Synthesis of N alpha-(1-phenyl-2-mercaptoethyl) amino acids, new building blocks for ligation and cyclization at non-cysteine sites: scope and limitations in peptide synthesis

A new and convenient method for the synthesis and incorporation of N(alpha)-(1-phenyl-2-mercaptoethyl)-derivatized amino acids applicable to chemical ligation at non-cysteine sites is presented. N(alpha)-Auxiliary derivatives of glycine and alanine were easily prepared using reductive amination approaches. Several strategies for the incorporation of these derivatives into peptide chains were investigated: coupling without protection, with acid-labile protection, with base-labile protection, and via a novel protection strategy using the thiazolidine derivative. All amino acid derivatives were successfully coupled to various peptide resins, and with the exception of those incorporating Boc-protected derivatives, all resins yielded the desired peptide fragments. However, the coupling of the two alanine derivative diastereomers generated some epimerization. Finally, N-terminal auxiliary glycine and alanine peptides were cyclized, and the corresponding native circular peptides were obtained upon successful removal of the auxiliary.     

Temperature selectivity effects in reversed-phase liquid chromatography due to conformation differences between helical and non-helical peptides

In order to characterize the effect of temperature on the retention behaviour and selectivity of separation of polypeptides and proteins in reversed-phase high-performance liquid chromatography (RP-HPLC), the chromatographic properties of four series of peptides, with different peptide conformations, have been studied as a function of temperature (5-80 degrees C). The secondary structure of model peptides was based on either the amphipathic alpha-helical peptide sequence Ac-EAEKAAKEX(D/L)EKAAKEAEK-amide, (position X being in the centre of the hydrophobic face of the alpha-helix), or the random coil peptide sequence Ac-X(D/L)LGAKGAGVG-amide, where position X is substituted by the 19 L- or D-amino acids and glycine. We have shown that the helical peptide analogues exhibited a greater effect of varying temperature on elution behaviour compared to the random coil peptide analogues, due to the unfolding of alpha-helical structure with the increase of temperature during RP-HPLC. In addition, temperature generally produced different effects on the separations of peptides with different L- or D-amino acid substitutions within the groups of helical or non-helical peptides. The results demonstrate that variations in temperature can be used to effect significant changes in selectivity among the peptide analogues despite their very high degree of sequence homology. Our results also suggest that a temperature-based approach to RP-HPLC can be used to distinguish varying amino acid substitutions at the same site of the peptide sequence. We believe that the peptide mixtures presented here provide a good model for studying temperature effects on selectivity due to conformational differences of peptides, both for the rational development of peptide separation optimization protocols and a probe to distinguish between peptide conformations.     

A structural study of model peptides derived from HIV-1 integrase central domain

The HIV-1 integrase (IN) catalyzes the integration of viral DNA in the human genome. In vitro the enzyme displays an equilibrium of monomers, dimers, tetramers and larger oligomers. However, its functional oligomeric form in vivo is not known. We report a study of the auto-associative properties of three peptides denoted K156, E156 and E159. These derive from the alpha4 helix of the IN catalytic core. The alpha4 helix is an amphipatic helix exposed at the surface of the protein and could be involved in the oligomerization process through its hydrophobic face. The peptides were obtained from the replacement of several amino acid residues by more helicogenic ones in the alpha4 helix peptide. K156 carries the basic residues Lys156 and Lys159, which have been shown important for the binding of IN to viral DNA. In E156 and E159 they are replaced with the acidic residue Glu. A fourth peptide K(E)156 obtained from the replacement of hydrophobic residues with Glu in K156 in order to abolish the auto-associative properties is used as a negative control. The capacity shown by peptides for alpha-helical formation is demonstrated by circular dichroism (CD) analysis performed in aqueous solution and in aqueous trifluoroethanol (TFE) mixtures. Both electrospray ionization mass spectrometry (ESI-MS) and glutaraldehyde chemical cross-linking show that peptides adopt different solvent-dependent equilibriums of monomers, dimers, trimers and tetramers. Oligomerization of peptides in aqueous solution is related to their ability to form helical structures. Addition of a small amount of TFE (<10%) stimulates helix stabilization and the interhelical hydrophobic contacts. Higher amounts of TFE alter the hydrophobic contacts and disrupt the oligomeric species. In addition to hydrophobic interactions, the patterns indicate that the biologically important Lys156 and Lys159 residues also participate in helix association. K(E)156 despite its ability to adopt a helical structure is unable to associate into oligomers, demonstrating the importance of hydrophobic contacts for oligomerization. Thus, the designed peptides provide us information on the functional properties of the alpha4 IN that seems to hold a dual role in DNA recognition and protein oligomerization.     

