Crystallographic characterization of the α,γ C12 helix in hybrid peptide sequences

The solid‐state conformations of two αγ hybrid peptides Boc‐[Aib‐γ⁴(R)Ile]₄‐OMe 1 and Boc‐[Aib‐γ⁴(R)Ile]₅‐OMe 2 are described. Peptides 1 and 2 adopt C₁₂‐helical conformations in crystals. The structure of octapeptide 1 is stabilized by six intramolecular 4 → 1 hydrogen bonds, forming 12 atom C₁₂ motifs. The structure of peptide 2 reveals the formation of eight successive C₁₂ hydrogen‐bonded turns. Average backbone dihedral angles for αγ C₁₂ helices are peptide 1 , Aib; φ (°) = ?57.2 ± 0.8, ψ (°) = ?44.5 ± 4.7; γ⁴(R)Ile; φ (°) = ?127.3 ± 7.3, θ₁ (°) = 58.5 ± 12.1, θ₂ (°) = 67.6 ± 10.1, ψ (°) = ?126.2 ± 16.1; peptide 2 , Aib; φ (°) = ?58.8 ± 5.1, ψ (°) = ?40.3 ± 5.5; ψ⁴(R)Ile; φ (°) = ?123.9 ± 2.7, θ₁ (°) = 53.3 θ 4.9, θ ₂ (°) = 61.2 ± 1.6, ψ (°) = ?121.8 ± 5.1. The tendency of γ⁴‐substituted residues to adopt gauche–gauche conformations about the C^α–C^β and C^β–C^γ bonds facilitates helical folding. The αγ C₁₂ helix is a backbone expanded analog of α peptide 3₁₀ helix. The hydrogen bond parameters for α peptide 3₁₀ and α‐helices are compared with those for αγ hybrid C₁₂ helix. Copyright © 2016 European Peptide Society and John Wiley & Sons.     

X-Pro peptides: Solution and solid-state conformation of benzyloxycarbonyl-(Aib-Pro)2methyl ester,a type I β-turn

The synthesis of the tetrapeptide benzyloxycarbonyl(α-aminoisobutyryl-L -prolyl)₂-methyl ester (Z-(Aib-Pro)₂-OMe) and an analysis of its conformation in solution and the solid state are reported. Stepwise synthesis using dicyclohexylcarbodiimide leads to racemization at Pro(2). Evidence for the presence of diastereomeric tetrapeptides is obtained from 270-MHz¹H-nmr and 67.89-MHz ¹³C-nmr. The all-L tetrapeptide is obtained by fractional crystallization from ethyl acetate. The NH of Aib(3) is shown to be involved in an intramo-lecular hydrogen bond by variable-temperature ¹H-nmr and the solvent dependence of NH chemical shifts. The results are consistent with a β-turn conformation with Aib(1) and Pro(2) at the corners stabilized by a 4 → 1 hydrogen bond. The molecule crystallizes in the space group P2₁2₁2₁, with a = 8.839, b = 14.938, and c = 22.015 Å. The structure has been refined to an R value of 0.051. The peptide backbone is all-trans, and a 4 → 1 hydrogen bond, between the CO group of the urethane moiety and Aib(3) NH, is observed. Aib(1) and Pro(2) occupy the corner positions of a type I β-turn with ? = ?55.4°, Ψ = ?31.3° for Aib(1) and ? = ?71.6°, Ψ = ?38° for Pro(2). The tertiary amide unit linking Pro(2) and Aib(3) is significantly distorted from planarity (Δω = 14.3°).     

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.     

Structural versatility of peptides from Cα,α-dialkylated glycines. I. A conformational energy computation and x-ray diffraction study of homo-peptides from Cα,α-diethylglycine

Conformational energy computations on a derivative and a homo-dipeptide of C^α,α-diethylglycine were performed. In both cases the N- and C-terminal groups are blocked as acetamido and methylamido moieties, respectively. It was found that the C^α,α-diethylglycine residues are conformationally restricted and that the minimum energy conformation corresponds to the fully extended C₅ structure when the N? C^α? C′ bond angle is smaller than 108° (as experimentally observed). The results of the theoretical analysis are in agreement with the crystal-state structural propensity of the complete series of N-trifluoroacetylated homo-peptides of this C^α,α-dialkylated residue from monomer to pentamer, determined by x-ray diffraction and also described in this work. Interestingly, for the first time, a crystallographically planar peptide backbone was observed (in the protected tripeptide). A comparison with peptides of C^α,α-dimethylglycine, C^α-methyl, C^α-ethylglycine, and C^α,α-di-n-propylglycine indicates that the fully extended conformation becomes more stable than the helical structures when both amino acid side-chain C^β atoms are substituted.     

