａ２ｌＯ３催化剂，其低温深度氧化性能明显优于Ａｇ或Ｐｄ单组分催化剂；Ｔ５０和Ｔ９５（ＣＨ３ＯＨ）分别为１２５℃和１７０℃。且不受Ｏ２及ＣＯ的阻抑。Ｏ２－ＴＰＯ的结果表明，随着在Ａｇ组分中加入Ｐｄ及Ｐｄ加入量的增加，Ｏ２的低温脱附峰逐渐减弱至消失；中温脱附峰则向低温位移；高温脱附峰峰面积逐渐增大，峰温比单组分钯的氧脱附峰高。Ｐｄ与Ａｇ的     

掺杂ZrO＿2的Pt／Al＿2O＿3表面氧性质及一氧化碳氧化性能研究

用ＴＰＤ－ＭＳ、ＴＰＳＲ－ＭＳ及ＣＯ氧化活性测定等方法研究了Ｐｔ／Ａｌ２Ｏ３和掺杂超细ＺｒＯ２的样品的表面氧脱出－恢复性能、ＣＯ表面氧化性能及催化氧化性能．结果表明，在Ｐｔ／Ａｌ２Ｏ３中掺杂ＺｒＯ２后，样品表面上的氧物种脱出和氧化恢复性能明显提高，脱氧量也明显增大；并发现在ＣＯ－ＴＰＳＲ过程中程脱物ＣＯ２的脱附量大小及峰顶温度次序与对ＣＯ的催化氧化活性也有一致的关系     

氧化铈对Pd／Al2O3表面上CO氧化性能的影响

采用ＴＰＤ－ＭＳ及ＴＰＳＲ－ＭＳ技术研究了添加ＣｅＯ２对Ｐｄ／Ａｌ２Ｏ３催化剂上ＣＯ脱附、表面反应及表面氧脱附等性能的影响，考察了不同含氧量的气氛下ＣＯ的氧化活性，结果表明，Ｐｄ－Ｃｅ间的相互作用有利于各自原子上表面氧的吸脱附及ＣＯ的表面反应，并发现ＣＯ２脱附量大小及峰温次序与对ＣＯ的催化氧化活性有一致的对应关系。     

SAPO－34分子筛表面酸性质的研究

以水热合成法制备了三个具有不同硅磷铝组成的硅磷酸铝分子筛ＳＡＰＯ－３４样品，采用红外光谱（ＩＲ）和氨法程序升温脱附（ＴＰＤ）两种方法考察了它们的表面酸性质。红外谱图中的３６００ｃｍ－１和３６２１ｃｍ－１谱峰归属于处于ＳＡＰＯ－３４分子筛结构中不同位置的两种桥联羟基（Ｓｉ－ＯＨ－Ａｌ）的振动。ＮＨ３－ＩＲ测定结果显示，这两种羟基具有较强的Ｂ酸特性，并且是分子筛酸性的主要来源；而分子筛具有的Ｌ酸中心的酸性较弱。比较三个样品的ＮＨ３－ＴＰＤ、ＮＨ_３－ＩＲ和骨架组成后发现，ＳＡＰＯ－３４的酸性受其骨架硅含量的强烈影响：当Ｓｉ／Ａｌ摩尔比小于１时，酸性随硅含量增高而变弱；当Ｓｉ／Ａｌ摩尔比大于１时，酸性将随硅含量增高而变强。     

Ag／γ—Al2O3催化剂的TPR和TPD研究

本文运用ＴＰＤ－ＭＳ，ＴＰＲ等方法研究了Ａｇ／γ－Ａｌ２Ｏ３催化剂的氧脱附和还原性能，结果表明，Ａｇ／γ－Ａｌ２Ｏ３催化剂表面银物相由Ａｇ＾０和Ａｈ＾＋（Ａｇ２Ｏ）组成，其中，Ａｇ＾＋所占比例随负载量增加而减少。     

掺杂ZrO2的Pt／Al2O3表面氧化性质及一氧化碳氧化性能研究

用ＴＰＤ－ＭＳ，ＴＰＳＲ－ＭＳ及ＣＯ氧化活性测定等方法研究了Ｐｔ／Ａｌ２Ｏ３和掺杂超细ＺｒＯ２的样品的表面氧脱－恢复性能，ＣＯ表面氧化性能有催化氧化性能。结果表明，在Ｐｔ／Ａｌ２Ｏ３中掺杂ＺｒＯ２后，样品表面上的氧物种脱出和氧经恢复性能明显提高，脱氧量也明显增大。     

