The first N‐terminal unprotected (Gly‐Aib)n peptide: H‐Gly‐Aib‐Gly‐Aib‐OtBu

Glycine (Gly) is incorporated in roughly half of all known peptaibiotic (nonribosomally biosynthesized antibiotic peptides of fungal origin) sequences and is the residue with the greatest conformational flexibility. The conformational space of Aib (α‐aminoisobutyric acid) is severely restricted by the second methyl group attached to the C_α atom. Most of the crystal structures containing Aib are N‐terminal protected. Deprotection of the N‐ or C‐terminus of peptides may alter the hydrogen‐bonding scheme and/or the structure and may facilitate crystallization. The structure reported here for glycyl‐α‐aminoisobutyrylglycyl‐α‐aminoisobutyric acid tert‐butyl ester, C₁₆H₃₀N₄O₅, describes the first N‐terminal‐unprotected (Gly‐Aib)_n peptide. The achiral peptide could form an intramolecular hydrogen bond between the C=O group of Gly1 and the N—H group of Aib4. This hydrogen bond is found in all tetrapeptides and N‐terminal‐protected tripeptides containing Aib, apart from one exception. In the present work, this hydrogen bond is not observed (N...O = 5.88 Å). Instead, every molecule is hydrogen bonded to six other symmetry‐related molecules with a total of eight hydrogen bonds per molecule. The backbone conformation starts in the right‐handed helical region (and the left‐handed helical region for the inverted molecule) and reverses the screw sense in the last two residues.     

Foldamer‐Mediated Remote Stereocontrol: >1,60 Asymmetric Induction

An N‐terminal L ‐α‐methylvaline dimer induces complete conformational control over the screw sense of an otherwise achiral helical peptide foldamer formed from the achiral quaternary amino acids Aib and Ac₆c. The persistent right‐handed screw‐sense preference of the helix enables remote reactive sites to fall under the influence of the terminal chiral residues, and permits diastereoselective reactions such as alkene hydrogenation or iminium ion addition to take place with 1,16‐, 1,31‐, 1,46‐ and even 1,61‐asymmetric induction. Stereochemical information may be communicated in this way over distances of up to 4 nm.     

Ambidextrous α,γ‐Hybrid Peptide Foldamers

Molecular chirality is ubiquitous in nature. The natural biopolymers, proteins and DNA, preferred a right‐handed helical bias due to the inherent stereochemistry of the monomer building blocks. Here, we are reporting a rare co‐existence of left‐ and right‐handed helical conformations and helix‐terminating property at the C‐terminus within a single molecule of α,γ‐hybrid peptide foldamers composed of achiral Aib (α‐aminoisobutyric acid) and 3,3‐dimethyl‐substituted γ‐amino acid (Adb; 4‐amino‐3,3‐dimethylbutanoic acid). At the molecular level, the left‐ and right‐handed helical screw sense of α,γ‐hybrid peptides are representing a macroscopic tendril perversion. The pronounced helix‐terminating behaviour of C‐terminal Adb residues was further explored to design helix–Schellman loop mimetics and to study their conformations in solution and single crystals. The stereochemical constraints of dialkyl substitutions on γ‐amino acids showed a marked impact on the folding behaviour of α,γ‐hybrid peptides.     

聚肽分子链构象转变研究

聚肽分子链在一定的条件下可发生右旋分子链构象与左旋分子链构象间的构象转变 .文中提出了一个有关这一现象的统计理论解释 ,理论包含了两个重要的参数γ、ρ .γ与右旋和左旋分子链构象间能量差别有关 ;ρ与在左旋分子链段中启始一个右旋构象的结构单元所需能量有关 .进一步考虑了体系中酸分子存在对构象转变的影响 ,以解释新近发现的聚肽左、右旋多重分子链构象转变现象 .此外 ,运用核磁共振 (NMR)手段还对这一现象做了进一步的研究 .通过与实验结果和文献中报告的数据比较 ,证明这一理论可以很好地解释聚肽分子链中发生的左、右旋分子链构象转变     

The peptide Z‐Aib‐Aib‐Aib‐L‐Ala‐OtBu

The title peptide, N‐benzyloxycarbonyl‐α‐aminoisobutyryl‐α‐aminoisobutyryl‐α‐aminoisobutyryl‐L‐alanine tert‐butyl ester or Z‐Aib‐Aib‐Aib‐L‐Ala‐OtBu (Aib is α‐aminoisobutyric acid, Z is benzyloxycarbonyl and OtBu indicates the tert‐butyl ester), C₂₇H₄₂N₄O₇, is a left‐handed helix with a right‐handed conformation in the fourth residue, which is the only chiral residue. There are two 4→1 intramolecular hydrogen bonds in the structure. In the lattice, molecules are hydrogen bonded to form columns along the c axis.     