Dehydroalanine-based inhibition of a peptide epimerase from spider venom

Ribosomally produced peptides that contain D-amino acids have been isolated from a number of vertebrate and invertebrate sources. In each case, the D-amino acids are introduced by a posttranslational modification of a parent peptide containing only amino acids of the L-configuration. The only known enzyme to catalyze such a reaction is the peptide epimerase (also known as peptide isomerase) from the venom of the funnel web spider, Agelenopsis aperta. This enzyme interconverts two 48-amino-acid-long peptide toxins that differ only by the stereochemistry at a single serine residue. In this paper we report the synthesis and testing of two pentapeptide analogues that contain modified amino acids at the site normally occupied by the substrate serine residue. When the L-chloroalanine-containing peptide 3 was incubated with the epimerase it was converted into the dehydroalanine-containing peptide 4 via an elimination of HCl. The dehydroalanine peptide 4 was independently synthesized and found to act as a potent inhibitor of the epimerase (IC50 = 0.5 microM). These results support a direct deprotonation/reprotonation mechanism in which a carbanionic intermediate is formed. The observed inhibition by 4 can be attributed to the sp(2)-hybridization of the alpha-carbon in the dehydroalanine unit that mimics the planar geometry of the anionic intermediate.     

Protease-catalyzed peptide synthesis for the site-specific incorporation of alpha-fluoroalkyl amino acids into peptides

Substitution of native amino acids by fluoroalkyl analogues represents a new approach for the design of biologically active peptides with increased metabolic stability as well as defined secondary structure and provides a powerful label for spectroscopic investigations. Here, we introduce a methodology for the incorporation of sterically demanding C(alpha)-fluoroalkyl amino acids into the P(1) position of peptides catalyzed by the commercially available proteases trypsin and alpha-chymotrypsin. The combination of 4-guanidinophenyl ester of C(alpha)-fluoroalkyl amino acids as substrate mimetics with frozen-state reaction conditions provided the most efficient strategy for protease-catalyzed site-specific introduction of this kind of nonnatural amino acids into peptide sequences. Consequently, a library of di-, tri-, and tetrapeptides containing alpha-methyl, alpha-difluoromethyl, and alpha-trifluoromethyl alanine, leucine, and phenylalanine in the P(1) position was synthesized catalyzed by trypsin as well as alpha-chymotrypsin. Trypsin was shown to be the more versatile protease.     

Quantitation of the nearest-neighbour effects of amino acid side-chains that restrict conformational freedom of the polypeptide chain using reversed-phase liquid chromatography of synthetic model peptides with L- and D-amino acid substitutions

Side-chain backbone interactions (or "effects") between nearest neighbours may severely restrict the conformations accessible to a polypeptide chain and thus represent the first step in protein folding. We have quantified nearest-neighbour effects (i to i+1) in peptides through reversed-phase liquid chromatography (RP-HPLC) of model synthetic peptides, where L- and D-amino acids were substituted at the N-terminal end of the peptide sequence, adjacent to a L-Leu residue. These nearest-neighbour effects (expressed as the difference in retention times of L- and D-peptide diastereomers at pHs 2 and 7) were frequently dramatic, depending on the type of side-chain adjacent to the L-Leu residue, albeit such effects were independent of mobile phase conditions. No nearest-neighbour effects were observed when residue i is adjacent to a Gly residue. Calculation of minimum energy conformations of selected peptides supported the view that, whether a L- or D-amino acid is substituted adjacent to L-Leu, its orientation relative to this bulky Leu side-chain represents the most energetically favourable configuration. We believe that such energetically favourable, and different, configurations of L- and D-peptide diastereomers affect their respective interactions with a hydrophobic stationary phase, which are thus quantified by different RP-HPLC retention times. Side-chain hydrophilicity/hydrophobicity coefficients were generated in the presence of these nearest-neighbour effects and, despite the relative difference in such coefficients generated from peptides substituted with L- or D-amino acids, the relative difference in hydrophilicity/hydrophobicity between different amino acids in the L- or D-series is maintained. Overall, our results demonstrate that such nearest-neighbour effects can clearly restrict conformational space of an amino acid side-chain in a polypeptide chain.     

