Anisotropic rotational motions of poly(L-glutamic acid) in the alpha-helix conformation

The anisotropic rotational motion of the backbone and the side chains of poly(L -glutamic acid) in the α-helical structure was investigated using the ¹³C-T₁ and T₂ relaxation times of all carbon atoms with directly attached protons, obtained at a ¹³C-Larmor frequency of 67.89 MHz. The evaluation of the nmr data was carried out according to the previously derived anisotropic diffusion model, in which the macromolecule is considered a rigid rod. The rotation of the backbone is characterized by two diffusion constants, D₁ and D₃, describing the rotation perpendicular to and around the symmetry axis. The additional internal motion of the C^β-methylene group is described as a jump process with a jump rate, k₁, between two allowed rotametric states. Steric considerations indicate that the occupation of the third rotameric position is forbidden. The rotation of the C^γ-methylene group is decribed as a one-dimensional diffusion process around the C^β–C^γ bond. Investigation of the temperature dependence of the relaxation parameters led to the temperature dependence of the dynamic parameters. Activation energies were determined from these data. The dynamic parameters obtained for poly(L -glutamic acid) at 291 K are compared with the corresponding results of a previous study of poly(L -lysine). The development of an anisotropic diffusion model for the motions of the rod-shaped poly(L -lysine) α-helix and its application to the interpretation of the ¹³C-relaxation data of this molecule have already been published previously. In this model, both the overall molecular tumbling and the various internal motions have been characterized by diffusion constants or jump rates typical for each process. These dynamic parameters can be calculated from the spin–lattice relaxation times, the spin–spin relaxation times and the NOE factors of the C^α, C^β, and C^γ nuclei of the polypetide. In the present paper, we describe the application of the above-mentioned dynamic model to the interpretation of ¹³C-relaxation studies of a further homopolypeptide, poly(L -glutamic acid), in the α-helical structure. Furthermore, we studied the temperature dependence of the relaxation times of this polymer and determined the anisotropic diffusion parameters at each temperature. From their temperature dependence and from comparison of our present results with the data of our previous study of poly(L -lysine), we were able to derive new insights into the intramolecular diffusion processes and the excitation of various motions.     

13C Nuclear magnetic resonance study of salt induced conformational change of basic polypeptides: Poly (L-lysine), poly (L-arginine), and poly (L-ornithine)

A ¹³C-nmr study of the salt-induced helix–coil transition of the basic polypeptides poly(L -lysine) [(Lys)_n], poly(L -arginine) [(Arg)_n], and poly (L -ornithine) [(Orn)_n] was performed to serve as a reference of the helical portion of histones and other proteins. As is the case with pH-induced helix–coil transition, the downfield displacement of the C^α and carbonyl carbon signals are observed in the helical state. The upfield shift of the C^β signals, on the other hand, is noted in the salt-induced transition. Regardless of the differences in the side chains and also the salts used, very similar helix-induced chemical shifts are obtained for (Lys)_n and (Arg)_n. However, the displacement of the C^α, C^β, and carbonyl carbons of (Orn)n in the presence of 4M NaClO₄ is found to be almost 50% of that of (Lys)_n and (Arg)_n. This is explained by the fact that the maximum helical content is about 50%, consistent with the ORD result. Further, the motion of the backbone and side chains of the helical from was estimated by measuring the spin-lattice relaxation time (T₁), nuclear Overhauser enhancement (NOE), and line width. In the case of (Lys)_n, the motion of the side chains is charged very little in comparison with that of the random coil. Indicating that the aggregation of the salt-induced helix is small in contrast to that of the pH-induced helix. For (Arg)_n, however, the precipitate of the helical polymers is mainly due to aggregation.     

Absorption and circular dichroism spectra of CuCl2 complexed with poly(S-carboxymethyl-L-cysteine) and poly(S-carboxyethyl-L-cysteine)

The interaction of CuCl₂ with poly(S-carboxymethyl-L -cysteine) (poly[Cys(CH₂COOH)]) and poly(S-carboxyethyl-L -cysteine) (poly[Cys(C₂H₄COOH)]) were studied by absorption spectra and circular dichroism (CD). On mixing CuCl₂ with polypeptide solutions, absorption bands appeared at 320–325 nm in both polypeptides, and at 255–260 nm in the case of poly[Cys(CH₂COOH)]. A stable bound species was formed in the case of poly[Cys(CH₂COOH)], since the apparent molar absorption coefficient of the bound species did not depend on the mixing ratio. From the absorption data, it was inferred that Cu²⁺ ions were complexed with the side chains, most probably with sulfur atoms and carboxyl groups. Induced optical activities were observed for the two polypeptides. The CD spectra of poly[Cys(CH₂COOH)] + CuCl₂ gave simpler aspects than those of poly[Cys(C₂H₄COOH)] + CuCl₂.     

