Optimized Biocatalytic Synthesis of 2-Selenopyrimidine Nucleosides by Transglycosylation**

Selenium-modified nucleosides are powerful tools to study the structure and function of nucleic acids and their protein interactions. The widespread application of 2-selenopyrimidine nucleosides is currently limited by low yields in established synthetic routes. Herein, we describe the optimization of the synthesis of 2-Se-uridine and 2-Se-thymidine derivatives by thermostable nucleoside phosphorylases in transglycosylation reactions using natural uridine or thymidine as sugar donors. Reactions were performed at 60 or 80 °C and at pH 9 under hypoxic conditions to improve the solubility and stability of the 2-Se-nucleobases in aqueous media. To optimize the conversion, the reaction equilibria in analytical transglycosylation reactions were studied. The equilibrium constants of phosphorolysis of the 2-Se-pyrimidines were between 5 and 10, and therefore differ by an order of magnitude from the equilibrium constants of any other known case. Hence, the thermodynamic properties of the target nucleosides are inherently unfavorable, and this complicates their synthesis significantly. A tenfold excess of sugar donor was needed to achieve 40−48 % conversion to the target nucleoside. Scale-up of the optimized conditions provided four Se-containing nucleosides in 6–40 % isolated yield, which compares favorably to established chemical routes.     

Synthesis and properties of aminopropyl nucleic acids

Oligonucleotides that contain up to three aminopropyl nucleoside analogues have been synthesized. Dimers of aminopropyl adenine and thymidine were prepared and used as building blocks by applying phosphoramidite chemistry. Both R and S isomers of the aminopropyl nucleosides were used. This incorporation led to a reduction of thermal stability of double-stranded DNA. Furthermore, the (R)-adenine analogue, which yielded (S)-APNA, can be considered as a candidate for universal base pairing.     

The Role of a Nonribosomal Peptide Synthetase in l‐Lysine Lactamization During Capuramycin Biosynthesis

Capuramycins are one of several known classes of natural products that contain an l ‐Lys‐derived l ‐α‐amino‐?‐caprolactam (l ‐ACL) unit. The α‐amino group of l ‐ACL in a capuramycin is linked to an unsaturated hexuronic acid component through an amide bond that was previously shown to originate by an ATP‐independent enzymatic route. With the aid of a combined in vivo and in vitro approach, a predicted tridomain nonribosomal peptide synthetase CapU is functionally characterized here as the ATP‐dependent amide‐bond‐forming catalyst responsible for the biosynthesis of the remaining amide bond present in l ‐ACL. The results are consistent with the adenylation domain of CapU as the essential catalytic component for l ‐Lys activation and thioesterification of the adjacent thiolation domain. However, in contrast to expectations, lactamization does not require any additional domains or proteins and is likely a nonenzymatic event. The results set the stage for examining whether a similar NRPS‐mediated mechanism is employed in the biosynthesis of other l ‐ACL‐containing natural products and, just as intriguingly, how spontaneous lactamization is avoided in the numerous NRPS‐derived peptides that contain an unmodified l ‐Lys residue.     

脂肪酶和醇腈酶在有机合成中的应用

有机相酶催化反应常用于高附加值产品的合成。脂肪酶是有机合成中一种重要的工具酶，可以催化酯交换反应。脂肪酶催化的酯的氨解反应已经用于手性酰胺的合成，烷氧羰基化也广泛应用于一些天然化合物的化学选择性修饰。以前在有机合成中很少实用的裂合酶如(R)-醇腈酶在合成具有旋光性的氰醇中的应用也开始引起人们的关注。     

HPLC测定海马中核苷和核苷酸的含量

为建立一种同时测定海马中5种核苷(胞苷、尿苷、肌苷、鸟苷和腺苷)和5种核苷酸(CMP、UMP、IMP、GMP和AMP)含量的方法,本研究采用HPLC,以Accalaim C30(2.1 mm×150 mm,3μm)为分析柱,以水(A)-100 mmol/L NH4OAC溶液(p H=5.0)(B)-乙腈(C)为流动相,梯度洗脱,流速0.4 m L/min,检测波长254 nm,柱温25℃。结果显示,5种核苷和5种核苷酸的浓度均在0.02~10 mg/L范围内线性关系良好。平均加样回收率在88.38%~96.30%(n=3)之间。本法能够快速、简便地测定海马中主要核苷和核苷酸的含量,为其质量控制提供保证。     