“Helix‐in‐Helix” Superstructure Formation through Encapsulation of Fullerene‐Bound Helical Peptides within a Helical Poly(methyl methacrylate) Cavity

A one‐handed 3₁₀‐helical hexapeptide is efficiently encapsulated within the helical cavity of st‐PMMA when a fullerene (C₆₀) derivative is introduced at the C‐terminal end of the peptide. The encapsulation is accompanied by induction of a preferred‐handed helical conformation in the st‐PMMA backbone with the same‐handedness as that of the hexapeptide to form a crystalline st‐PMMA/peptide‐C₆₀ inclusion complex with a unique optically active helix‐in‐helix structure. Although the st‐PMMA is unable to encapsulate the 3₁₀‐helical peptide without the terminal C₆₀ unit, the helical hollow space of the st‐PMMA is almost filled by the C₆₀‐bound peptides. This result suggests that the C₆₀ moiety can serve as a versatile molecular carrier of specific molecules and polymers in the helical cavity of the st‐PMMA for the formation of an inclusion complex, thus producing unique supramolecular soft materials that cannot be prepared by other methods.     

Synthesis of Z‐Protected Aib‐ and Phe(2Me)‐Containing Pentapeptides and Their Crystal Structures

A series of pentapeptide derivatives containing α,α‐disubstituted α‐amino acids have been prepared by a combination of the ‘azirine/oxazolone method’ and segment condensations. X‐Ray crystal‐structure determinations of the molecular structures confirmed the presence of helical conformations stabilized by β‐turns of type III or III′. Pentapeptides containing (R)‐Phe(2Me) form a right‐handed helix, whereas those containing (S)‐Phe(2Me) adopt a left‐handed helical structure.     

The Propensity of α‐Aminoisobutyric Acid (=2‐Methylalanine; Aib) to Induce Helical Secondary Structure in an α‐Heptapeptide: A Computational Study

We present a molecular‐dynamics simulation study of an α‐heptapeptide containing an α‐aminoisobutyric acid (=2‐methylalanine; Aib) residue, Val¹‐Ala²‐Leu³‐Aib⁴‐Ile⁵‐Met⁶‐Phe⁷, and a quantum‐mechanical (QM) study of simplified models to investigate the propensity of the Aib residue to induce 3₁₀/α‐helical conformation. For comparison, we have also performed simulations of three analogues of the peptide with the Aib residue being replaced by L ‐Ala, D ‐Ala, and Gly, respectively, which provide information on the subtitution effect at C(α) (two Me groups for Aib, one for L ‐Ala and D ‐Ala, and zero for Gly). Our simulations suggest that, in MeOH, the heptapeptide hardly folds into canonical helical conformations, but appears to populate multiple conformations, i.e., C₇ and 3₁₀‐helical ones, which is in agreement with results from the QM calculations and NMR experiments. The populations of these conformations depend on the polarity of the solvent. Our study confirms that a short peptide, though with the presence of an Aib residue in the middle of the chain, does not have to fold to an α‐helical secondary structure. To generate a helical conformation for a linear peptide, several Aib residues should be present in the peptide, either sequentially or alternatively, to enhance the propensity of Aib‐containing peptides towards the helical conformation. A correction of a few of the published NMR data is reported.     

Crystal and NMR Structures of a Peptidomimetic β‐Turn That Provides Facile Synthesis of 13‐Membered Cyclic Tetrapeptides

Herein we report the unique conformations adopted by linear and cyclic tetrapeptides (CTPs) containing 2‐aminobenzoic acid (2‐Abz) in solution and as single crystals. The crystal structure of the linear tetrapeptide H₂N‐d ‐Leu‐d ‐Phe‐2‐Abz‐d ‐Ala‐COOH ( 1 ) reveals a novel planar peptidomimetic β‐turn stabilized by three hydrogen bonds and is in agreement with its NMR structure in solution. While CTPs are often synthetically inaccessible or cyclize in poor yield, both 1 and its N ‐Me‐d ‐Phe analogue ( 2 ) adopt pseudo‐cyclic frameworks enabling near quantitative conversion to the corresponding CTPs 3 and 4 . The crystal structure of the N ‐methylated peptide ( 4 ) is the first reported for a CTP containing 2‐Abz and reveals a distinctly planar 13‐membered ring, which is also evident in solution. The N ‐methylation of d ‐Phe results in a peptide bond inversion compared to the conformation of 3 in solution.     