Length dependence of the coil <--> beta-sheet transition in a membrane environment

The most abundant structural element in protein aggregates is the beta-sheet. Designed peptides that fold into a beta-sheet structure upon binding to lipid membranes are useful models to elucidate the thermodynamic characteristics of the random coil <-->beta-structure transition. Here, we examine the effect of strand length on the random coil <--> beta-sheet transition of the (KIGAKI)n peptide with the total chain length varying between 7 and 30 amino acids. The beta-sheet content of the peptides in the presence and absence of membranes was measured with circular dichroism spectroscopy. The peptides were titrated with small unilamellar lipid vesicles, and the thermodynamic binding parameters were determined with isothermal titration calorimetry (ITC). Membrane binding includes at least two processes, namely (i) the transfer of the peptide from the aqueous phase to the lipid surface and (ii) the conformational change from a random coil conformation to a beta-sheet structure. CD spectroscopy and ITC analysis demonstrate that beta-sheet formation depends cooperatively on the peptide chain length with a distinct increase in beta-structure for n > 10-12. Binding to the lipid membrane is an entropy-driven process as the binding enthalpy is always endothermic. The contribution of the beta-sheet folding reaction to the overall process was determined with analogues of the KIGAKI repeat where two adjacent amino acids were replaced by their D-enantiomers. The folding reaction for peptides with n >or= 12 is characterized by a negative free folding energy of DeltaG(degree)beta approximately equal -0.15 kcal/mol per amino acid residue. The folding step proper is exothermic with DeltaH(degree)(beta) approximately equal -0.2 to -0.6 kcal/mol per residue and counteracted by a negative entropy term TDeltaS(degree)(beta) = -0.1 to -0.5 kcal/mol per residue, depending on the chain length (18 相似文献   

Isoxazole‐Derived Amino Acids are Bromodomain‐Binding Acetyl‐Lysine Mimics: Incorporation into Histone H4 Peptides and Histone H3

A range of isoxazole‐containing amino acids was synthesized that displaced acetyl‐lysine‐containing peptides from the BAZ2A, BRD4(1), and BRD9 bromodomains. Three of these amino acids were incorporated into a histone H4‐mimicking peptide and their affinity for BRD4(1) was assessed. Affinities of the isoxazole‐containing peptides are comparable to those of a hyperacetylated histone H4‐mimicking cognate peptide, and demonstrated a dependence on the position at which the unnatural residue was incorporated. An isoxazole‐based alkylating agent was developed to selectively alkylate cysteine residues in situ. Selective monoalkylation of a histone H4‐mimicking peptide, containing a lysine to cysteine residue substitution (K12C), resulted in acetyl‐lysine mimic incorporation, with high affinity for the BRD4 bromodomain. The same technology was used to alkylate a K18C mutant of histone H3.     

Oligomerization of uniquely folded mini-protein motifs: development of a homotrimeric betabetaalpha peptide.

The discovery of a discretely folded homotrimeric betabetaalpha motif (BBAT1) was recently reported (J. Am. Chem. Soc. 2001, 123, 1002-1003). Herein the design, synthesis, and analysis of a small library of peptides which led to the isolation of BBAT1 is described. betabetaalpha peptides based on the monomeric sequence of BBA5 (Folding Des. 1998, 120, 95-103) were synthesized to include the anthranilic acid/nitrotyrosine fluorescence quenching pair to rapidly detect interpeptide association. In the first generation of peptides synthesized, truncations in the loop region connecting the beta-hairpin to the alpha-helix revealed that a two-residue deletion in the loop promoted an interpeptide association as detected by fluorescence quenching. An additional library of 22 loop-truncated betabetaalpha peptides was subsequently synthesized to include a variety of sequence mutations in an effort to enhance the observed peptide-peptide binding. From the fluorescence quenching screen, peptide B2 was found to possess the strongest fluorescence-quenching response, indicative of a strong peptide-peptide association. Due the poor solubility of peptide B2, the S-methylated cysteine at position 9 in the loop was substituted with a glycine to generate peptide BBAT1 which possessed greatly improved water solubility and formed discrete trimers. The successful design of this oligomeric betabetaalpha structure will likely aid the design of more complex alpha-beta superstructures and further our understanding of the factors controlling protein-protein interactions at alpha-beta protein interfaces.     

