In vivo determination of amino acid bioavailability in humans and model animals

Because the digestion of many dietary proteins is incomplete, and because there is a continuous (but variable) entry into the intestinal lumen of endogenous protein and amino acid nitrogen that is also subject to digestion, the fluxes of nitrogen, amino acids, and protein in the gut exhibit a rather complicated pattern. Methods to distinguish and quantitate the endogenous and dietary components of nitrogen and amino acids in ileal chyme or feces include the use of a protein-free diet, the enzyme-hydrolyzed protein method, different levels of protein intake, multiple regression methods, and stable-isotope labelling of endogenous or exogenous amino acids. Assessment of bioavailability can be made, with varying degrees of difficulty, in man directly but, for routine evaluation of foods, the use of model animals is attractive for several reasons, the main ones being cost and time. Various animals and birds have been proposed as models for man but, in determining their suitability as a model, their physiological, enzymological, and microbiological differences must be considered. Fecal or ileal digestibility measurements, as well as apparent and true nitrogen and amino acid digestibility measurements, have very different nutritional significance and can, thus, be used for different objectives. Measurements at the ileal level are critical for determining amino acid losses of both dietary and endogenous origin, whereas measurements at the fecal level are critical in assessing whole-body nitrogen losses. A complementary and still unresolved aspect is to take into account the recycling of intestinal nitrogen and bacterial amino acids to the body.     

CHANGES IN THE PATTERNS OF BIOSYNTHESIS and COMPOSITION OF AMINO ACIDS IN A MARINE PHYTOPLANKTER EXPOSED TO ULTRAVIOLET-B RADIATION: NITROGEN LIMITATION IMPLICATED

Abstract— The impact of ultraviolet-B radiation (UVBR) on the biosynthesis and the composition of intracellular dissolved free and combined amino acids was examined in nitrate-replete and nitrate-deficient cultures of Tetraselmis sp. Several similarities were observed in the response of Tetraselmis sp. to nitrogen deficiency and exposure to UVBR, in support of a view that UVBR affects amino acid synthesis in phytoplankton via its inhibitory effects on nitrogen assimilation into the cells. At levels that still permitted the uptake of carbon into the cells, both nitrogen deficiency and UVBR exposure resulted in a reduction in the overall rates of carbon incorporated into amino acids, an increase in the absolute concentrations of amino acids within the intracellular dissolved free amino acid (INDFAA) pool and a decrease in the total cellular amino acid (TCAA) pool. An examination of the patterns of carbon assimilation into individual amino acids in cells exposed to UVBR revealed similarities with the patterns in cells subject to nitrogen deficiency. The most conspicuous changes from the controls included an increased incorporation of ¹³C into glutamic' acid (glutamic acid + glutamine) and aspartic acid and a marked reduction into alanine and valine. Changes in the concentrations of amino acids within the INDFAA and TCAA pools were also similar in nitrate-deficient and UVBR-exposed cells and resembled the carbon assimilation patterns. These results strongly suggest that UVBR-induced changes in the biosynthesis and composition of amino acids are probably via its suppression of nitrogen assimilation into the cells.     

氟硼二吡咯类pH荧光探针PET光谱特性的理论计算

氟硼二吡咯(BODIPY)类pH荧光探针分子是基于光诱导电子转移(PET)的荧光探针分子, 识别基团氮原子上引入不同取代基可呈现不同的光学灵敏度. 本文应用密度泛函理论(DFT)及含时密度泛函理论(TD-DFT)方法对六种含不同取代基的探针分子进行了几何构型优化及激发态计算, 探讨了不同取代基对PET效应影响. 计算结果表明: 基态时这些探针分子的最高占有分子轨道(HOMO)和最低未占有分子轨道(LUMO)都在荧光母体BODIPY的π, π^*轨道, 而识别基团上氮原子孤对电子所在的轨道为HOMO-1轨道. 但是在激发态, 当氮原子上有两个取代基时, HOMO-1→LUMO跃迁的激发能都小于荧光团的HOMO→LUMO跃迁, 这将有可能产生PET效应并导致荧光熄灭, 而当氮原子上有一个取代基时不会出现这种现象. 通过激发态结构优化可以发现, 无论识别基团氮原子上有一个还是两个取代基, N原子的轨道对称性都发生变化, 由sp³→sp², 孤对电子占据在p轨道上, 其轨道能级升高至荧光团的HOMO和LUMO轨道之间, 将导致不同程度的PET效应, 与实验结果一致.     