Conformational Study of Heteropentapeptides Containing an α‐Ethylated α,α‐Disubstituted Amino Acid: (S)‐Butylethylglycine (=2‐Amino‐2‐ethylhexanoic Acid) within a Sequence of Dimethylglycine (=2‐Aminoisobutyric Acid) Residues

Heteropentapeptides containing the α‐ethylated α,α‐disubstituted amino acid (S)‐butylethylglycine and four dimethylglycine residues, i.e., CF₃CO‐[(S)‐Beg]‐(Aib)₄‐OEt ( 4 ) and CF₃CO‐(Aib)₂‐[(S)‐Beg]‐(Aib)₂‐OEt ( 7 ), were synthesized by conventional solution methods. In the solid state, the preferred conformation of 4 was shown to be both a right‐handed (P) and a left‐handed (M) 3₁₀‐helical structure, and that of 7 was a right‐handed (P) 3₁₀‐helical structure. IR, CD, and ¹H‐NMR spectra revealed that the dominant conformation of both 4 and 7 in solution was the 3₁₀‐helical structure. These conformations were also supported by molecular‐mechanics calculations.     

N‐(tert‐Butoxycarbonyl)‐α‐aminoisobutyryl‐α‐aminoisobutyric acid methyl ester: two polymorphic forms in the space group P21/n

The title compound (systematic name: methyl 2‐{2‐[(tert‐butoxycarbonyl)amino]‐2‐methylpropanamido}‐2‐methylpropanoate), C₁₄H₂₆N₂O₅, (I), crystallizes in the monoclinic space group P2₁/n in two polymorphic forms, each with one molecule in the asymmetric unit. The molecular conformation is essentially the same in both polymorphs, with the α‐aminoisobutyric acid (Aib) residues adopting ϕ and ψ values characteristic of α‐helical and mixed 3₁₀‐ and α‐helical conformations. The helical handedness of the C‐terminal residue (Aib2) is opposite to that of the N‐terminal residue (Aib1). In contrast to (I), the closely related peptide Boc‐Aib‐Aib‐OBn (Boc is tert‐butoxycarbonyl and Bn is benzyl) adopts an α_L‐P_II backbone conformation (or the mirror image conformation). Compound (I) forms hydrogen‐bonded parallel β‐sheet‐like tapes, with the carbonyl groups of Aib1 and Aib2 acting as hydrogen‐bond acceptors. This seems to represent an unusual packing for a protected dipeptide containing at least one α,α‐disubstituted residue.     

Conformational structure of peptides containing dehydroalanine: Formation of β‐bend ribbon structure

α,β‐Unsaturated amino acids (dehydroamino acids) have been found in naturally occurring antibiotics of microbial origin and in some proteins. Due to the presence of the C_αC_β double bond, the dehydroamino acids influence the main‐chain and the side‐chain conformations. The lowest‐energy conformational state of the model tripeptides, Ac–X–ΔAla–NHMe, (X=Ala, Val, Leu, Abu, or Phe) corresponds to ϕ₁=−30°, ψ₁=120° and ϕ₂=ψ₂=30°. This structure is stabilized by the hydrogen bond between CO of the acetyl group and the NH of the amide group, resulting in the formation of a 10‐membered ring. In the model heptapeptide containing ΔAla at alternate position with Ala, Abu, and Leu, the lowest‐energy conformation corresponds to ϕ=−30° and ψ=120° for all the Ala, Abu, and Leu residues and ϕ=ψ=30° for all ΔAla residues. A graphical view of the molecule in this conformation reveals the formation of three hydrogen bonds involving the CO moiety of the ith residue and the NH moiety of the i+3th residue, resulting in a 10‐membered ring formation. In this structure, only alternate peptide bonds are involved in the intramolecular hydrogen‐bond formation unlike the helices and it has been named the β‐bend ribbon structure. The helical structures were predicted to be the most stable structures in the heptapeptide Ac–(Aib–ΔAla)₃–NHMe with ϕ=±30°, ψ=±60° for Aib residues and ϕ=ψ=±30° for ΔAla residues. The computational results reveal that the ΔAla residue does not induce an inverse γ‐turn in the preceding residue. It is the competitive interaction of small solvent molecules with the hydrogen‐bonding sites of the peptide which gives rise to the formation of an inverse γ‐turn (ϕ₁=−54°, ψ₁=82°; ϕ₂=44°, ψ₂=3°) in the preceding residue to ΔAla. The computational studies for the positional preference of ΔAla in the peptide containing one ΔAla and nine Ala residues reveals the formation of a 3₁₀ helical structure in all the cases with the terminal preferences for ΔAla, consistent with the position of ΔAla in the natural antibiotics. The extended structures is found to be the most stable for poly‐ΔAla. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 15–23, 1999     