ZrO2对Mn—O／γ—Al2O3催化剂中表面氧性能的影响

用ＴＰＤ－ＭＳ和ＴＰＲ技术研究了Ｍｎ－Ｏ／γ－Ａｌ２Ｏ３和Ｍｎ－Ｏ／ＺｒＯ＋γ－Ａｌ２Ｏ３催化剂中表面氧的脱附、还原性能和再生氧化恢复性能，并用ＸＲＤ对催化剂的固相结构进行了表征。结果表明，Ｍｎ－Ｏ／γ－Ａｌ２Ｏ３或Ｍｎ－Ｏ／ＺｒＯ２＋γ－Ａｌ２Ｏ３催化剂上存在ＭｎＯ２、Ｍｎ２Ｏ３和少量Ｍｎ３Ｏ４物种。ＺｒＯ２的存在不影响Ｍｎ－Ｏ／γ－Ａｌ２Ｏ３的ＴＰＲ的ＴＰＤ的特征峰，但使ＭｎＯ２的量明显增加，     

添加Ag或Ag,Ni的V2O5表面氧性质及催化性能研究

用ＴＰＤ－ＭＳ及甲苯选择氧化活性测定等方法研究了Ｖ２Ｏ５和添加Ａｇ或Ａｇ，Ｎｉ的３种样品的表面氧和晶格氧的热脱附性能及催化活性。结果表明，在Ｖ２Ｏ５中加入Ｎｇ或Ａｇ，Ｎｉ后，样品表面上的Ｏ＾－和Ｏ＾２－氧物种的脱附活化能明显降低。添加Ａｇ，Ｎｉ样品表面氧的脱附活化能最低，而苯甲醛生成的选择性最高，在Ｖ２Ｏ５和含有Ａｇ，Ｎｉ的样品中，温度为９６８和７３４℃时，均依次出现以晶格氧（Ｏ＾２－）脱附为主的     

Al、Bi及P改性β分子筛的表面酸性及对烷基转移反应的影响

本文系统地考察了甲苯和１，３，５－三甲苯在以Ｈβ为基质的改性沸石分子筛上烷基转移反应。探讨了金属Ａｌ，Ｂｉ和非金属Ｐ改性Ｈβ沸石对甲苯和１，３，５－３甲苯的转化率和对产物二甲苯的选择性影响规律。用氨程序升温脱附（ＮＨ３－ＴＰＤ）和吡啶吸附红外（ＩＲ）方法考察了Ｈβ沸石表面酸性和酸中心分布以及改性对β沸石表面酸性的影响。发现：１）用Ａｌ改性后，催化活性大大超过未改性的Ｈβ分子筛；２）Ｌ酸中心对改性Ｈβ分子筛的烷基转移反应活性起重要作用。     

Amino-acid-dependent shift in tRNA synthetase editing mechanisms

Many aminoacyl-tRNA synthetases prevent mistranslation by relying upon proofreading activities at multiple stages of the aminoacylation reaction. In leucyl-tRNA synthetase (LeuRS), editing activities that precede or are subsequent to tRNA charging have been identified. Although both are operational, either the pre- or post-transfer editing activity can predominate. Yeast cytoplasmic LeuRS (ycLeuRS) misactivates structurally similar noncognate amino acids including isoleucine and methionine. We show that ycLeuRS has a robust post-transfer editing activity that efficiently clears tRNA(Leu) mischarged with isoleucine. In comparison, the enzyme's post-transfer hydrolytic activity against tRNA(Leu) mischarged with methionine is weak. Rather, methionyl-adenylate is cleared robustly via an enzyme-mediated pre-transfer editing activity. We hypothesize that, similar to E. coli LeuRS, ycLeuRS has coexisting functional pre- and post-transfer editing activities. In the case of ycLeuRS, a shift between the two editing pathways is triggered by the identity of the noncognate amino acid.     

CHEMICAL MODIFICATION OF SULFHYDRYL GROUPS OF E.coli LEUCYL-tRNA SYNTHE-TASE AND SEQUENCING OF [~(14)C]NEM-LABELED PEPTIDES

The chemical modification of the sulfhydryl groups of E. coli Leucyl--tRNA synthetase(LeuRS) by DTNB, NEM and IAA resulted in a time-dependent loss of both amino-acid acti-vation and aminoacylation activities in parallel. The second-order reaction constants of DTNB,NEM and IAA were 1700, 150 and 0.46 mol/L~(-1) min~(-1) respectively. Chemical stoichiometryshowed that only one sulfhydryl group of LeuRS was essential for both activities. Substratesleucine and Leu-AMP protected the active sulfhydryl group from modification, suggestingthat the modified sulfhydryl group is located in or near the active site region responsiblefor amino-acid activation. [~(14)C]NEM--labeled LeuRS was subjected to tryptic digestion, andpeptides were separated and sequenced. 179 Cys~*-Asp-Thr-Leu182 was identified as the major[~(14)C]NEM-labeled site in LeuRS. This result is consistent with the previous observationthat the region for Leu--AMP formation was located at the N--terminal part of LeuRS.     

