Photoreaction of rac‐Leucine in Ice by Circularly Polarized Synchrotron Radiation: Temperature‐Induced Mechanism Switching from Norrish Type II to Deamination

The delivery of extraterrestrial organics to primitive Earth is considered to have triggered the origin and subsequent evolution of life. Indeed, enantiomerically enriched amino acids of nonterrestrial origin have been found in carbonaceous meteorites, and enantioselective photodecomposition by circularly polarized light (CPL) in outer space has been proposed to have played some role in the initial enantiomeric bias. To experimentally examine this possibility and elucidate the photoreaction mechanisms, we have studied the photolysis of racemic leucine (rac‐Leu) in acidic and neutral ice/water media at 21–298 K with left‐ and right‐CPL in an attempt to detect enantiomerically enriched D ‐ and L ‐Leu, respectively. Comprehensive product analyses revealed that the CPL‐induced deracemization of Leu proceeds in both acidic and neutral ice matrices even at 21 K, and that the main mechanism switches from Norrish‐type II γ‐hydrogen abstraction to S_Ni deamination on lowering the temperature. The potential role of the CPL‐induced photodecomposition of amino acids as a source of the enantiomer imbalance in meteorites is discussed.     

Mechanism of pH-dependent photolysis of aliphatic amino acids and enantiomeric enrichment of racemic leucine by circularly polarized light

It has been proposed that the origin of biological homochirality may be the result of irradiation of a racemic sample of amino acids by circularly polarized light (CPL). To determine the mechanism of enantiomeric enrichment, the irradiation of aliphatic amino acids by CPL was undertaken. An enantiomerically enriched sample (e.g., L isomer enrichment from r-CPL) was found to result from the preferential excitation/decomposition of one enantiomer over another via a Norrish Type II mechanism (leucine, valine, and isoleucine), with the enantiomeric excess dependent on the degree of protonation of the amino/carboxylic acid moiety.     

Stereoselective synthesis of 2-aminocyclobutanols via photocyclization of alpha-amido alkylaryl ketones: mechanistic implications for the Norrish/Yang reaction.

A series of chiral N-acylated alpha-amino p-methylbutyrophenone derivatives 1a-1h was synthesized from alpha-amino acids via a three-step procedure. These substrates were photolyzed in benzene and gave Norrish II and Norrish I cleavage products as well as the N-acylated 2-aminocyclobutanols that derive from gamma-hydrogen abstraction and 1,4-triplet biradical combination (Yang cyclization). The products were formed with characteristic Yang/cleavage ratios. The quantum yields for the photodecomposition of the N-acetyl-protected substrates 1b,e,f were moderate (12-26%); the diastereoselectivities of the cyclobutanol formation were remarkably high for all substrates. High diastereospecificity was observed for the isoleucine derivatives (2S,3S)-1g and allo-(2S,3R)-1g; the Yang reaction dominated the photochemistry of allo-1g, whereas 1ggave preferentially Norrish II cleavage. The role of hydrogen bonding as one of the stereodirecting effects was demonstrated by comparison of the cyclization efficiency of the valine derivative 1e with 1h,i,j. Also, aromatic beta-keto esters gave the Yang cyclization products in low yields. The diastereoselectivity of the cyclobutanol formation was rationalized by a three-step mechanism where every step is connected with one distinct stereochemical induction mechanism: (a) diastereoselective hydrogen abstraction, (b) conformational equilibration of the 1,4-tetramethylene biradicals (as calculated by semiempiric methods) controlled by hydrogen bonding, and (c) diastereoselective biradical combination (versus cleavage) influenced by spin-orbit coupling controlled intersystem crossing geometries.     