Novel Antimicrobial Peptides from a Cecropin-Like Region of Heteroscorpine-1 from Heterometrus laoticus Venom with Membrane Disruption Activity

The increasing antimicrobial-resistant prevalence has become a severe health problem. It has led to the invention of a new antimicrobial agent such as antimicrobial peptides. Heteroscorpine-1 is an antimicrobial peptide that has the ability to kill many bacterial strains. It consists of 76 amino acid residues with a cecropin-like region in N-terminal and a defensin-like region in the C-terminal. The cecropin-like region from heteroscorpine-1 (CeHS-1) is similar to cecropin B, but it lost its glycine-proline hinge region. The bioinformatics prediction was used to help the designing of mutant peptides. The addition of glycine-proline hinge and positively charged amino acids, the deletion of negatively charged amino acids, and the optimization of the hydrophobicity of the peptide resulted in two mutant peptides, namely, CeHS-1 GP and CeHS-1 GPK. The new mutant peptide showed higher antimicrobial activity than the native peptide without increasing toxicity. The interaction of the peptides with the membrane showed that the peptides were capable of disrupting both the inner and outer bacterial cell membrane. Furthermore, the SEM analysis showed that the peptides created the pore in the bacterial cell membrane resulted in cell membrane disruption. In conclusion, the mutants of CeHS-1 had the potential to develop as novel antimicrobial peptides.     

Femtomolar Ln(III) affinity in peptide-based ligands containing unnatural chelating amino acids

The incorporation of unnatural chelating amino acids in short peptide sequences leads to lanthanide-binding peptides with a higher stability than sequences built exclusively from natural residues. In particular, the hexadentate peptide P(22), which incorporates two unnatural amino acids Ada(2) with aminodiacetate chelating arms, showed picomolar affinity for Tb(3+). To design peptides with higher denticity, expected to show higher affinity for Ln(3+), we synthesized the novel unnatural amino acid Ed3a(2) which carries an ethylenediamine triacetate side-chain and affords a pentadentate coordination site. The synthesis of the derivative Fmoc-Ed3a(2)(tBu)(3)-OH, with appropriate protecting groups for direct use in the solid phase peptide synthesis (Fmoc strategy), is described. The two high denticity peptides P(HD2) (Ac-Trp-Ed3a(2)-Pro-Gly-Ada(2)-Gly-NH(2)) and P(HD5) (Ac-Trp-Ada(2)-Pro-Gly-Ed3a(2)-Gly-NH(2)) led to octadentate Tb(3+) complexes with femtomolar stability in water. The position of the high denticity amino acid Ed3a(2) in the hexapeptide sequence appears to be critical for the control of the metal complex speciation. Whereas P(HD5) promotes the formation of polymetallic species in excess of Ln(3+), P(HD2) forms exclusively the mononuclear complex. The octadentate coordination of Tb(3+) by both P(HD) leads to total dehydration of the metal ion in the mononuclear complexes with long luminescence lifetimes (>2 ms). Hence, we demonstrated that unnatural amino acids carrying polyaminocarboxylate side-chains are interesting building blocks to design high affinity Ln-binding peptides. In particular the novel peptide P(HD2) forms a unique octadentate Tb(3+) complex with femtomolar stability in water and an improvement of the luminescence properties with respect to the trisaquo TbP(22) complex by a factor of 4.     

Helix formation in alpha,gamma- and beta,gamma-hybrid peptides: theoretical insights into mimicry of alpha- and beta-peptides

Alpha,gamma- and beta,gamma-hybrid peptides, which are composed of two different homologous amino acid constituents in alternate order, are suggested as novel classes of peptide foldamers. On the basis of a systematic conformational search employing the methods of ab initio MO theory, the possibilities for the formation of periodic secondary structures in these systems are described. The conformational analysis provides a great number of helix conformers widely differing in energy, which can be arranged into three groups: (i) helices with all hydrogen bonds formed in forward direction along the sequence, (ii) helices with all hydrogen bonds in backward direction, and (iii) helices with alternate hydrogen-bond directions (mixed or beta-helices). Most stable are representatives of beta-helices, but their stability decreases considerably in more polar environments in comparison to helix conformers from the other two classes. There is a great similarity between the overall topology of the most stable hybrid peptide helices and typical helices of peptides which are exclusively composed of a single type of homologous amino acids. Thus, the helices of the beta,gamma-hybrid peptides mimic perfectly those of the native alpha-peptides as, for instance, the well-known alpha-helix, whereas the most stable helix conformers of alpha,gamma-hybrid peptides correspond well to the overall structure of beta-peptide helices. The two suggested novel hybrid peptide classes expand considerably the pool of peptide foldamers and may be promising tools in peptide design and in material sciences.     