One‐Handed Single Helicates of Dinickel(II) Benzenehexapyrrole‐α,ω‐diimine with an Amine Chiral Source

Benzenehexapyrrole‐α,ω‐dialdehyde, composed of a pair of formyltripyrrole units with a 1,3‐phenylene linker, was metallated to give dinuclear single‐stranded helicates. X‐ray studies of the bis‐nickel(II) complex showed a helical C₂ form with a pair of helical–metal coordination planes of a 3N+O donor set. The terminal aldehyde was readily converted into the imine by optically active amines, whereby helix‐sense bias was induced. Bis‐nickel(II) and bis‐palladium(II) complexes of the benzenehexapyrrole‐α,ω‐diimines were studied to show that an enantiomer pair of the helical C₂ form are interchanged by slow flipping of each coordination plane and fast rotation around the C(benzene)?C(pyrrole) bond. The helical screw in the bis‐nickel(II) complexes was biased to one side in more than 95 % diastereoselectivity, which was achieved by using a variety of optically active amines, such as (R)‐1‐cyclohexylethylamine, (S)‐1‐ phenylethylamine, L ‐Phe(OEt) (Phe=phenylalanine), and (R)‐valinol. The nickel complexes showed much better diastereoselectivity than the corresponding palladium complexes.     

Remote Control of the Planar Chirality in Peptide‐Bound Metallomacrocycles and Dynamic‐to‐Static Planar Chirality Control Triggered by Solvent‐Induced 310‐to‐α‐Helix Transitions

The dynamic planar chirality in a peptide‐bound Ni^II‐salphen‐based macrocycle can be remotely controlled. First, a right‐handed (P)‐3₁₀‐helix is induced in the dynamic helical oligopeptides by a chiral amino acid residue far from the macrocyclic framework. The induced planar chirality remains dynamic in chloroform and acetonitrile, but is almost completely locked in fluoroalcohols as a result of the solvent‐induced transition of the peptide chains from a 3₁₀‐helix to a wider α‐helix, which freezes the rotation of the pendant peptide units around the macrocycle.     

Trismaleimide Dendrimers: Helix‐to‐Superhelix Supramolecular Transition Accompanied by White‐Light Emission

Reported here are unprecedented fluorescent superhelices composed of primary, supramolecular polymers of the opposite helical twist. A new class of functional dendrimers was synthesized by amino‐ene click reactions, and they demonstrate an alternating OFF/ON fluorescence with generation growth. A peripherally alkyl‐modified dendrimer displays helix‐sense‐selective supramolecular polymerization, which predominantly forms right‐handed (or left‐handed) helical supramolecular polymers in the solution containing chiral solvents. With increasing the concentration, these primary helical supramolecular polymers spontaneously twist around themselves in the opposite direction to form superhelical structures. Atomic force microscopy and circular dichroism measurements were used to directly observe the helix‐to‐superhelix transition occurring with a reversal in the helical direction. Exceptional white‐light emission was observed during superhelix formation.     

Controlling the Helix Handedness of ααβ‐Peptide Foldamers through Sequence Shifting

Peptide foldamers containing both cis ‐β‐aminocyclopentanecarboxylic acid and α‐amino acid residues combined in various sequence patterns (ααβ, αααβ, αβααβ, and ααβαααβ) were screened using CD and NMR spectroscopy for the tendency to form helices. ααβ‐Peptides were found to fold into an unprecedented and well‐defined 16/17/15/18/14/17‐helix. By extending the length of the sequence or shifting a fragment of the sequence from one terminus to another in ααβ‐peptides, the balance between left‐handed and right‐handed helix populations present in the solution can be controlled. Engineering of the peptide sequence could lead to compounds with either a strong propensity for the selected helix sense or a mixture of helical conformations of opposite senses.     