Solvent extraction of amino acids into a room temperature ionic liquid with dicyclohexano-18-crown-6

Amino acids Trp, Gly, Ala, Leu are extracted efficiently from aqueous solution at pH 1.5–4.0 (Lys and Arg at pH 1.5–5.5) into the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BmimPF₆) with dicyclohexano-18-crown-6 (CE). The most hydrophilic amino acids such as Gly are extracted as efficiently as the less hydrophilic (92–96%). The influence of pH, amino acid and crown ether concentration, volume ratio of aqueous and organic phases, and presence of some cations on amino acid recovery were studied. The ratio of amino acid to crown ether in the extracted species is 1:1 for cationic Trp, Leu, Ala, and Gly and to 1:2 for dicationic Arg and Lys. This ionic liquid extraction system was used successfully for the recovery of amino acids from pharmaceutical samples and fermentation broth, and was followed by fluorimetric determination.These results were published in part in Smirnova SV (2002) Ph.D. Thesis, Moscow State University.     

Amino acid adsorption on mesoporous materials: influence of types of amino acids, modification of mesoporous materials, and solution conditions

In order to disclose the dominant interfacial interaction between amino acids and ordered mesoporous materials, the adsorption behaviors of five amino acids on four mesoporous materials were investigated in aqueous solutions with adjustable amino acid concentration, ion strength, and pH. The selected amino acids were acidic amino acid glutamic acid (Glu), basic amino acid arginine (Arg), and neutral amino acids phenylalanine (Phe), leucine (Leu), and alanine (Ala), and the selected mesoporous materials were SBA-15, Al-SBA-15, CH3(10%)-SBA-15, and CH3(20%)-SBA-15. The adsorption capacities of Glu and Arg were strongly dependent on pH and surface charge of the mesoporous adsorbent. The adsorption of Phe showed pH insensitivity but depended on the surface organic functionalization of mesoporous adsorbent. On the basis of the theoretical analysis about the interaction between amino acid and adsorbent, such a remarkable difference was attributed to the different nature of the interaction between amino acid and adsorbent. Arg could be readily adsorbed on the surface of SBA-15, especially Al-SBA-15, under appropriate pH in which the electrostatic interaction was predominant. The driving force of Phe adsorption on mesoporous adsorbent mainly came from the hydrophobic interaction. Therefore, the adsorption capability of Arg decreased with increasing ion strength of solution, while the adsorption capability of Phe increased with the increasing degree of CH3 functionalization on SBA-15. For neutral amino acid Phe, Ala, and Leu, the adsorption capability increased with the increase of the length of their side chains, which was another evidence of hydrophobic effect. Thus, all the adsorption of amino acids on mesoporous silica materials can be decided by the combined influence of two fundamental interactions: electrostatic attraction and hydrophobic effect.     

Amino acids of ricin and its polypeptides

Ricin and its corresponding polypeptides (A & B chain) were purified from castor seed. The molecular weight of ricin subunits were 29,000 and 28,000 daltons. The amino acids in ricin determined were Asp45 The22 Ser40 Glu53 Cys4 Gly96 His5 Ile21 Leu33 Lys20 Met4 Phe13 Pro37 Tyr11 Ala45 Val23 Arg20 indicating that ricin contains approximately 516 amino acid residues. The amino acids of the two subunits of ricin A and B chains were Asp23 The12 Ser21 Glu29 Cys2 Gly48 His3 Ile12, Leu17 Lys10 Met2 Phe6 Pro17 Tyr7 Ala35 Val13 Arg13 while in B chain the amino acids were Asp22 The10 Ser19 Glu25 Cys2 Gly47 His1 Ile10, Leu15 Lys11 Met1 Phe7 Pro6 Tyr5 Ala32Val11 Arg10. The total helical content of ricin came around 53.6% which is a new observation.     

