Structural versatility of peptides from Cα,αdialkylated glycines: Linear Ac3c homo-oligopeptides

The crystal-state molecular structures of five linear Ac₃c homo-oligopeptides to the tetramer were determined by x-ray diffraction. The oligomers are H-(Ac₃c)₂-OMe, Fmoc-(Ac₃c)₂-OMe MeOH, Ac-(Ac₃c)₂-OMe, pBrBz-(Ac₃c)₃-OMe · H₂O, and t-Boc-(Ac₃c)₄-OMe · 2H₂O. The results indicate the propensity of the tri- and tetrapeptides to fold into type I β-bends and distorted 3₁₀-helices, respectively, in partial contrast to Aib, Ac₅c, and Ac₆c homo-peptides of comparable main-chain length, where regular type III β-bends and 3₁₀-helical structures were found. When the influence of the constraints produced by the intramolecular H bonds of the C₁₀-type is absent, other less common structural features may be observed. The average geometry of the cyclopropyl group of the Ac₃c residue is found to be asymmetric and the N? C^α? C′ bond angle significantly expanded from the regular tetrahedral value.     

Structural versatility of peptides from Cα,α-disubstituted glycines: Preferred conformation of the chiral isovaline residue

The molecular structures of four protected isovaline- (Iva-) containing peptides to the pentamer level have been determined by x-ray diffraction. The peptides are t-Boc-Ala-(S)-Iva-Ala-OMe (t-Boc : tert-butyloxycarbonyl; OMe : methoxy) and its (R)-Iva diastereomer, and t-Boc-[Ala-(R)-Iva]₂-Ala-OH and its (S)-Iva diastereomeric methyl ester analogue. The two tripeptides are folded in an open type II β-bend conformation. The fully developed right-handed 3₁₀-helix formed by the (R)-Iva pentapeptide, which includes an unusual intramolecular (acid) O? H ?O?C(peptide) H bond, is partially unfolded (near the C-terminus) in the (S) -Iva pentapeptide. ¹H-nmr and Fourier transform ir absorption studies suggest that in CDCl₃ solution (a) the two tripeptides maintain a type II β-bend conformation of comparable stability and (b) both diastereomeric pentapeptide sequences adopt a fully developed 3₁₀-helix. A comparison with the preferred conformation of other extensively investigated C^α,α-disubstituted glycines is made and the implications for the use of the Iva residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.     

Structural versatility of peptides from Cα, α-disubstituted glycines: Crystal-state conformational analysis of homopeptides from Cα-methyl,Cα-benzylglycine [(αMe)Phe]n

The molecular and crystal structures of one derivative and three homopeptides (from the di-to the tetrapeptide level) of the chiral, C^{α, α}-disubstituted glycine C^α-methyl, C^α-benzylglycine [(αMe)Phe], have been determined by x-ray diffraction. The derivative is mClAc-D -(αMe)Phe-OH, and the peptides are pBrBz-[D -(αMe)Phe]₂-NHMe, pBrBz-[D -(αMe)Phe]₃-OH hemihydrate, and pBrBz-[D -(αMe)Phe]₄-OtBu sesquihydrate. All (αMe)Phe residues prefer ?,ψ torsion angles in the helical region of the conformational map. The dipeptide methylamide and the tripeptide carboxylic acid adopt a β-turn conformation with a 1 ← 4 C?O…?H? N intramolecular H bond. The structure of the tripeptide carboxylic acid is further stabilized by a 1 ← 4 C?O…?H? O intramolecular H bond, forming an “oxy-analogue” of a β-turn. The tetrapeptide ester is folded in a regular (incipient) 3₁₀-helix. In general, the relationship between (αMe)Phe chirality and helix screw sense is opposite to that exhibited by protein amino acids. A comparison is made with the conclusions extracted from published work on homopeptides from other C^α-methylated α-amino acids. © 1993 John Wiley & Sons, Inc.     

Conformational behavior of α,α-dialkylated peptides

The preferred conformations of N-acetyl-N′-methyl amides of some dialkylglycines have been determined by empirical conformational-energy calculations; minimum-energy conformations were located by minimizing the energy with respect to all the dihedral angles of the molecules. The conformational space of these compounds is sterically restricted, and low-energy conformations are found only in the regions of fully extended and helical structures. Increasing the bulkiness of the substituents on the C^α, the fully extended conformation becomes gradually more stable than the helical structure preferred in the cases of dimethylglycine. This trend is, however, strongly dependent on the bond angles between the substituents on the C^α atom: In particular, helical structures are favored by standard values (111°) of the N-C^α-C′ angle, while fully extended conformations are favored by smaller values of the same angle, as experimentally observed, for instance, in the case of α,α-di-n-propylglycine.     