Electron transfer reaction and mechanism of oxidation of Inosine

Electrochemical oxidation of Inosine has been studied in the phosphate buffers of pH range 3.3-10.9 at pyrolytic graphite electrode. In the entire pH range a single well-defined oxidation peak (I_a) was observed, when the sweep was initiated in the positive direction. In the reverse sweep no cathodic peak was obtained. The peak potential of the oxidation peak was dependent on pH and shifted to less positive potential with increase in pH. The kinetics of the UV absorbing intermediate was followed spectrophotometrically and the decay occurred in a pseudo first order reaction having k values in the range 0.50-0.92 × 10⁻³ s⁻¹ in the entire pH range studied. The value of n was found to be 2.95 ± 0.3. The products of oxidation were silylated and characterized by using GC-Mass. Two tetramers having CC, CN, NN, CON and COOC linkages were identified. A plausible mechanism for the electrooxidation of Inosine has been suggested.     

Comparative study of purine and pyrimidine nucleoside analogues acting on the thymidylate kinases of Mycobacterium tuberculosis and of humans

Thymidine monophosphate kinase (TMPK) from Mycobacterium tuberculosis (TMPKmt) is an attractive target for the design of specific inhibitors. This fact is the result of its key role in the thymidine pathway and of unique structural features in the active site observed by X-ray crystallography, especially in comparison to its human counterpart (TMPKh). Different 5-modified thymidine derivatives, as well as purine and pyrimidine analogues or C-nucleosides were tested on TMPKmt and TMPKh, and the results were rationalized by docking studies. 5-Halogenated 2'-deoxyuridines are the best inhibitors of TMPKmt found and present the highest selectivity indexes in favor of TMPKmt.     

pH‐Cleavable Nucleoside Lipids: A New Paradigm for Controlling the Stability of Lipid‐Based Delivery Systems

Lipid‐based delivery systems are an established technology with considerable clinical acceptance and several applications in human. Herein, we report the design, synthesis and evaluation of novel orthoester nucleoside lipids (ONLs) for the modulation of liposome stability. The ONLs contain head groups with 3′‐orthoester nucleoside derivatives featuring positive or negative charges. The insertion of the orthoester function in the NL structures allows the formation of pH‐sensitive liposomes. ONL‐based liposomes can be hydrolyzed to provide nontoxic products, including nucleoside derivatives and hexadecanol. To allow the release to be tunable at different hydrolysis rates, the charge of the polar head structure is modulated, and the head group can be released at a biologically relevant pH. Crucially, when ONLs are mixed with natural phosphocholine lipids (PC), the resultant liposome evolves toward the formation of a hexadecanol/PC lamellar system. Biological evaluation shows that stable nucleic acid lipid particles (SNALPs) formulated with ONLs and siRNAs can effectively enter into tumor cells and release their nucleic acid payload in response to an intracellular acidic environment. This results in a much higher antitumor activity than conventional SNALPs. The ability to use pH‐cleavable nucleolipids to control the stability of lipid‐based delivery systems represents a promising approach for the intracellular delivery of drug cargos.     

Design,Synthesis, and Biological Evaluation of Unconventional Aminopyrimidine,Aminopurine, and Amino‐1,3,5‐triazine Methyloxynucleosides

Herein we describe a class of unconventional nucleosides (methyloxynucleosides) that combine unconventional nucleobases such as substituted aminopyrimidines, aminopurines, or aminotriazines with unusual sugars in their structures. The allitollyl or altritollyl derivatives were pursued as ribonucleoside mimics, whereas the tetrahydrofuran analogues were pursued as their dideoxynucleoside analogues. The compounds showed poor, if any, activity against a broad range of RNA and DNA viruses, including human immunodeficiency virus (HIV). This inactivity may be due to lack of an efficient metabolic conversion into their corresponding 5′‐triphosphates and poor affinity for their target enzymes (DNA/RNA polymerases). Several compounds showed cytostatic activity against proliferating human CD4⁺ T‐lymphocyte CEM cells and against several other tumor cell lines, including murine leukemia L1210 and human prostate PC3, kidney CAKI‐1, and cervical carcinoma HeLa cells. A few compounds were inhibitory to Moloney murine sarcoma virus (MSV) in C3H/3T3 cell cultures, with the 2,6‐diaminotri‐O‐benzyl‐D ‐allitolyl‐ and ‐D ‐altritolyl pyrimidine analogues being the most potent among them. This series of unconventional nucleosides may represent a novel family of potential antiproliferative agents.