Crystal and molecular structure of benzyloxycarbonyl-α-aminoisobutyryl-L-prolyl methylamide: The observation of an X2-Pro3 Type III β-Turn

The crystal and molecular structure of N-benzyloxycarbonyl-α-aminoisobutyryl-L -prolyl methylamide, the amino terminal dipeptide fragment of alamethicin, has been determined using direct methods. The compound crystallizes in the orthorhombic system with the space group P2₁2₁2₁. Cell dimensions are a = 7.705 Å, b = 11.365 Å, and c = 21.904 Å. The structure has been refined using conventional procedures to a final R factor of 0.054. The molecular structure possesses a 4 → 1 intramolecular N-H—O hydrogen bond formed between the CO group of the urethane moiety and the NH group of the methylamide function. The peptide backbone adopts the type III β-turn conformation, with ?₂ = ?51.0°, ψ₂ = ?39.7°, ?₃ = ?65.0°, ψ₃ = ?25.4°. An unusual feature is the occurrence of the proline residue at position 3 of the β-turn. The observed structure supports the view that Aib residues initiate the formation of type III β-turn conformations. The pyrrolidine ring is puckered in C^γ-exo fashion.     

Vibrational analysis of peptides,polypeptides, and proteins. XVIII. Conformational sensitivity of the α-helix spectrum: αI- and αII-Poly(L-alanine)

The α_II-helix (? = ?70.47°, ψ = ?35.75°) is a structure having the same n and h as the (standard) α_I-helix (? = ?57.37°, ψ = ?47.49°). Its conformational angles are commonly found in proteins. Using an improved α-helix force field, we have compared the vibrational frequencies of these two structures. Despite the small conformational differences, there are significant predicted differences in frequencies, particularly in the amide A, amide I, and amide II bands, and in the conformation-sensitive region below 900 cm^?1. This analysis indicates that α_II-helices are likely to be present in bacteriorhodopsin [Krimm, S. & Dwivedi, A. M. (1982) Science 216 , 407–408].     

Synthesis,and crystal and molecular structure of the 310-helical α,β-dehydro pentapeptide Boc-Leu-Phe-Ala-ΔPhe-Leu-Ome

α,β-Dehydro amino acid residues are known to constrain the peptide backbone to the β-bend conformation. A pentapeptide containing only one α,β dehydrophenylalanine (ΔPhe) residue has been synthesized and crystallized, and its solid state conformation has been determined. The pentapeptide Boc-Leu-Phe-Ala-ΔPhe-Leu-OMe (C₃₉H₅₅N₅O₈, M_w = 721.9) was crystallized from aqueous methanol. Monoclinic space group was P2₁, a = 10.290(2)°, b = 17.149(2)°, c = 12.179(2) Å, β = 96.64(1)° with two molecules in the unit cell. The x-ray (Mo K_α, λ = 0.7107A) intensity data were collected using a CAD4 diffractometer. The crystal structure was determined by direct methods and refined using least-squares technique. R = 4.4% and R_w = 5.4% for 4403 reflections having |F₀| ≥ 3σ(|F₀|). All the peptide links are trans and the pentapeptide molecule assumes 3₁₀-helical conformation. The mean ?,ψ values, averaged over the first four residues, are ?64.4°, ?22.4° respectively. There are three 4 → 1 intramolecular hydrogen bonds, characteristic of 3₁₀,-helix. In the crystal, the peptide helices interact through two head-to-tail. N? H? O intermolecular hydrogen bonds. The peptide molecules related by 2₁, screw symmetry form a skewed assembly of helices. © 1995 John Wiley & Sons, Inc.     

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.     