Metal complexs of poly (α-amino acids): Cu(II) chelates of acetoacetylated poly(L-lysine), poly(L-ornithine), and poly(L-diaminobutyric acid)

The cupric complexes of poly(N^ε-acetoacetyl-L -lysine), [Lys(Acac)]_n′ poly(N^δ-acetoacetyl-L -ornithine), [Orn(Acac)]_n′ and poly(N^γ-acetoacetyl-L -diaminobutyric acid), [A₂bu-(Acac)]_n, as well as of the model compound n-hexyl acetoacetamide, have been investigated by means of absorption, potentiometric, equilibrium dialysis, and CD measurements. While in the complex of the model compound, one chelating group is bound to one cupric ion, in the polymeric complexes two β-ketoamide groups are bound to Cu(II) under the same experimental conditions. The binding constant of cupric ions to the three polymers and the formation constant of the Cu(II)-nhexylacetoacetamide complex have been evluated. Investigation on the chiroptical properties of the three polymeric complexes shows that the peptide backbone does not undergo conformational transitions, remaining α-helical when up to 20% of the side chains are bound to Cu(II). The optical activity of the β-ketoamide chromophores is substantially affected by complex formation and is discussed in terms of asymmetric induction from the chiral backbone.     

Molecular mobility of polypeptides containing proline as determined by 13C magnetic resonance

The molecular conformations and dynamics of poly(L -prolyl), poly(hydroxyl-L -prolyl), poly(L -prolyl-glycyl), poly(hydroxyl-L -prolyl), and poly(glycyl-glycyl-L -prolyl-glycyl), in aqueous solution, have been studied using ¹³C pulse Fourier transform nmr spectroscopy. From a measurement of the intensities of major and minor resonances in the spectra of the copolypeptides, it was determined that 15–20% of the glycyl-prolyl and glycyl-hydroxyprolyl peptide bonds are cis. Effective rotational correlation times (τ_eff), obtained from measurements of spin-lattice relaxation times (T₁) of individual backbone and side-chain carbons, demonstrated that backbone reorientation is approximately isotropic for the five polypeptides and is characterized by correlation times of ca. 0.3–0.6 nanoseconds as a result of rapid segmental motion. In a given polypeptide glycyl and pyrrolidine residues were found to have the same backbone correlation times, but backbone carbon τ_eff values did decrease as the glycyl content of the peptides increased. A semi-quantitative analysis of C^β, C^γ, and C^δ correlation times suggests that rapid ring motion in both prolyl and hydroxyprolyl involves primarily C^γ and C^β, with the prolyl ring being more mobile than the hydroxyprolyl ring.     

Cyclo(D-Pro-L-Pro-D-Pro-L-Pro): Structural properties and cis/trans isomerization of the cyclotetrapeptide backbone

Combinations of L - and D -proline residues are useful compounds for finding new structures and properties of cyclic peptides. This is demonstrated with one striking example, the cyclic tetrapeptide c(D -Pro-L -Pro-D -Pro-L -Pro). For this molecule composed of strictly alternating D - and L -configurated residues, a highly symmetrical structure is expected, which should be an optically inactive meso-form. Cyclization of the enantiomeric pure linear precursor D -Pro-L -Pro-D -Pro-L -Pro, however, yields a racemic mixture of two enantiomeric cyclotetrapeptides, both with twofold symmetry and a cis–trans–cis–trans sequence of the peptide bonds. Remarkably, this formation of a racemate was not caused by racemization, but by cis/trans isomerization of all peptide bonds in the ring. This process may occur in the linear precursor during the ring formation (cyclization of conformers with trans–cis–trans or cis–trans–cis arrangement of the amide bonds) as well as in the enantiomeric pure cyclic tetrapeptide at higher temperature. In the latter case, an all-cis structure should exist as the intermediate, which can form a cis–trans–cis–trans sequence in two equivalent ways, leading finally to two enantiomeric cyclotetrapeptides. In the first one, the cis peptide bonds are attributed to the L -residues and the trans peptide bonds to the D -residues; in the second one, the cis bonds belong to the D and the trans bonds to the L -residues. The mixture of these two enantiomers does not crystallize in the racemic form, but in enantiomeric pure separate crystals. The structural properties could be proved by ¹H- and ¹³C-nmr spectroscopy and x-ray analysis. The cis/trans isomerization process was confirmed by optical rotation measurements and CD spectroscopy, as well as DREIDING model studies. Calorimetric measurements in the solid state suggest the existence of the expected all-cis intermediate. The backbone conformation of the 12-membered medium-sized ring shows only slight deviations—up to 6° —from the planarity of the peptide bonds. On the other hand, the four pyrrolidine rings show different types of puckering of the C_γ or the C_β atoms.     