Microscopic Imaging of Chiral Amino Acids in Agar Gel through Circularly Polarized Luminescence of EuIII Complex

We recently found that [Eu(pda)₂]^? (pda: 1,10‐phenanthroline‐2,9‐dicarboxylic acid), which has an achiral structure in crystals, exhibits circularly polarized luminescence (CPL) in aqueous solutions containing chiral amino acids such as arginine and histidine. CPL measurements were performed for agar gel, which includes an aqueous solution of [Eu(pda)₂]^? and chiral arginine or histidine. The spectral shape, concentration, and pH dependences on CPL intensity in the agar gels were very close to those in aqueous solutions, indicating that the CPL of the Eu^III complex in the agar gels was induced by mechanism similar to that in aqueous solutions. We performed spatially resolved CPL measurements using a laboratory‐built microscopic CPL spectroscopic system for agar‐gel samples, where d ‐ and l ‐ amino acids were separately dispersed. We successfully recorded CPL imaging maps showing spatial dispersions of d ‐ and l ‐amino acid in the agar gels.     

Chiral sensing using an achiral europium(III) complex by induced circularly polarized luminescence

[Eu(bda)(2)](-) (bda = 2,2'-bipyridine-6,6'-dicarboxylic acid) produces intense circularly polarized luminescence (CPL) in aqueous solutions in the presence of (S)-2-pyrrolidone-5-carboxylic acid upon UV irradiation, although the molecular structure of the europium(III) complex is achiral. The mechanism for the induction of CPL was preliminarily attributed to distortions induced by association with an amino acid to generate chirality in the achiral complex. The optical anisotropy factor (g(lum) value) for the (5)D(0) → (7)F(1) transition was 0.03 in the presence of 1.0 mol dm(-3) of the amino acid. Analysis of the CPL intensity as a function of the amino acid concentration gave an association constant between those of [Eu(bda)(2)](-) and the amino acid, K(aso) = 0.55 ± 0.09 mol(-1) dm(3). These results demonstrate the potential of [Eu(bda)(2)](-) to act as a luminescent chiral-sensing reagent in microscopic spectroscopy.     

Essential of Proline and Valine Residues in the Peptide Derived from Lactoferrin for Angiotensin Converting Enzyme Inhibition

We synthesized Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu, the peptide contained in lactoferrin (Lf), to identify the angiotensin converting enzyme (ACE) inhibition. In an attempt to know the structure‐activity relationship of this peptide, we replaced Pro (the third amino acid residues from N‐terminal) or Val (the fourth amino acid residues from N‐terminal) with Ala (neutral amino acid), Glu (acidic amino acid) or Lys (basic amino acid) to produce six peptides. From the in vitro ACE inhibition (IC₅₀) of these synthesized peptides, the original peptide (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu) showed higher ACE inhibition than the replaced six peptides. Thus, replacement of Pro at the third amino acid residues or Val at the fourth position with Ala, Glu or Lys revealed the ACE inhibition to be lower than the original form of Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu. Otherwise, we added one peptide at the C‐terminal of Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu and found both products with an addition of Val (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu‐Val) or Ile (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu‐Ile) showing a lower ACE inhibition than the original one. The ACE inhibitions produced by both replaced peptides were without significance. Also, deletion of the last peptide at the C‐terminal (Leu‐Arg‐Pro‐Val‐Ala‐Ala) failed to produce a marked change of ACE inhibition as compared to the original one. These results suggest that Pro and Val are essential in the peptide for inhibition of ACE activity.     

Adsorption and photocatalytic decomposition of amino acids in TiO2 photocatalytic systems

The adsorption and photodecomposition of seven kinds of amino acids on a TiO2 surface were investigated by zeta potential measurements and 1H NMR spectroscopy in TiO2 aqueous suspension systems. The decomposition rates increased in the order of Phe < Ala < Asp < Trp < Asn < His < Ser. For Phe, Trp, Asn, His, and Ser, the isoelectric point (IEP) of TiO2 shifted to a lower pH with increasing decomposition rates upon adsorption on TiO2, suggesting that the effective adsorption and photocatalytic sites for these amino acids should be the basic terminal OH on the solid surface. Since the amino acids that decomposed faster than the others contain -OH (Ser), -NH (Trp, His), or -NH2 (Asn) in their side chain, they are considered to interact with the basic terminal OH groups more preferably by the side chain and are vulnerable to photocatalytic oxidation. On the other hand, Ala interacts with the acidic bridged OH on TiO2 to cause an IEP shift to a higher pH. The correlation of the surface hydroxyl groups with the photocatalysis of amino acids was verified by the use of calcined TiO2 without surface hydroxyl groups.     