Relationship between conformation and geometry as evidenced by molecular dynamics simulation of Cα,α-dialkylated glycines

The relationship between the local backbone conformation and bond angles at C^α of symmetrically substituted C^α,α-dialkylated glycines (C^α,α-dimethylglycine or α-aminoisobutyric acid, Aib; C^α,α-diethylglycine, Deg; C^α,α-di-n-propylglycine, Dpg) has been investigated by molecular dynamics (MD) simulation adopting flat bottom harmonic potentials, instead of the usual harmonic restraints, for the C^α bond angles. The MD simulations show that the C^α bond angles are related to the local backbone conformation, irrespectively of the side-chain length of Aib, Deg, and Dpg residues. Moreover, the N-C^α-C′ (τ) angle is the most sensitive conformational parameter and, in the folded form, is always larger and more flexible than in the extended one. © 1998 John Wiley & Sons, Inc. Biopoly 46: 239–244, 1998     

Preferred conformation of peptides from Cα,α-symmetrically disubstituted glycines: Aromatic residues

The conformational preference of C^α,α-diphenylglycinc (Døg) and C^α,α-dibenzylglycine (Dbz) residues was assessed in selected derivatives and small peptides by conformational energy computations, ir absorption, ¹H-nmr, and x-ray diffraction. Conformational energy computations on the two monopeptides strongly support the view that these C^α,α-symmetrically disubstituted glycines are conformationally restricted and that their minimum energy conformation falls in the fully extended (C₅) region. The results of the theoretical analyses appear to be in agreement with the solution and crystal-state structural propensities of three derivatives and seven di-and tripeptides.     

Conformational behavior of α,α-dialkylated peptides: Ab initio and empirical results for cyclopropylglycine

The preferred conformations of model cyclopropylglycine peptides have been investigated by means of ab initio and empirical methods. Empirical computations performed with fixed bond lengths and valence angles using two well-known force fields show that only values of ? in the ranges ±70° ± 20° are sterically allowed, and that the C₇-conformation corresponds to the absolute energy minimum irrespective of the terminal groups used. Also, ab initio computations give similar results, but suggest greater stabilities for bridge and, especially, extended structures. These discrepancies can be removed, adding to the empirical force field a twofold torsional potential on ψ and using softer steric repulsive potentials. Complete geometry optimization using both ab initio and empirical methods does not affect the relative stabilities of folded conformations, but leads to a further significant stabilization of the fully extended structure via large modifications of some valence angles.     

Contrasting solution conformations of peptides containing α,α-dialkylated residues with linear and cyclic side chains

The conformational properties of α,α-dialkylated amino acid residues possessing acyclic (diethylglycine, Deg: di-n-propylglycine, Dpg; di-n-butylglycine, Dbg) and cyclic (1-amino-cycloalkane-1-carboxylic acid, Ac_nc) side chains have been compared in solution. The five peptides studied by nmr and CD spectroscopy are Boc-Ala-Xxx-Ala-OMe, where Xxx = Deg(I). Dpg (II), Dbg (III), Ac₆c (IV), and Ac₇c (V). Delineation of solvent-shielded NH groups have been achieved by solvent and temperature dependence of NH chemical shifts in CDCl₃ and (CD₃)₂SO and by paramagnetic radical induced line broadening in pepiide III. In the Dxg peptides the order of solvent exposure of NH groups is Ala(1) > Ala(3) > Dxg(2), whereas in the Ac_nc peptides the order of solvent exposure of NH groups is Ala(1) > Ac_nc(2) > Ala(3). The nmr results suggest that Ac_nc peptides adopt folded β-turn conformations with Ala(1) and Ac_nc(2) occupying i + 1 and i + 2 positions. In contrast, the Dxg peptides favor extended C₅ conformations. The conformational differences in the two series are clearly borne out in CD studies. The solution conformations of peptides I-III are distinctly different from the β-turn structure observed in crystals. Low temperature nmr spectra recorded immediately after dissolution of crystals of peptide II provide evidence for a structural transition. Introduction of an additional hydrogen-bonding function in Boc-Ala-Dpg-Ala-NHMe (VI) results in a stabilization of a consecutive β-turn or incipient 3₁₀-helix in solution. © 1995 John Wiley & Sons, Inc.     