Kinetic studies of the helix–coil transition in aqueous solutions of poly(L-lysine)

The rate of conformational change of aqueous poly(α-L -lysine) solutions was measured using the electric field pulse relaxation method with conductivity detection. The relaxation time as a function of pH exhibits two maxima. One is assigned to a proton transfer reaction and the other to the helix–coil conformational transition. The helix nucleation parameter and the maximum relaxation time yield the rate constant of helix growth process (k_F) according to Schwarz's kinetic theory as k_F = 2 × 10⁷ sec^?1, which is comparable to that of the poly(glutamic acid) solution. The thermodynamic parameters of the helix growth process are compared with those of poly(glutamic acid).     

In vitro correction of antigen-induced immune suppression: effects of poly(A) poly(U) and prostaglandin E

Incubation of SJL or DBA/1 mouse spleen cells with poly(lTyr, lGlu)-polylPro—polylLys, (T, G)-Pro—L in vitro reduced the immune response potential of the cells to this immunogen as tested by adoptive transfer into irradiated, syngeneic recipients, followed by immunization with (T, G)-Pro—L in complete Freund's adjuvant. This reduction in immunocompetence was antigen-specific, since incubation with another antigen (rabbit immunoglobulin G) did not result in a suppression of responsiveness of the cells to subsequent in vivo immunization with (T, G)-Pro—L. Incubation of the spleen cell-(T, G)-Pro—L mixture in the presence of either prostaglandin E₁(PGE₁) or polyadenylic-polyuridylic acid (poly(A)·poly (U)) restored the immune response potential to the normal level. Incubation of (T, G)-Pro—L with spleen cells had no effect on cyclic AMP accumulation, whereas incubation of PGE₁ with the cells stimulated cyclic AMP production, irrespective of the presence of antigens. In contrast, the level of cyclic AMP was not affected by poly(A) · poly(U). The difference in cyclic AMP accumulation suggests that PGE₁ and poly(A) · poly(A) modify immune responsiveness by different mechanisms. The above observations were verified both in SJL and DBA/1 mice, which are the respective genetic high and low responders to (T, G) -Pro—L. This implies that the modifications of responsiveness described are not related to the genetic control of immune response to this immunogen.     

A UV resonance Raman investigation of poly(rI): Evidence for cation-dependent structural perturbations

Poly(rI) stabilized by either Na⁺ or K⁺ was investigated using uv resonance Raman (UVRR) spectroscopy. Raman excitation profiles of inosine 5′-monophosphate demonstrated the 250 nm excitation selectively enhances base stacking interactions, while ribose and carbonyl stretching vibrations are preferentially enhanced with 210 nm excitation. These wavelengths were used to examine the structure of poly(rI) in the presence of either K⁺ or Na⁺ as a function of temperature. UVRR studies revealed that the K⁺ stabilized form is more thermally stable, yielding a T_m of ∼ 47°C compared to a T_m of ∼ 30°C for the Na⁺ stabilized form. We observed that both the ribosyl conformation and the coordination of the carbonyl groups depend on the nature of the cation. The C₆O stretching frequency indicates that Na⁺ coordinates much more strongly to the carbonyl groups than K⁺ (1672 cm⁻¹ Na⁺ vs 1684 cm⁻¹ K⁺ at 4°C). Conformationally sensitive modes of the phosphate backbone and the ribosyl ring indicate that Na⁺ stabilized poly(rI) predominantly exists in a C_3′-endo ribose conformation, whereas K⁺ stabilized poly(rI) adopts a C_2′-endo conformation possibly as a consequence of the larger ionic radius of the K⁺ ion. © 1998 John Wiley & Sons, Inc. Biopoly 46: 475–487, 1998     