LIMITED TRYPTIC DIGESTION OF LEUCYL-tRNA SYNTHETASE AND CHARACTERIZATION OF ITS ACTIVE FRAGMENT

Leucyl-tRNA synthetase (LeuRS, EC 6.1.1.4) from E. coli underwent limitedproteolysis by trypsin which cut off 6K peptide and converted the intact LeuRS into a 96K fragment. The truncated enzyme retained the PPi exchange activity with the same kinetic parameters as those of native LeuRS but lost the tRNA~(Leu)charging, binding and other tRNA~(Leu)-related activities. N-terminus analysis showed that the 6K peptide was located at the C-terminus of LeuRS. This small part played a crucial role in tRNA~(Leu) binding. Our results suggest that the two activities, PPi exchange and tRNA charging are independent of each other and correspond to different structural regions of LeuRS. The C-terminal region might be the tRNA~(Leu)binding site of LeuRS.     

In situ chemical aminoacylation with amino acid thioesters linked to a peptide nucleic acid

tRNA-specific chemical aminoacylation was achieved with nonnatural amino acids. A nonnatural amino acid was activated as a thioester derivative, and the latter was linked through a spacer to the N-terminal of a 9-mer peptide nucleic acid that is complementary to the 3'-terminal region of yeast phenylalanine tRNA. Efficient aminoacylation was observed when the amino acid thioester-spacer-PNA conjugate was mixed with the tRNA. The PNA-assisted aminoacylation was also successful in an Escherichia coli in vitro protein synthesizing system that contained an orthogonalized tRNA. The in situ aminoacylation/in vitro translation gave a mutant protein in which the nonnatural amino acid was incorporated into the position directed by a CGGG 4-base codon/anticodon pair.     

Orientation and distance dependent chiral discrimination in the first step of the aminoacylation reaction: Integrated molecular orbital and semi-empirical method (ONIOM) based calculation

Aminoacylation is a vital step in natural biosynthesis process of peptide and is the key step in correlating the realm of protein with the RNA world. Incorrect aminoacylation might lead to misacylation of d-amino acid in the tRNA which might cause synthesis of a hetero-peptide rather than natural homopeptide leading to the altered functionality of the peptide. However, the accuracy of this process is remarkable and leads to the attachment of the correct enantiomer of the amino acid with their cognate tRNA. Thus, the chiral discrimination is stringent. In the present work, we presented a combined ONIOM (ab initio/semi-empirical) study of the chiral discrimination in the first step of aminoacylation reaction based on a model of crystal structure of the oligomeric complex of histidyl-tRNA synthetase (HisRS) from Escherichia coli complexed with ATP and histidinol and histidyl-adenylate. The study reveals that the molecular mechanism of the chiral discrimination involves the amino acid, ATP as well as surrounding residues of the synthetase. Several factors are noted to be responsible for discrimination and explain the high level of stereospecificity of the process. The chirality of the amino acid of the substrate and its (principally) electrostatic interaction with the ATP is important for discrimination. The distance and orientational changes involved in the approach of the d-His towards the ATP is energetically unfavorable. The charge distributions on the His and ATP are important for the discrimination. Removal of the charges in the model drastically reduces the discrimination. Restricted nature of the mutual orientation within the cavity of the active site where the His and ATP are located during the change in orientation for the approach to form the adenylate makes the resultant interaction profile as different for l-His and d-His also influences chiral discrimination. The analysis of the transition state structure revealed that alteration of the chirality of the His destabilize the transition state by removing the favorable electrostatic interaction between the Glu-83 and NH₃⁺ group of the His substrate. The proximity of the surrounding residues as present in the active site of the synthetase with the His and ATP (the separation is of nanometer range) has influence of discrimination. The study provides a molecular mechanism of the retention of biological homochirality.     

Crystal Structure of an EMAP‐II‐Like Cytokine Released from a Human tRNA Synthetase

Aminoacyl‐tRNA synthetases catalyze the first step of protein synthesis by aminoacylation of tRNAs. Remarkably, biological fragments of two human enzymes – tyrosyl‐tRNA synthetase (TyrRS) and tryptophanyl‐tRNA synthetase – are active cytokines produced by proteolysis or alternative splicing. One is a C‐terminal fragment of TyrRS (C‐TyrRS) that has potent activity for chemotaxis of leukocytes and monocytes and for stimulating production of other cytokines. Significantly, the cytokine activity of C‐TyrRS is absent in the context of the full‐length native protein. Unknown is the mechanism by which domain‐release from the dimeric native protein activates the cytokine. Here, the crystal structure of C‐TyrRS is presented at 2.2 Å resolution. This structure is similar to that of endothelial monocyte‐activating protein II (EMAP‐II), with critical residues of a heptapeptide element important for chemotaxis activity exposed on the first strand of a β‐barrel of the monomeric unit. In contrast, the same residues of C‐TyrRS are buried in an operational model for native TyrRS. Importantly, C‐TyrRS is shown here to be monomeric when released from dimeric native TyrRS. Further analysis suggests that the critical residues are exposed when tRNA is bound. Thus, tRNA binding to native TyrRS may be an additional or alternative way to activate cytokine signaling.     