Structural versatility of peptides containing C-dialkylated glycines. An X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac₃c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac₃c)₂-OMe·CH₃OH, Ac-(Ac₃c)₂-OMe, t-Boc-Ac₃c-l-Phe-OMe, pBrBz-(Ac₃c)₃-OMe·H₂O, Z-Gly-Ac₃c-Gly-OTmb·(CH₃₂CO, andt-Boc-(Ac₃c)₄-OMe·2H₂O. Type-I (I′) β-bends and distorted 3₁₀-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form β-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac₃c residue is asymmetric and the N-C^α-C′ bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C^α,α-dialkylated α-amino acids is made and the implications for the use of the Ac₃c residue in conformational design are examined.     

β-Turn conformations in crystal structures of model peptides containing α,α-Di-n-propylglycine and α,α-Di-n-butylglycine

The crystal state conformations of three peptides containing the α,α-dialkylated residues. α,α-di-n-propylglycine (Dpg) and α,α-di-n-butylglycine (Dbg), have been established by x-ray diffraction. Boc-Ala-Dpg-Alu-OMe (I) and Boc-Ala-Dbg-Ala-OMe (III) adopt distorted type II β-turn conformations with Ala (1) and Dpg/Dbg (2) as the corner residues. In both peptides the conformational angles at the Dxg residue (I: ? = 66.2°, ψ = 19.3°; III: ? = 66.5°. ψ = 21.1°) deviate appreciably from ideal values for the i + 2 residue in a type II β-turn. In both peptides the observed (N…O) distances between the Boc CO and Ala (3) NH groups are far too long (1: 3.44 Å: III: 3.63 Å) for an intramolecular 4 → 1 hydrogen bond. Boc-Ala-Dpg-Ata-NHMe (II) crystallizes with two independent molecules in the asymmetric unit. Both molecules HA and HB adopt consecutive β-turn (type III-III in HA and type III-I in IIB) or incipient 3₁₀-helical structures, stabilized by two intramolecular 4 → 1 hydrogen bonds. In all four molecules the bond angle N-C^α-C′ (τ) at the Dxg residues are ≥ 110°. The observation of conformational angles in the helical region of ?,ψ space at these residues is consistent with theoretical predictions. © 1995 John Wiley & Sons, Inc.     

Synthesis and conformational studies of peptides containing TOAC,a spin-labelled Cα,α-disubstituted glycine

A variety of host L -alanine homo-peptides (to the pentamer) containing one or two spin-labelled TOAC (2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid) residues were synthesized by solution methods and fully characterized. The conformational features of the terminally blocked, doubly spin-labelled–TOAC–(Ala)₂–TOAC–Ala– pentapeptide were examined in the crystal state by X-ray diffraction and in solution using a combination of techniques (Fourier transform infrared, circular dichroism, cyclic voltammetry and electron spin resonance) in comparison with singly labelled shorter peptides. The 3₁₀-helical structure of the pentapeptide, promoted by the two C^α,α-disubstituted glycines under favourable experimental conditions, allows an interaction to take place between the two nitroxide TOAC side chains spaced by one turn of the helix. Taken together, these results suggest that TOAC is an excellent probe for exploring bends and helices in doubly labelled peptides.     

Conformational characterization of peptides rich in the cycloaliphatic Cα,α-disubstituted glycine 1-amino-cyclononane-1-carboxylic acid

A series of N- and C-protected, monodispersed homo-oligopeptides (to the pentamer level) from the cycloaliphatic C^α,α,-dialkylated glycine 1-aminocyclononane-1-carboxylic acid (Ac₉c) and two Ala/Ac₉c tripeptides have been synthesized by solution methods and fully characterized. The conformational preferences of all the model peptides were determined in deuterochloroform solution by FT-IR absorption and ¹H-NMR. The molecular structures of the amino acid derivatives mClAc-Ac₉c-OH and Z-Ac₉c-OtBu, the dipeptide pBrBz-(Ac₉c)₂-OtBu, the tetrapeptide Z-(Ac₉c)₄-OtBu, and the pentapeptide Z-( Ac₉c)₅-OtBu were determined in the crystal state by X-ray diffraction. Based on this information, the average geometry and the preferred conformation for the cyclononyl moiety of the Ac₉c residue have been assessed. The backbone conformational data are strongly in favour of the conclusion that the Ac₉c residue is a strong β-turn and helix former. A comparison with the structural propensity of α-aminoisobutyric acid, the prototype of C^α,α-dialkylated glycines, and the other extensively investigated members of the family of 1-aminocycloalkane-1-carboxylic acids (Ac_nc, with n=3−8) is made and the implications for the use of the Ac₉c residue in conformationally constrained analogues of bioactive peptides are briefly examined. © 1997 European Peptide Society and John Wiley & Sons, Ltd. J. Pep. Sci. 3: 367–382 No. of Figures: 10. No. of Tables: 6. No. of References: 62     