Homochirality and life

Before the emergence of life, left-handed amino acids (L-enantiomers) were selected and right-handed amino acids (D-enantiomers) were eliminated on the primal earth. Nevertheless, with the progress of analytical methods, D-amino acids have recently been found in higher order living organisms in the form of free amino acids, peptides, and proteins. Free D-amino acids have numerous physiological functions. D-amino acids containing animal peptides are well known as opioid peptides. D-amino acids in protein are related to aging. In this review, we describe the D-amino acids that are present and function as D-amino acid biosystems in our bodies.     

Sequencing Lys-N Proteolytic Peptides by ESI and MALDI Tandem Mass Spectrometry

In this study, we explored the MS/MS behavior of various synthetic peptides that possess a lysine residue at the N-terminal position. These peptides were designed to mimic peptides produced upon proteolysis by the Lys-N enzyme, a metalloendopeptidase issued from a Japanese fungus Grifola frondosa that was recently investigated in proteomic studies as an alternative to trypsin digestion, as a specific cleavage at the amide X-Lys chain is obtained that provides N-terminal lysine peptide fragments. In contrast to tryptic peptides exhibiting a lysine or arginine residue solely at the C-terminal position, and are thus devoid of such basic amino acids within the sequence, these Lys-N proteolytic peptides can contain the highly basic arginine residue anywhere within the peptide chain. The fragmentation patterns of such sequences with the ESI-QqTOF and MALDI-TOF/TOF mass spectrometers commonly used in proteomic bottom-up experiments were investigated.     

Different mechanisms of oxidative stress and neurotoxicity for Alzheimer's A beta(1--42) and A beta(25--35)

Oxidative stress induced by amyloid beta-peptide (A beta) has been implicated in the neurodegeneration observed in Alzheimer's disease (AD) brain. However, the mechanism by which the predominant form of A beta found in AD brains, A beta(1--42), causes oxidative stress and neurotoxicity remains unknown. Numerous laboratories have used the smaller 11-amino acid fragment of the full-length peptide, A beta(25--35), as a convenient alternative in AD investigations since the smaller peptide mimics several of the toxicological and oxidative stress properties of the native full-length peptide. Our observation that the truncated peptide is more rapidly toxic and causes more oxidative damage than the parent A beta(1--42) led us to investigate the cause for this enhanced toxicity of A beta(25--35) in order to gain insight into the mechanism of action of these peptides. These studies reveal that two different mechanisms may be operative in the two peptides; however, the single methionine residue in the peptides appears to play a crucial role in both mechanisms. That methionine is C-terminal in A beta(25--35) seems to be the cause for its exaggerated effects. When the next amino acid in the sequence of A beta(1--42) (valine) is appended to A beta(25--35), the resultant peptide, A beta(25--36), in which methionine is no longer C-terminal, is neither toxic to cultured neurons nor does it cause oxidative damage. Additionally, oxidizing the sulfur of methionine to a sulfoxide abrogates the damaging effects of both A beta(25--35) and A beta(1--42). The putative mechanistic role of methionine in the observed properties of A beta peptides is discussed in the context of the obtained results as is the role of A beta(1--42)-induced oxidative stress in the neurodegeneration found in AD brain.     

Synthetic collagen heterotrimers: structural mimics of wild-type and mutant collagen type I

Collagen type I is an AAB heterotrimer assembled from two alpha1 chains and one alpha2 chain. Missense mutations in either of these chains that substitute a glycine residue in the ubiquitous X-Y-Gly repeat with a bulky amino acid leads to osteogenesis imperfecta (OI) of varying severity. These mutations have been studied in the past using collagen-like peptide homotrimers as a model system. However, homotrimers, which by definition will contain glycine mutations in all the three chains, do not accurately mimic the mutations in their native form and result in an exaggerated effect on stability and folding. In this article, we report the design of a novel model system based upon collagen-like heterotrimers that can mimic the glycine mutations present in either the alpha1 or alpha2 chains of type I collagen. This design utilizes an electrostatic recognition motif in three chains that can force the interaction of any three peptides, including AAA (all same), AAB (two same and one different), or ABC (all different) triple helices. Therefore, the component peptides can be designed in such a way that glycine mutations are present in zero, one, two, or all three chains of the triple helix. With this design, we for the first time report collagen mutants containing one or two glycine substitutions with structures relevant to native forms of OI. Furthermore, we demonstrate the difference in thermal stability and refolding half-life times between triple helices that vary only in the frequency of glycine mutations at a particular position.     