Detection of glucosamine in the acid hydrolysis solution of humic substances

Several humic substances isolated from water and soil have been characterized applying acid hydrolysis to release defined components bound in these high molecular organic compounds. The hydrolysis solution have been analyzed by HPLC after precolumn derivatization with o-phthalaldehyde and 9-fluorenylmethylchloroformate. Under these conditions the nitrogen containing carbohydrate glucosamine could be detected and separated besides seventeen amino acids. The results show that 8.0 to 40.1% of the total nitrogen content of the investigated samples contributes to amino acids and up to 7.9% to the carbohydrate glucosamine. It could be deduced that defined amino acid carbohydrate building blocks were bound in humic substances, because the glucosamine nitrogen content correlates with the calculated amino acid nitrogen. Furthermore the total amino acid nitrogen amount of natural organic matter can be estimated from the results of the determination of glucosamine.     

Detection of glucosamine in the acid hydrolysis solution of humic substances

Several humic substances isolated from water and soil have been characterized applying acid hydrolysis to release defined components bound in these high molecular organic compounds. The hydrolysis solution have been analyzed by HPLC after precolumn derivatization with o-phthalaldehyde and 9-fluorenylmethylchloroformate. Under these conditions the nitrogen containing carbohydrate glucosamine could be detected and separated besides seventeen amino acids. The results show that 8.0 to 40.1% of the total nitrogen content of the investigated samples contributes to amino acids and up to 7.9% to the carbohydrate glucosamine. It could be deduced that defined amino acid carbohydrate building blocks were bound in humic substances, because the glucosamine nitrogen content correlates with the calculated amino acid nitrogen. Furthermore the total amino acid nitrogen amount of natural organic matter can be estimated from the results of the determination of glucosamine.     

Wie entstand das Leben auf der Erde?: Ammoniak aus Stickstoff unter präbiotischen Bedingungen

This article discusses natural and technical nitrogen fixation. Additionally, a possible scenario of prebiotic nitrogen fixation is presented. The way how ammonia is generated in this scenario could have been the basis for the formation of early amino acids, which are the fundamental building blocks of all nitrogen‐containing bio‐molecules.     

N-Aminosulfamide peptide mimic synthesis by alkylation of aza-sulfurylglycinyl peptides

N-Aminosulfamides are peptidomimetics in which the C(α)H and the carbonyl of an amino acid residue are both respectively replaced by a nitrogen atom and a sulfonyl group. Aza-sulfurylglycinyl tripeptide analogs were effectively synthesized from amino acid building blocks by condensations of N-protected amino hydrazides and p-nitrophenylsulfamidate esters. The installation of N-alkyl chains and access to other aza-sulfuryl amino acid residues were effectively achieved by chemoselective alkylation.     

Characterization of natural and synthetic humic substances (melanoidins) by stable carbon and nitrogen isotope measurements and elemental compositions

Stable carbon isotope ratios of the starting sugars (SG) and amino acids (AA) are mostly preserved in laboratory synthetic melanoidins. Stable nitrogen isotope ratios may not be “imprinted” in melanoidins by SG and AA precursors in the same way, indicating that nitrogen fractionation could occur during the rate-determining step in the Maillard reaction. Carbon isotope ratios support the stoichiometric ratios for combination of sugars with amino acids, which are based on the elemental composition data of melanoidins. These findings may provide clues to asses the role of the Maillard reaction in the formation of natural humic substances.     

Protonation of glycine and alanine: proton affinities, intrinsic basicities and proton transfer path

Proton affinities and intrinsic basicities for nitrogen and oxygen protonation in the gas phase of the amino acids glycine and alanine were calculated using density functional theory (DFT) and ab initio methods at different levels of theory from Hartree-Fock (HF) to G2 approximations. All methods gave good agreement for proton affinities for nitrogen protonation for both amino acids. However, dramatic differences were found between DFT, MP4//MP2, and G2 results on one hand, and MP4//HF results on the other to the calculation of structural and energetic characteristics of oxygen protonation in glycine and alanine. An investigation into the source of these differences revealed that electron correlation effects are chiefly responsible for the differences in calculated oxygen proton affinities between the various methods. It has been found that proton transfer between nitrogen and oxygen protonation sites in both amino acids occurs without a transfer path barrier when correlated methods were used to calculate the path energetics.     