Characterization of amino acids in silk sericin protein from Gonometa rufobrunnae by MEKC with phenyl isothiocyanate derivatization

An MEKC method was developed for the separation and characterization of phenyl-isothiocyanate (PITC)-labeled amino acids derived from Gonometa rufobrunnae silkworm after microdialysis sample cleanup. The influence of the buffer and SDS concentration on the resolution of the amino acids was investigated. A buffer system consisting of 25 mM phosphate, 10 mM borate buffer at pH 9.00, and 70 mM SDS showed the best results, with 13 PITC-amino acid derivatives being resolved out of 15 possible amino acids that were under study. Microdialysis sampling demonstrated its efficiency as a sample cleanup technique. Sericin protein from G. rufobrunnae was found to be characterized by at least 11 positively identified amino acids. These included His, Tyr, Ser, Ala, Phe, Lys, Gly, Arg, Cys, Glu, and Asp. Leu/Met and Val/Thr were coeluting pairs and hence could not be positively confirmed.     

低浓度下离子交换树脂吸附氨基酸的机理

详细地研究了在低浓度下, 酸(碱)式或盐式强酸性阳离子和强碱性阴离子交换树脂吸附氨基酸的机理·结果表明, 氨基酸通过离子交换、离子转移和物理吸附等三种机理被离子交换树脂吸附。     

分析测试新方法(197~203)毛细管电泳-间接紫外法用于太子参中的非衍生化氨基酸检测

建立了一种简单、快速测定赖氨酸(Lys)、脯氨酸(Pro)、亮氨酸(Leu)、丙氨酸(Ala)、组氨酸(His)、苏氨酸(Thr)、蛋氨酸(Met)、丝氨酸(Ser)和甘氨酸(Gly)的毛细管电泳-间接紫外检测方法.通过研究缓冲液的种类和浓度、缓冲液的p H等分离条件对被测组分分离度和灵敏度的影响,从而优化了分析条件.实验结果表明,用0.1mol/L氢氧化钠溶液调节14 mmol/L的对氨基苯磺酸至p H为11.2做为运行缓冲液,当分离电压为20 k V时,9组分在12 min内实现了完全分离.实验结果表明,方法能成功用于不同产地太子参药材中9种氨基酸的含量测定.方法重现性良好,迁移时间和峰高的RSD分别小于2.6%和4.5%(n=7).     

Photodegradation of polyvinylacetophenone. I. Long-wave photolysis

Polyvinylacetophenone (PVAP) films were exposed to long-wave (λ ≥ 300 nm) ultraviolet radiation in the absence of oxygen. The gaseous products, determined quantitatively by mass spectrometry, were methane, ethane, carbon monoxide, and acetaldehyde. These indicated that the polymer was undergoing photodecomposition via a Norrish type I reaction. The polymer also undergoes crosslinking reactions, becoming insoluble after 100 hr irradiation, and the ultraviolet and phosphorescence spectra indicate loss of carbonyl chromophores. The mechanism of photolysis is discussed, and the implications of the effects of photodegradation on the efficiency of the polymer as a sunlight energy transfer agent are examined.     