A microscopic view of a helical poly(γ‐peptide): Molecular dynamics simulations of a 20‐residue un‐ionized poly(γ‐D‐glutamic acid) in water

We present a molecular dynamics study of the helical conformation of the naturally occurring poly(γ‐D ‐glutamic acid) in the un‐ionized state. The study was conducted in both aqueous solution and gas‐phase considering a 20 residue polypeptide. The results indicated that the left‐handed helix with 19‐membered ring hydrogen bonds set between the CO of the amide group i and the NH of amide group i + 3 is very stable in aqueous solution. This conformation was recently proposed for this poly(γ‐amino acid) from a conformational search study. A detailed picture of the most relevant structural details of the helical conformation of poly(γ‐D ‐glutamic acid) is provided.     

A Chirally Stable,Atropoisomeric, Cα‐Tetrasubstituted α‐Amino Acid: Incorporation into Model Peptides and Conformational Preference

A variety of model peptides, including four complete homologous series, to the pentamer level, characterized by the recently proposed binaphthyl‐based, axially chiral, C^α‐tetrasubstituted, cyclic α‐amino acid Bin, in combination with Ala, Gly, or Aib residues, was synthesized by solution methods and fully characterized. The solution conformational propensity of these peptides was determined by FT‐IR absorption and ¹H‐NMR techniques. Moreover, the molecular structures of the free amino acid (S)‐enantiomer and an N^α‐acylated dipeptide alkylamide with the heterochiral sequence ‐(R)‐Bin‐Phe‐ were assessed in the crystal state by X‐ray diffraction. Taken together, the results point to the conclusion that β‐bends and 3₁₀ helices are preferentially adopted by Bin‐containing peptides, although the fully extended conformation would also be adopted in solution by the short oligomers to some extent. We also confirmed the tendency of (R)‐Bin to fold a peptide chain into right‐handed bend and helical structures. The absolute configuration of the Bin residue(s) was correlated with the typically intense exciton‐split Cotton effect of the ¹B_b binaphthyl transition near 225 nm.     

Helical Structures of Bicyclic α‐Amino Acid Homochiral Oligomers with the Stereogenic Centers at the Side‐Chain Fused‐Ring Junctions

Chiral bicyclic α‐amino acid (R,R)‐Ab_5,6=c with stereogenic centers at the γ‐position of fused‐ring junctions, and its enantiomer (S,S)‐Ab_5,6=c, were synthesized. The CD spectra of (R,R)‐Ab_5,6=c oligomers indicated that the (R,R)‐Ab_5,6=c hexapeptide formed a mixture of right‐handed (P)‐ and left‐handed (M)‐3₁₀‐helices, while, in the (R,R)‐Ab_5,6=c nonapeptide, a right‐handed (P)‐3₁₀‐helix slightly dominated over the (M)‐helix. X‐Ray crystallographic analyses of (S,S)‐tripeptide and (R,R)‐hexapeptide revealed that both the tripeptide and hexapeptide formed a mixture of (P)‐ and (M)‐3₁₀‐helices, respectively. These results indicated that the side‐chain environments around the stereogenic centers are particularly important to control the helical‐screw handedness of foldamers.     

The amide-aromatic-ring system : An inherently dissymmetric chromophore

Two cyclic peptide like compounds, cyclo-anthranoyl-L- prolyl (1) and cyclo-homoanthranoyl-L-prolyl (2) have been synthesized and investigated by UV spectroscopy and measurement of circular dichroism. Compound 1 of entirely rigid conformation with two N-CO groups conjugated to the aromatic ring, Ar-NH-CO left handed helical, Ar-CO-N right handed helical, shows a very strong positive Cotton effect centered around 250 nm and a strong negative one centered around 227 nm. Compound 2, which has only one conjugated skewed electron system, Ar-NH-CO, can exist in two different stable conformations, one of them being left handed, the other one right handed helical. It also shows a strong positive Cotton effect centered around 235 nm. Since from earlier ¹³C NMR data the latter was concluded to be the preferred conformation, a right handed helical sense of the amide bond-aromatic ring system is likely to correlate with a positive Cotton effect of this inherently dissymmetric chromophore.