Accommodation of a D-Phe residue into a right-handed 310-helix: Structure of Boc-D-Phe-(Aib)4-Gly-L-Leu-(Aib)2-Ome,an analogue of the amino terminal segment of antiamoebins and emerimicins

The crystal structure of the nonapeptide Boc-D -Phe-Aib-Aib-Aib-Aib-Gly-Leu-Aib-AibOMe (I), which is an analogue of the N-terminal sequence of antiamoebins and emerimicins, establishes a completely 3₁₀-helical conformation with seven successive intramolecular 4 → 1 hydrogen bonds. The average, ?, ψ values for residues 1–8 are ?59° and ?32°, respectively. Crystal parameters are C₄₇H₇₇N₉O₁₂, space group P1, a = 10.636(4) Å, b = 11.239(4) Å, c = 12.227(6) Å, α = 101.17(4)°, β = 97.22(4)°, γ = 89.80(3)°, Z = 1, R = 5.95% for 3018 data with |F₀| > 3α(F), resolution 0.93 Å. The use of the torsion angle κ = C(i ? 1)N(i)C^α(i)C^β(i), where κ = 68° for D -Phe and κ = 164° for L -Leu, confirms the opposite configurations of these residues. The ?, ψ values of ?62° and ?32° at D -Phe are unusual, since this region is characteristic of residues with L configurations. Peptide I possesses only two chiral residues of opposing configuration. The observed right-handed 3₁₀-helical structure suggests that helix sense has probably been determined by the stereo-chemical preferences of the Leu residue. © 1993 John Wiley & Sons, Inc.     

The effect of β‐methylation on the conformation of α, β‐dehydrophenylalanine: a DFT study

Dehydroamino acids are non‐coded amino acids that offer unique conformational properties. Dehydrophenylalanine (ΔPhe) is most commonly used to modify bioactive peptides to constrain the topography of the phenyl ring in the side chain, which commonly serves as a pharmacophore. The Ramachandran maps (in the gas phase and in CHCl₃ mimicking environments) of ΔPhe analogues with methyl groups at the β position of the side chain as well as at the C‐terminal amide were calculated using the B3LYP/6‐31 + G** method. Unexpectedly, β‐methylation alone results in an increase of conformational freedom of the affected ΔPhe residue. However, further modification by introducing an additional methyl group at C‐terminal methyl amide results in a steric crowding that fixes the torsion angle ψ of all conformers to the value 123°, regardless of the Z or E position of the phenyl ring. The number of conformers is reduced and the accessible conformational space of the residues is very limited. In particular, (Z)‐Δ(βMe)Phe with the tertiary C‐terminal amide can be classified as the amino acid derivative that has a single conformational state as it seems to adopt only the β conformation. Copyright © 2009 European Peptide Society and John Wiley & Sons, Ltd.     

Type II β-turn conformation of pivaloyl-L-prolyl-α-aminoisobutyryl-N-methylamide: Theoretical,spectroscopic, and X-ray studies

Pivaloyl-L -Pro-Aib-N-methylamide has been shown to possess one intramolecular hydrogen bond in (CD₃)₂SO solution, by ¹H-nmr methods, suggesting the existence of β-turns, with Pro-Aib as the corner residues. Theoretical conformational analysis suggests that Type II β-turn conformations are about 2 kcal mol^?1 more stable than Type III structures. A crystallographic study has established the Type II β-turn in the solid state. The molecule crystallizes in the space group P2₁ with a = 5.865 Å, b = 11.421 Å, c = 12.966 Å, β = 97.55°, and Z = 2. The structure has been refined to a final R value of 0.061. The Type II β-turn conformation is stabilized by an intramolecular 4 → 1 hydrogen bond between the methylamide NH and the pivaloyl CO group. The conformational angles are ?_Pro = ?57.8°, ψ_Pro = 139.3°, ?_Aib = 61.4°, and ψ_Aib = 25.1°. The Type II β-turn conformation for Pro-Aib in this peptide is compared with the Type III structures observed for the same segment in larger peptides.     

Design of Peptides Using α,β-Dehydro-residues: Synthesis,Crystal Structure and Molecular Conformation of N-Boc-L-Val-ΔPhe–ΔPhe-L-Ala-OCH3

To obtain general rules of peptide design using α,β-dehydro-residues, a sequence with two consecutive ΔPhe-residues, Boc-L -Val-ΔPhe–ΔPhe- L -Ala-OCH₃, was synthesized by azlactone method in solution phase. The peptide was crystallized from its solution in an acetone/water mixture (70:30) in space group P6₁ with a=b=14.912(3) Å, c= 25.548(5) Å, V=4912.0(6) ų. The structure was determined by direct methods and refined by a full matrix least-squares procedure to an R value of 0.079 for 2891 observed [I?3σ(I)] reflections. The backbone torsion angles ?₁=?54(1)°, ψ₁= 129(1)°, ω₁=?177(1)°, ?₂ =57(1)°, ψ₂=15(1)°, ω₂ =?170(1)°, ?₃=80(1)°, ψ₃ =7(2)°, ω₃=?177(1)°, ?₄ =?108(1)° and ψ^T₄=?34 (1)° suggest that the peptide adopts a folded conformation with two overlapping β-turns of types II and III′. These turns are stabilized by two intramolecular hydrogen bonds between the CO of the Boc group and the NH of ΔPhe³ and the CO of Val¹ and the NH of Ala⁴. The torsion angles of ΔPhe² and ΔPhe³ side chains are similar and indicate that the two ΔPhe residues are essentially planar. The folded molecules form head-to- tail intermolecular hydrogen bonds giving rise to continuous helical columns which run parallel to the c-axis. This structure established the formation of two β-turns of types II and III′ respectively for sequences containing two consecutive ΔPhe residues at (i+2) and (i+3) positions with a branched β-carbon residue at one end of the tetrapeptide.     