Uncharged water-soluble Co(II)-porphyrin: a receptor for aromatic alpha-amino acids

Changes in the UV-vis spectra and induced circular dichroism (ICD) signals observed, in correspondence with the porphyrin Soret region, for aqueous solutions of achiral 5,10,15,20-tetrakis{p-[omega-methoxy poly(oxy-ethylene)]phenyl}porphyrin cobalt (II) (Co-P) and aromatic alpha-L-amino acids (Trp and Phe) give direct evidence for the coordination between the Co-P and amino acids. Considering that Co-P, besides the Co atom (one-fixation-point system), does not contain in the molecule active ligand groups and that no ICD signals have been observed in the case of Co-P/Ala, it has been concluded that hydrophobic interactions or stacking interactions between the aromatic rings of the porphyrin and those of Trp or Phe, acting as further amino acid (AA) fixation points, can strongly reduce the mobility of the chiral guest, thus permitting the generation of ICD signals. The effects of changes of both pH (in the range 2-9) and amino acid structure on the ICD phenomenon have also been investigated. In particular, the following have been observed: (i) strong ICD signals for all of the Co-P/N-acetyl amino acid aqueous solutions at pH 7, (ii) an unexpected ICD band with a bisignate form for the Co-P/Ala solution at pH 9 after long aging, and (iii) an opposite ICD signal when alpha-D-Phe and alpha-D-Trp enantiomers have been used. The data reported in this paper show how the binding mechanism between receptor and AAs changes by modulating properly the pH or the molecular structures and indicate that in these aqueous solutions the coordination Co-N is not the fundamental mechanism giving rise to the formation of the complexes and that the binding can be driven by hydrophobic interactions. These occurrences, through the analysis of the spectroscopic response (and, in particular, the form of the ICD band), can allow the recognition of AAs.     

Chiral Sensing of Various Amino Acids Using Induced Circularly Polarized Luminescence from Europium(III) Complexes of Phenanthroline Dicarboxylic Acid Derivatives

Circularly polarized luminescence (CPL) was observed from [Eu(dppda)₂]^? (dppda=4,7‐diphenyl‐1,10‐phenanthroline‐2,9‐dicarboxylic acid) and [Eu(pzpda)₂]^? (pzpda=pyrazino[2,3‐f][1,10]phenanthroline‐7,10‐dicarboxylic acid) in aqueous solutions containing various amino acids. The selectivity of these complexes towards amino acids enabled them to be used as chiral sensors and their behavior was compared with that of [Eu(pda)₂]^? (pda=1,10‐phenanthroline‐2,9‐dicarboxylic acid). As these Eu^III complexes have achiral D_2d structures under ordinary conditions, there were no CPL signals in the emission assigned to f–f transitions. However, when the solutions contained particular amino acids they exhibited detectable CPL signals with g_lum values of about 0.1 (g_lum=CPL/2 TL; TL=total luminescence). On examining 13 amino acids with these three Eu^III complexes, it was found that whether an amino acid induced a detectable CPL depended on the Eu^III complex ligands. For example, when ornithine was used as a chiral agent, only [Eu(dppda)₂]^? exhibited intense CPL in aqueous solutions of 10^?2 mol dm^?3. Steep amino acid concentration dependence suggested that CPL in [Eu(dppda)₂]^? and [Eu(pzpda)₂]^? was induced by the association of four or more amino acid molecules, whereas CPL in [Eu(pda)₂]^? was induced by association of two arginine molecules.     

Spectroscopy of hydrothermal reactions,part 26: Kinetics of decarboxylation of aliphatic amino acids and comparison with the rates of racemization

The kinetics of decarboxylation of six α‐amino acids (glycine, alanine, aminobutyric acid, valine, leucine, and isoleucine) and β‐aminobutyric acid were studied in aqueous solution at 310–330ˆC and 275 bar over the pH₂₅ range 1.5–8.5 by using an in situ FT‐IR spectroscopy flow reactor. Based on the rate of formation of CO₂, the first‐order or pseudo‐first‐order rate constants were obtained along with the Arrhenius parameters. The decarboxylation rates of amino acids follow the order Gly > Leu ≈ Ile ≈ Val > Ala > α‐Aib > β‐Aib. Differences in the concentration between 0.05 and 0.5 m had only a minor effect on the decarboxylation rate. The effect of the position of the amino group on the decarboxylation rate was investigated for α‐, β‐, and γ‐aminobutyric acid and the order was found to be α > β ≫ γ. Although the pH dependence is complex, the decarboxylation rates of α‐amino acids qualitatively have the inverse trend of the racemization rates. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 602–610, 2003     