Sterically hindered C(alpha, alpha)-disubstituted alpha-amino acids: synthesis from alpha-nitroacetate and incorporation into peptides

The preparation of sterically hindered and polyfunctional C(alpha,alpha)-disubstituted alpha-amino acids (alpha alpha AAs) via alkylation of ethyl nitroacetate and transformation into derivatives ready for incorporation into peptides are described. Treatment of ethyl nitroacetate with N,N-diisopropylethylamine (DIEA) in the presence of a catalytic amount of tetraalkylammonium salt, followed by the addition of an activated alkyl halide or Michael acceptor, gives the doubly C-alkylated product in good to excellent yields. Selective nitro reduction with Zn in acetic acid or hydrogen over Raney Ni gives the corresponding amino ester that, upon saponification, can be protected with the fluorenylmethyloxycarbonyl (Fmoc) group. The first synthesis of an orthogonally protected, tetrafunctional C(alpha,alpha)-disubstituted analogue of aspartic acid, 2,2-bis(tert-butylcarboxymethyl)glycine (Bcmg), is described. Also, the sterically demanding C(alpha,alpha)-dibenzylglycine (Dbg) has been incorporated into a peptide using solid-phase synthesis. It was found that once sterically congested Dbg is at the peptide N-terminus, further chain extension becomes very difficult using uronium or phosphonium salts (PyAOP, PyAOP/HOAt, HATU). However, preformed amino acid symmetrical anhydride couples to N-terminal Dbg in almost quantitative yield in nonpolar solvent (dichloroethane-DMF, 9:1).     

Complex-formation reactions of dicholoro(S-methyl-L-cysteine)palladium(II) with bio-relevant ligands. Labilization induced by S-donor chelates

The complex-formation equilibria of [Pd(SMC)(H2O)2]+, where SMC = S-methyl-l-cysteinate, with bio-relevant ligands such as amino acids, peptides, dicarboxylic acids and DNA constituents were studied and their formation constants were determined. The binding mode of the ligands containing various functional groups was studied and the speciation diagrams were evaluated. The kinetics of base hydrolysis of amino acid esters bound to [Pd(SMC)(H2O)2]+ was studied in aqueous solution at 25 degrees C and 0.1 M ionic strength. The effect of solvent polarity and temperature on the hydrolysis of coordinated glycine methyl ester was investigated. The activation parameters are evaluated and discussed.     

Amino Acid‐Specific,Ribonucleotide‐Promoted Peptide Formation in the Absence of Enzymes

Nucleic acids and polypeptides are at the heart of life. It is interesting to ask whether the monomers of these biopolymers possess intrinsic reactivity that favors oligomerization in the absence of enzymes. We have recently observed that covalently linked peptido RNA chains form when mixtures of monomers react in salt‐rich condensation buffer. Here, we report the results of a screen of the 20 proteinogenic amino acids and four ribonucleotides. None of the amino acids prevent phosphodiester formation, so all of them are compatible with genetic encoding through RNA chain growth. A reactivity landscape was found, in which peptide formation strongly depends on the structure of the amino acid, but less on the nucleobase. For example, proline gives ribonucleotide‐bound peptides most readily, tyrosine favors pyrophosphate and phosphodiester formation, and histidine gives phosphorimidazolides as dominant products. When proline and aspartic acid were allowed to compete for incorporation, only proline was found at the N‐terminus of peptido chains. The reactivity described here links two fundamental classes of biomolecules through reactions that occur without enzymes, but with amino acid specificity.     

Amino Acid‐Specific,Ribonucleotide‐Promoted Peptide Formation in the Absence of Enzymes

Nucleic acids and polypeptides are at the heart of life. It is interesting to ask whether the monomers of these biopolymers possess intrinsic reactivity that favors oligomerization in the absence of enzymes. We have recently observed that covalently linked peptido RNA chains form when mixtures of monomers react in salt‐rich condensation buffer. Here, we report the results of a screen of the 20 proteinogenic amino acids and four ribonucleotides. None of the amino acids prevent phosphodiester formation, so all of them are compatible with genetic encoding through RNA chain growth. A reactivity landscape was found, in which peptide formation strongly depends on the structure of the amino acid, but less on the nucleobase. For example, proline gives ribonucleotide‐bound peptides most readily, tyrosine favors pyrophosphate and phosphodiester formation, and histidine gives phosphorimidazolides as dominant products. When proline and aspartic acid were allowed to compete for incorporation, only proline was found at the N‐terminus of peptido chains. The reactivity described here links two fundamental classes of biomolecules through reactions that occur without enzymes, but with amino acid specificity.