Conformational energy studies on N-methylated analogs of thyrotropin releasing hormone, enkephalin, and luteinizing hormone-releasing hormone

Empirical conformational energy calculations have been carried out for N-methyl derivatives of alanine and phenylalanine dipeptide models and N-methyl-substituted active analogs of three biologically active peptides, namely thyrotropin-releasing hormone (TRH), enkephalin (ENK), and luteinizing hormone-releasing hormone (LHRH). The isoenergetic contour maps and the local dipeptide minima obtained, when the peptide bond (ω) preceding the N-methylated residue is in the trans configuration show that (1) N-methylation constricts the conformational freedom of both the ith and (i + 1)th residues; (2), the lowest energy position for both residues occurs around ? = ?135° ± 5° and ψ = 75° ± 5°, and (3) the α_L conformational state is the second lowest energy state for the (i + 1)th residue, whereas for the ith residue the C₅ (extended) conformation is second lowest in energy. When the peptide bond (ω_i) is in the cis configuration the ith residue is energetically forbidden in the range ? = 0° to 180° and ψ = ?180° to +180°. Conformations of low energy for ω_i = 0° are found to be similar to those obtained for the trans peptide bond. In all the model systems (irrespective of cis or trans), the α_R conformational state is energetically very high. Significant deviations from planarity are found for the peptide bond when the amide hydrogen is replaced by a methyl group. Two low-energy conformers are found for [(N-Me)His²]TRH. These conformers differ only in the ? and ψ values at the (N-Me)His² residue. Among the different low-energy conformers found for each of the ENK analogs [D -Ala²,(N-Me)Phe⁴, Met⁵]ENK amide and [D -Ala²,(N-Me)Met⁵]ENK amide, one low-energy conformer was found to be common for both analogs with respect to the side-chain orientations. The stability of the low-energy structures is discussed in the light of the activity of other analogs. Two low-energy conformers were found for [(N-Me)Leu⁷]LHRH. These conformations differ in the types of bend around the positions 6 and 7 of LHRH. One bend type is eliminated when the active analog [D -Ala⁶,(M-Me)Leu⁷]LHRH is considered.     

Experimental conformational study of two peptides containing α-aminoisobutyric acid. Crystal structure of N-acetyl-α-aminoisobutyric acid methylamide

Some theoretical studies have predicted that the conformational freedom of the α-aminoisobutyric acid (H-Aib-OH) residue is restricted to the α-helical region of the Ramachandran map. In order to obtain conformational experimental data, two model peptide derivatives, MeCO-Aib-NHMe 1 and Bu^tCO-LPro-Aib-NHMe 2 , have been investigated. The Aib dipeptide 1 crystallizes in the monoclinic system (a = 12.71 Å, b = 10.19 Å, c = 7.29 Å, β = 110.02°, Cc space group) and its crystal structure was elucidated by x-ray diffraction analysis. The azimuthal angles depicting the molecular conformation (? = ?55.5°, ψ = ?39.3°) fall in the α-helical region of the Ramachandran map and molecules are hydrogen-bonded in a three-dimensional network. In CCl₄ solution, ir spectroscopy provides evidence for the occurrence of the so-called ₅ and C₇ conformers stabilized by the intramolecular i → i and i + 2 → i hydrogen bonds, respectively. The tripeptide 2 was studied in various solvents [CCl₄, CD₂Cl₂, CDCl₃, (CD₃)₂SO, and D₂O] by ir and pmr spectroscopies. It was shown to accommodate predominantly the βII folded state stabilized by the i + 3 → i hydrogen bond. All these experimental findings indicate that the Aib residue displays the same conformational behavior as the other natural chiral amino acid residues.     