On the sodium and lithium ion affinities of some α-amino acids

The decomposition of 59 different cluster ions (generated by fast atom bombardment) consisting of two different amino acids and a sodium ion was analysed. The only fragment ions of significant abundance could be assigned to sodium ion-bound amino acids. Assuming that the most abundant ion in the fragment ion spectrum corresponds to the amino acid with the highest sodium ion affinity (SIA), the 20 common α-amino acids could be ordered with increasing sodium ion affinity as follows: Gly, Ala, Cys, Val, (Leu, Ile), Ser, Met, Thr, (Phe, Pro), Asp, Tyr, (Glu, Lys), Trp, Asn, Gln, His, Arg. Quantitative determinations were carried out by comparison of the lithium ion affinity (LIA) of Ala with that of dimethylformamide (DMF) in a fragment ion scan of the ion-bound dimer Ala—Li⁺—DMF. LIA(Ala) was calculated from LIA(Ala) = LIA(DMF) – (1/C)ln[I(AlaLi⁺)/I(DMF—Li⁺)], where the constant C was estimated from measurements of proton-bound amine–amino acid clusters. From fragment ion analysis of nine other Li⁺-bound α-amino acid dimers, the following lithium ion affinities were obtained: Gly 51.0, Ala 52.6, Sar 53.5, α-aminobutyric acid 53.7, glycine methyl ester 54.7 and Val 54.8. SIA(Ala) was estimated to be 75% of the lithium ion affinity and from fragment ion analysis of ten Na⁺-bound α-amino acid dimers the following sodium ion affinities were obtained: Gly 37.9, Ala 39.4, α-aminobutyric acid 40.3, Val 41.0, glycine methylster 41.0 and Sar 41.2.     

Enantioselective fluorescent recognition of chiral acids by cyclohexane-1,2-diamine-based bisbinaphthyl molecules

The cyclohexane-1,2-diamine-based bisbinaphthyl macrocycles (S)-/(R)-5 and their cyclic and acyclic analogues are synthesized. The interactions of these compounds with various chiral acids are studied. Compounds (S)-/(R)-5 exhibit highly enantioselective fluorescent responses and high fluorescent sensitivity toward alpha-hydroxycarboxylic acids and N-protected amino acids. Among these interactions, (S)-mandelic acid (10(-3) M) led to over 20-fold fluorescence enhancement of (S)-5 (1.0 x 10(-5) M in benzene/0.05% DME) at the monomer emission, and (S)-hexahydromandelic acid (10(-3) M) led to over 80-fold fluorescence enhancement. These results demonstrate that (S)-5 is useful as an enantioselective fluorescent sensor for the recognition of the chiral acids. On the basis of the study of the structures of (S)-5 and the previously reported 1,2-diphenylethylenediamine-based bisbinaphthyl macrocycle (S)-4, the large fluorescence enhancement of (S)-5 with a chirality-matched alpha-hydroxycarboxylic acid is attributed to the formation of a structurally rigidified host-guest complex and the further interaction of this complex with the acid to suppress the photoinduced electron-transfer fluorescent quenching caused by the nitrogens in (S)-5.     

Structure and energy of formation of β- and γ-cyclodextrin complexes with amino acid enantiomers

The interaction between cyclodextrins (CyD), β-CyD, and γ-CyD, and the L- and D-optical isomers of several amino acids (Ala, Leu, His, Phe) are calculated using DFT. It is found that the L-forms of the investigated amino acids bond more strongly to CyD, due to the different numbers of hydrogen bonds that form. The structures of the resulting complexes are analyzed.