Design and synthesis of novel nonpolar host peptides for the determination of the 310- and α-helix compatibilities of α-amino acid buildig blocks: An assessment of α,α-disubstituted glycines

The present work describes three novel nonpolar host peptide sequences that provide a ready assessment of the 3₁₀- and α-helix compatibilities of natural and unnatural amino acids at different positions of small- to medium-size peptides. The unpolar peptides containing Ala, Aib, and a C-terminal p-iodoanilide group were designed in such a way that the peptides could be rapidly assembled in a modular fashion, were highly soluble in solvent mixtures of triflouroethanol and H₂O for CD- and two-dimensional (2D) nmr spectroscopic analyses, and showed excellent crystallinity suited for x-ray structure analysis. To validate our approach we synthesized 9-mer peptides 79a–96 (Table IV), 12-mer peptides 99–110c (Table V), and 10-mer peptides 120a–125d and 129–133 (Table VI and Scheme 8) incorporating a series of optically pure cyclic and open-chain (R)- and (S)-α,α-disubstituted glycines 1–10 (Figure 2). These amino acids are known to significantly modulate the conformations of small peptides. Based on x-ray structures of 9-mers 79a, 80, and 87 (Figures 4–7), 10-mers 124c, 131, and 132 (Figures 9–12), and 12-mer peptide 102b (Figure 13), CD spectra of all peptides recorded in acidic, neutral, and basic media and detailed 2D-nmr analyses of 9-mer peptide 86 and 12-mer 102b, several interesting conformational observations were made. Especially interesting results were obtained using the convex constraint CD analysis proposed by Fasman on 9-mer peptides 79a–d, 80, 81, 86, and 87, which allowed us to determine the relative content of 3₁₀- and α-helical conformations. These results were fully supported by the corresponding x-ray and 2D-nmr analyses. As a striking example we found that the (S)- and (R)-β-tetralin derived amino acids (R)- and (S)-1 show excellent α-helix stabilisation, more pronounced than Aib and Ala. These novel reference peptide sequences should help establish a scale for natural and unnatural amino acids concerning their intrinsic 3₁₀- and α-helix compatibilities at different positions of medium-sized peptides and thus improve our understanding in the folding processes of peptides. © 1997 John Wiley & Sons, Inc. Biopoly 42: 575–626, 1997     

Structural versatility of peptides containing C alpha, alpha-dialkylated glycines. An X-ray diffraction study of six 1-aminocyclopropane-1-carboxylic acid rich peptides

The molecular and crystal structures of six fully blocked, Ac3c-rich peptides to the tetramer level were determined by X-ray diffraction. The peptides are Fmoc-(Ac3c)2-OMe-CH3OH, Ac-(Ac3c)2-OMe, t-Boc-Ac3c-L-Phe-OMe, pBrBz-(Ac3c)3-OMe.H2O, Z-Gly-Ac3c-Gly-OTmb.(CH3)2CO, and t-Boc-(Ac3c)4-OMe.2H2O. Type-I (I') beta-bends and distorted 3(10)-helices were found to be typical of the tri- and tetrapeptides, respectively. In the dipeptides, too short to form beta-bend conformations, other less common structural features may be observed. The average geometry of the cyclopropyl moiety of the Ac3c residue is asymmetric and the N-C alpha-C' bond angle is significantly expanded from the regular tetrahedral value. A comparison with the structural preferences of other extensively investigated C alpha, alpha-dialylated alpha-amino acids is made and the implications for the use of the Ac3c residue in conformational design are examined.     

Chain conformation in polyretropeptides III: Design of a 310 helix using α,α-dialkylated amino acids and retropeptide bonds

Computer simulations have been used to design a polypeptide with a 3₁₀ helix conformation. The study has been been performed taking advantage of the intrinsic helix forming tendency of α-Aminoisobutyric acid. In order to avoid the formation of the α helix, which is the other common helical conformation adopted by α-Aminoisobutyric acid-based peptides, retropeptide bonds have been included in the sequence. Thus, retropeptides are not able to form the intramolecular hydrogen bonding interactions characteristic of the α helix. The influences of both the peptide length and the solvent have been examined and compared with those of the polypeptide without retropeptide bonds. Proteins 29:575–582,1997. © 1997 Wiley-Liss, Inc.     