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     

Assignment of secondary structure from Cα coordinates

A multiple regression analysis has established a nonlinear relationship between the backbone dihedral angles and the C^α coordinates obtained from the x-ray crystal structures of 14 proteins. The regression equations have been applied to predict specific dihedral angles of each residue in the backbone of 24 proteins. Overall this method (Nonlinear Regression Distance Torsion) predicts values of ϕ and ψ within a ±45° window of those found in the x-ray structure with an accuracy of 94 and 91% and within a ±30° window of 88 and 81%. Two methods for the assignment of motif from C^α coordinates are reported. For the first method, motif is assigned from the dihedral angles predicted using the regression equations. By the second method, motif of the ith residue is assigned from the distance C^α_i-1 to C^α_i+2 (v₆) and torsional angle C^α_i-1, C^α_i, C^α_i+1, C^α_i+2 (v₁₃). For the 24 proteins, 23.7% of the residues by the former method and 19.6% by the latter method are assigned differently than in the Protein Data Bank. © 1997 John Wiley & Sons, Inc.     

A fluorescent probe for biothiols based on the conjugate addition of thiols to α,β‐unsaturated ester

A sensitive fluorogenic probe 1 for biothiols was developed based on the Michael addition reaction. The probe 1 was readily synthesized via the reaction of 2‐(4′‐hydroxyphenyl) benzimidazole (HPBI) with acryloyl chloride and shows weak fluorescence emission. Upon mixing with biothiols, the fluorescence of 1 is significantly enhanced due to the conjugate addition of thiols to the α,β‐unsaturated carbonyl moiety, thus eliminating the photoinduced electron transfer (PET) quenching of the fluorophore by the intramolecular carbon–carbon double bond. Cysteine (Cys) was selected as the representative thiol in the spectral experiment. A good linear relationship was obtained from 1.0 to 30.0 µmol L^?1 for Cys and the detection limit was 0.17 µmol L^?1. Furthermore, probe 1 was highly selective for biothiols without the interference of some biologically relevant analytes and has been applied to detecting biothiols in human urine samples. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A Supramolecular “Double‐Cable” Structure with a 12944 Helix in a Columnar Porphyrin‐C60 Dyad and its Application in Polymer Solar Cells

A novel porphyrin‐C₆₀ dyad (PCD1) is designed and synthesized to investigate and manipulate the supramolecular structure where geometrically isotropic [such as [60]fullerene (C₆₀)] and anisotropic [such as porphyrin (Por)] units coexist. It is observed that PCD1 possesses an enantiomeric phase behavior. The melting temperature of the stable PCD1 thermotropic phase is 160 °C with a latent heat (ΔH) of 18.5 kJ mol^?1. The phase formation is majorly driven by the cooperative intermolecular Por–Por and C₆₀–C₆₀ interactions. Structural analysis reveals that this stable phase possesses a supramolecular “double‐cable” structure with one p‐type Por core columnar channel and three helical n‐type C₆₀ peripheral channels. These “double‐cable” columns further pack into a hexagonal lattice with a = b = 4.65 nm, c = 41.3 nm, α = β = 90°, and γ = 120°. The column repeat unit is determined to possess a 129₄₄ helix. With both donor (D; Pro) and acceptor (A; C₆₀) units having their own connecting channels as well as the large D/A interface within the supramolecular “double‐cable” structure, PCD1 has photogenerated carriers with longer lifetimes compared to the conventional electron acceptor [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester. A phase‐separated columnar morphology is observed in a bulk‐heterojunction (BHJ) material made by the physical blend of a low band‐gap conjugated polymer, [poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta [2,1‐b;3,4‐b′]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothia‐diazole)] (PCPDTBT), and PCD1. With a specific phase structure in the solid state and in the blend, PCD1 is shown to be a promising candidate as a new electron acceptor in high performance BHJ polymer solar cells.     

Conformation of N-formyl-1,3-dihydroxy-Δ31-pentene,a model compound of sphingomyelin

The conformations of 2S,3R-2-(N-formyl)amino-l,3-dihydroxy-Δ³¹-pentene, a model compound of sphingomyelin, have been studied both by the classical potential function and by the INDO molecular orbital method. The results suggest that the preferred conformation of sphingomyelin in the membrane is such that the olefinic double bond of the γ-hydrocarbon chain and the planar amide group of the β-chain are parallel and stack in an antiparallel manner with dihedral angles β₁′(C3-C2-N21-C21) = ?100 ° and γ₁(C2-C3-C31-C32) = ?100 °.