A Synthetic Adenylation‐Domain‐Based tRNA‐Aminoacylation Catalyst

The incorporation of non‐proteinogenic amino acids represents a major challenge for the creation of functionalized proteins. The ribosomal pathway is limited to the 20–22 proteinogenic amino acids while nonribosomal peptide synthetases (NRPSs) are able to select from hundreds of different monomers. Introduced herein is a fusion‐protein‐based design for synthetic tRNA‐aminoacylation catalysts based on combining NRPS adenylation domains and a small eukaryotic tRNA‐binding domain (Arc1p‐C). Using rational design, guided by structural insights and molecular modeling, the adenylation domain PheA was fused with Arc1p‐C using flexible linkers and achieved tRNA‐aminoacylation with both proteinogenic and non‐proteinogenic amino acids. The resulting aminoacyl‐tRNAs were functionally validated and the catalysts showed broad substrate specificity towards the acceptor tRNA. Our strategy shows how functional tRNA‐aminoacylation catalysts can be created for bridging the ribosomal and nonribosomal worlds. This opens up new avenues for the aminoacylation of tRNAs with functional non‐proteinogenic amino acids.     

Biomimetic monoacylation of diols in water. Lanthanide-promoted reactions of methyl benzoyl phosphate

The direct monoacylation of diols by acyl phosphate monoesters in water is a biomimetic analogy to the enzymic aminoacylation of tRNA by aminoacyl adenylates. Without catalysis, acyl phosphate monoesters react rapidly with amines but very slowly with water and alcohols. Lanthanide ions dramatically and selectively facilitate the base-catalyzed monoacylation of diols in water by methyl benzoyl phosphate (MBP), a typical acyl phosphate monoester, in neutral solutions where reactive amines are protonated and unreactive. The reaction patterns and reactivity of various diols with MBP in the presence of lanthanides are consistent with a mechanism that involves internal addition from the conjugate base of the bis-bidentate complex of the lanthanide with the diol and MBP. The method is also applicable to reactions of nucleosides as evidenced by the selective monoacylation of the 2'- or 3'-hydroxyl group of adenosine, without reaction of the 5'-hydroxyl group or the 6-amino group. Analogues of adenosine without the diol are unreactive. This suggests that the method will selectively monoacylate the hydroxyl groups at the unique diol in tRNA that forms the 3'-terminus.     

Using a solid-phase ribozyme aminoacylation system to reprogram the genetic code

Here, we report a simple and economical tRNA aminoacylation system based upon a resin-immobilized ribozyme, referred to as Flexiresin. This catalytic system features a broad spectrum of activities toward various phenylalanine (Phe) analogs and suppressor tRNAs. Most importantly, it allows users to perform the tRNA aminoacylation reaction and isolate the aminoacylated tRNAs in a few hours. We coupled the Flexiresin system with a high-performance cell-free translation system and demonstrated protein mutagenesis with seven different Phe analogs in parallel. Thus, the technology developed herein provides a new tool that significantly simplifies the procedures for the synthesis of aminoacyl-tRNAs charged with nonnatural amino acids, which makes the nonnatural amino acid mutagenesis of proteins more user accessible.     

Magnesium isotope effects in enzymatic phosphorylation

Recent discovery of magnesium isotope effect in the rate of enzymatic synthesis of adenosine triphosphate (ATP) offers a new insight into the mechanochemistry of enzymes as the molecular machines. The activity of phosphorylating enzymes (ATP-synthase, phosphocreatine, and phosphoglycerate kinases) in which Mg(2+) ion has a magnetic isotopic nucleus 25Mg was found to be 2-3 times higher than that of enzymes in which Mg(2+) ion has spinless, nonmagnetic isotopic nuclei 24Mg or 26Mg. This isotope effect demonstrates unambiguously that the ATP synthesis is a spin-dependent ion-radical process. The reaction schemes, suggested to explain the effect, imply a reversible electron transfer from the terminal phosphate anion of ADP to Mg(2+) ion as a first step, generating ion-radical pair with singlet and triplet spin states. The yields of ATP along the singlet and triplet channels are controlled by hyperfine coupling of unpaired electron in 25Mg+ ion with magnetic nucleus 25Mg. There is no difference in the ATP yield for enzymes with 24Mg and 26Mg; it gives evidence that in this reaction magnetic isotope effect (MIE) operates rather than classical, mass-dependent one. Similar effects have been also found for the pyruvate kinase. Magnetic field dependence of enzymatic phosphorylation is in agreement with suggested ion-radical mechanism.