Marine sterols. XIII. Isolation and synthesis of 1β,3β,5,6β-tetrahydroxy-5α-androstan-17-one from the soft coral

A minor C₂₇ sterol, glaucasterol, was isolated from the soft coral Based on the spectroscopic evidence and the correlation to cholestanol and 26-nor-27-homocholestanol, its structure was proposed to be 24ξ, 25ξ-24,26-cyclocholesta-5,22E-dien-3β-ol ( ), the first example of a natural C₂₇ sterol having a cyclopropane ring in the side chain.     

Mixed conformation in Cα,α-disubstituted tripeptides: X-ray crystal structures of Z-Aib-Dph-Gly-Ome and Bz-Dph-Dph-Gly-Ome

We report here the synthesis and molecular structure in the solid state of fully protected tripeptides containing C^α,α-diphenylglycine (Dph), namely Z-Aib-Dph-Gly-OMe (Aib: C^α,α-dimethylgrycine) and Bz-Dph-Dph-Gly-OMe. The molecular conformation around the Dph residue, containing two bulky substituents, is fully extended, while the Aib residue, containing two smaller groups on the C^α atom, adopts the typical 3₁₀/α-helical conformation. Gly residues, without substituents on the C^α atom, show different conformational preferences. Each residue seems to behave, from a conformational point of view, independently from the presence of the other residues, and thus mixed local conformations (folded and extended) are present in the crystals. The nonconventional peptide synthesis, using the Ugi reaction, is also reported. © 1994 John Wiley & Sons, Inc.     

Studies on the 5α-androstane-3β, 17β-diol-6α-hydroxylase and on the 5α-androstane-3β, 17β-diol-7α-hydroxylase in anterior hypophysis of male rats.

In anterior pituitaries from male rats, it appeared that 5α-androstane-3β, 17β-diol was quickly metabolized into 5α-androstane-3β,6α-17β-triol and 5α-androstane-3β,7α, 17β-triol by action of 6α- and 7α-hydroxylases. Hydroxysteroid hydroxylases were located in endoplasmic reticulum and were dependent on NADPH⁺. Their optimum pH was 8.0, optima temperature, 37°C, and their apparent Km was 2.7 μM. Hydroxylative reactions were not reversible and not modified by gonadectomy. Hydroxylation seemed an efficient control of the pituitary level of 5α-andros-tane-3β, 17β-diol.     

Conformational behaviour of Cα,α-diphenylglycine: folded vs. extended structures in DϕG-containing tripeptides

The crystal structures of three fully protected tripeptides containing the Dϕg residue (C^α,α-diphenylglycine) in the central position are reported, namely Z-Gly-Dϕg-Gly-OMe ( a ), Z-Gly-Dϕg-Aib-OMe ( b ) and Z-Aib-Dϕg-Aib-OMe ( c ). The molecular conformations are quite unusual because the Dϕg residue adopts a folded conformation in the 3₁₀-helical region when the following residue adopts a folded conformation of opposite handedness (peptides b and c ). In contrast, the Dϕg residue adopts the more frequently observed fully extended conformation when the following residue adopts a semi-extended conformation (peptide a ). These findings are in agreement with the theoretical calculations on Ac-Dϕg-Aib-NHCH₃ and Ac-Aib-Dϕg-NHCH₃ also reported in this work. © 1998 European Peptide Society and John Wiley & Sons, Ltd.     

Synthesis,conformational study,and spectroscopic characterization of the cyclic Cα,α‐disubstituted glycine 9‐amino‐9‐fluorenecarboxylic acid

A series of terminally blocked peptides (to the pentamer level) from l ‐Ala and the cyclic C^α,α‐disubstituted Gly residue Afc and one Gly/Afc dipeptide have been synthesized by solution method and fully characterized. The molecular structure of the amino acid derivative Boc‐Afc‐OMe and the dipeptide Boc‐Afc‐Gly‐OMe were determined in the crystal state by X‐ray diffraction. In addition, the preferred conformation of all of the model peptides was assessed in deuterochloroform solution by FT‐IR absorption and ¹H‐NMR. The experimental data favour the conclusion that the Afc residue tends to adopt either the fully‐extended (C₅) or a folded/helical structure. In particular, the former conformation is highly populated in solution and is also that found in the crystal state in the two compounds investigated. A comparison with the structural propensities of the strictly related C^α,α‐disubstituted Gly residues Ac₅c and Dϕg is made and the implications for the use of the Afc residue in conformationally constrained analogues of bioactive peptides are briefly examined. A spectroscopic (UV absorption, fluorescence, CD) characterization of this novel aromatic C^α,α‐disubstituted Gly residue is also reported. Copyright © 1999 European Peptide Society and John Wiley & Sons, Ltd.