14N nuclear quadrupole resonance in N-acetyl amino acids

¹⁴N Nuclear quadrupole resonance (NQR) spectra of several N-acetyl amino acids and related compounds are reported and analyzed within the framework of the Townes and Dailey theory. The inductive effect of the chloroacetyl group on the nitrogen is discussed. A positive correlation between the π-σ_NC electron density at the nitrogen and the Taft inductive parameter σ* is observed, suggesting that the nitrogen π-charge density in the N-acetyl amino acids does not vary appreciably. NQR data for hippuric acid are compared to the ¹⁴N quadrupole parameters extracted from ENDOR data reported for X-irradiated hippuric acid.     

Basicity of azoles. VII. Basicity of C-aminopyrazoles in relation to tautomeric and protonation studies

The pK_a values of five aminopyrazoles [3(5)-amino, 1-methyl-3-amino, 1-methyl-5-amino, 4-amino and 1-methyl-4-amino] were determined. The aqueous basicities are discussed in terms of tautomerism (72% of 3-amino tautomer), protonation site (only 4-aminopyrazoles protonate on the amino group) and amino substituent effects. The results of theoretical calculations, carried out at the semiempirical INDO level, indicate that in the gas phase 3- and 5-aminopyrazoles protonate on the pyrazolic nitrogen atom, whereas 4-aminopyrazoles possess similar proton affinities for both nitrogen atoms (pyrazolic and amino).     

Selective Nitrogen Functionalization of Graphene by Bucherer‐Type Reaction

Nitrogen functionalization of graphene offers new hybrid materials with improved performance for important technological applications. Despite studies highlighting the dependence of the performance of nitrogen‐functionalized graphene on the types of nitrogen functional groups that are present, precise synthetic control over their ratio is challenging. Herein, the synthesis of nitrogen‐functionalized graphene rich in amino groups by a Bucherer‐type reaction under hydrothermal conditions is reported. The efficiency of the synthetic method under two hydrothermal conditions was examined for graphite oxide produced by Hummers and Hofmann oxidation routes. The morphological and structural properties of the amino‐functionalized graphene were fully characterized. The use of a synthetic method with a well‐known mechanism for derivatization of graphene will open new avenues for highly reproducible functionalization of graphene materials.     

Isolation and desalting with cation‐exchange chromatography for compound‐specific nitrogen isotope analysis of amino acids: application to biogeochemical samples

We have established a procedure for removing interfering materials from extracts of geological and biological samples, in order to determine precise compound‐specific nitrogen isotopic compositions of amino acids. We employed cation‐exchange chromatography of protein and non‐protein amino acids prior to derivatization for gas chromatographic separation. The average recovery of a standard amino acid solution was better than 94%, without nitrogen isotope fractionation during the cation‐exchange chromatography. We applied the procedure to various environmental samples including ‘hard’ (calcareous, siliceous, rock and sediment samples) and ‘soft’ materials (aggregated microbial samples and biological soft tissue samples). We conclude that cation‐exchange chromatography is a pre‐treatment procedure which should be widely useful for the determination of compound‐specific nitrogen isotopic compositions of amino acids. Copyright © 2010 John Wiley & Sons, Ltd.     

酱油中氨基酸态氮含量测量不确定度的评定

分析容量分析法测定酱油中氨基酸态氮含量的测量不确定度的影响因素，建立数学模型，对各不确定度分量进行分析和计算，通过合成和扩展得到酱油中氨基酸态氮测定结果的扩展不确定度为0．014g/(100mL)。     

氮之琐谈

氮元素是空气的主要成分,也是构成有机化合物的重要元素;生命离不开氮元素,化学工业也需要众多含氮化合物作为基础的工业原料。地球上氮元素的循环起始于闪电或生物固氮,而发展节能高效的固氮方法依然是化学家的长期任务。氮氧化物如一氧化氮和二氧化氮等是重要的空气污染源,控制其排放是治理空气污染的重要途径。对含氮有机化合物如氨基酸与蛋白质的研究至关重要,有望为人类揭示生命的奥秘或提供重要的材料如尼龙等。     

Polymerization in liquid ammonia: Acrylate esters and vinyl thiolacetate

Methyl ethyl and n-propyl acrylates when polymerized by sodium in liquid ammonia, contain an average of one amino group per polymer chain, which is indicative of amide anion initiation and chain transfer to the solvent ammonia. Isopropyl acrylate and the higher homologs contain considerably less nitrogen than required by one amino group per chain indicative of another initiating and terminating mechanism. Three alternate mechanisms are discussed. Of these, alkoxide initiation seems to be the most plausible. Vinyl thiolacetate polymers prepared under the same conditions also contain less nitrogen than required by one amino group per chain. The polymer structure is predominantly poly(vinyl thiol) with intramolecular or intermolecular disulfide bridges. The acetate part of the molecule is concomitantly split off to form acetamide.     

硼酸酯偶联剂的合成与表征

通过分子结构设计，合成了一种氮一硼内配位结构的硼酸酯偶联剂，解决了硼酸酯水解稳定性差的问题，同时分子结构中含有氨基、丁氧基等活性基团。用傅立叶变换红外光谱对硼酸酯进行了结构表征，讨论了原料配比、合成工艺等对硼酸酯收率及反应速率的影响，并通过实验确定了最佳配方及工艺。对硼酸酯的物理性能、溶解性、水解稳定性及对硼酸铝晶须的表面处理效果进行了表征。结果表明：合成的硼酸酯偶联剂由于具有氮-硼内配位作用，表现出优良的水解稳定性，并且硼酸酯偶联剂对硼酸铝晶须具有良好的表面改性效果。     

Catalytic α-allylation of unprotected amino acid esters

Catalytic α-allylation of unprotected amino acid esters to produce α-quaternary α-allyl amino acid esters is reported. Catalytic loadings of picolinaldehyde and Ni(II) salts induce preferential reactivity at the enolizable α-carbon of amino acid esters over the free nitrogen with electrophilic palladium π-allyl complexes. Fourteen examples are given. Additionally, the use of chiral ligands to access enantioenriched α-quaternary amino acid esters from racemic precursors is demonstrated by the enantioselective synthesis of α-allyl phenylalanine methyl ester from racemic phenylalanine methyl ester.     

An Investigation of Protonation Sites and Conformations of Protonated Amino Acids by IRMPD Spectroscopy

The protonation sites and structures of a series of protonated amino acids (Gly, Ala, Pro, Phe, Lys and Ser) are investigated by means of infrared multiple‐photon dissociation (IRMPD) spectroscopy and electronic‐structure calculations. The IRMPD spectra of the protonated species are recorded using the combination of a free‐electron laser (FEL) and an electrospray‐ion‐trap mass spectrometer. The structures of different possible isomers of these protonated species are optimized at the B3LYP/6‐311+G(d, p) level of theory and the IR spectra calculated using the same computational method. For every amino acid studied herein, the current results indicate that a proton is bound to the α‐amino nitrogen, except for lysine, in which the protonation site is the amino nitrogen in the side chain. According to the calculated and experimental IRMPD results, the structures of the protonated amino acids may be assigned unambiguously. For Gly, Ala, and Pro, in each of the most stable isomers the protonated amino group forms an intramolecular hydrogen bond with the adjacent carbonyl oxygen. In the case of Gly, the isomer containing a proton bound to the carbonyl oxygen is theoretically possible. However, it does not exist under the experimental conditions because it has a significantly higher energy (i.e. 26.6 kcal mol^?1) relative to the most stable isomer. For Ser and Phe, the protonated amino group forms two intramolecular hydrogen bonds with both the adjacent carbonyl oxygen and the side‐chain group in each of the most stable isomers. In protonated lysine, the protonated amino group in the side chain forms two hydrogen bonds with the α‐amino nitrogen and the carbonyl oxygen, which is a cyclic structure. Interestingly, for protonated lysine the zwitterionic structure is a local minimum energy isomer, but the experimental spectrum indicates that it does not exist under the experimental conditions. This is consistent with the fact that the zwitterionic isomer is 9.2 kcal mol^?1 higher in free energy at 298 K than the most stable isomer. The carbonyl stretching vibration in the range of 1760–1800 cm^?1 is especially sensitive to the structural change. In addition, IRMPD mechanisms for the protonated amino acids are also investigated.     

Tautomeric conjugate acids of 2-aminopyrroles: effect of substituents, solvation and cosolute

The tautomeric preferences of the conjugated acids of 2-aminopyrrole derivatives have been examined both in the gas phase and in aqueous solution by using a combination of quantum mechanical, self-consistent reaction field and Monte Carlo–free-energy perturbation methods. The results show that the nature of substituents, the solvent and the presence of cosolute are relevant factors in modulating the relative stability between the tautomeric conjugate acids protonated at the heterocyclic ring and at the exocyclic amino nitrogen. Thus, attachment of electron-withdrawing groups to the ring, solvation in polar solvents, and the presence of negatively charged cosolutes tend to favor protonation at the exocyclic amino nitrogen. Nevertheless, none of these factors alone suffice to change the tautomeric preference for the ring-protonated forms. The results point out that the concerted occurrence of the three factors is necessary to shift the tautomeric preference towards the conjugated species protonated at the exocyclic nitrogen.Contribution to the Jacopo Tomasi Honorary Issue