Synthesis and characterization of stereoregular poly(D-methionyl-L-methionine) and poly(L-methionyl-D-methionyl-L-methionine)

By use of a polycondensation procedure free of racemization, stereoregular polymethionines have been synthesized from C-activated D -methionyl-L -methionine and L -methionyl-D -methionyl-L -methionine. The poly(D -methionyl-L -methionine) and poly(L -methionyl-D -methionyl-L -methionine) so prepared are soluble in chloroform and can be purified through dissolution in this solvent and precipitation by ligroin. Poly(D -Met-L -Met)which is obtained in a 25% yield, is about 5000 in average molecular weight. It has no discernible optical activity when examined between 400 and 600 nm in a trifluoroacetic acid solution. Poly(L -Met-D -Met-L -Met) (40% yield, M. W. = 10,000) is an optically active polymer. [α]₄₃₆²⁴ ≈ + 170° for a chloroformic solution (c = 0.2 CHCl₃).     

Antinociceptive and antidepressant-like action of endomorphin-2 analogs with proline surrogates in position 2

In our efforts to develop new candidate drugs with antinociceptive and/or antidepressant-like activity, two novel endomorphin-2 (EM-2, Tyr-Pro-Phe-Phe-NH₂) analogs, containing proline surrogates in position 2 were synthesized using commercially available racemic trans-4-phenylpyrrolidine-3-carboxylic acid (4-Ph-β-Pro). The obtained mixture of two diastereoisomeric peptides (2a and 2b) was separated by HPLC and both enantiopure analogs were used in the in vitro and in vivo studies. To assign the absolute configuration to the 4-Ph-β-Pro residues in both peptides, the stereoselective synthesis of (3R,4S)-4-phenylpyrrolidine-3-carboxylic acid was performed and this enantiomer was introduced into position 2 of EM-2 sequence. Based on the HPLC retention times we were able to assign the absolute configuration of 4-Ph-β-Pro residues in both peptide analogs. Analog 2a incorporating (3R,4S)-4-Ph-β-Pro residue produced strong analgesia in mice after intracerebroventricular (icv) administration which was antagonized by the μ-opioid receptor (MOR) antagonist, β-funaltrexamine (β-FNA). This analog also influenced an emotion-related behavior of mice, decreasing immobility time in the forced swimming and tail suspension tests, without affecting locomotor activity. The antidepressant-like effect was reversed by the δ-selective antagonist, naltrindole (NLT) and κ-selective nor-binaltorphimine (nor-BNI). Thus, the experiments with selective opioid receptor antagonists revealed that analgesic action of analog 2a was mediated through the MOR, while the δ- and κ-receptors (DOR and KOR, respectively) were engaged in the antidepressant-like activity. Analog 2b with (3S,4R)-4-Ph-β-Pro in position 2 showed no antinociceptive or antidepressant-like activity in animal studies.     

Synthesis and conformation of the cyclic octapeptides cyclo(Phe-Pro)4, cyclo(Leu-Pro)4, and cyclo[Lys(Z)-Pro]4

Cyclic octapeptides, cyclo(X-Pro)₄, where X represents Phe, Leu, or Lys(Z), were synthesized and their conformations investigated. A C₂-symmetric conformer containing two cis peptide bonds was found in all of these cyclic octapeptides. The numbers of available conformations due to the cis–trans isomerization of Pro peptide bonds depended on the nature of the solvent and X residue: they decreased in the following order: cyclo[Lys(Z)-Pro]₄ > cyclo(Leu-Pro)₄ > cyclo(Phe-Pro)₄ in CDCl₃. ¹³C spin-lattice relaxation times (T₁) of these cyclic octapeptides were measured, and the contribution of segmental mobility to T₁ was found to vary with the nature of the X residue.     

Ferric complexes of acetoacetyl derivatives of poly(L-lysine), poly(L-ornithine), and poly(L-diaminobutyric acid). I. Preparation and absorption properties in solution

Poly(N^ε-acetoacetyl-L -lysine), poly(N^δ-acetoacetyl-L -ornithine) and poly(N^γ-acetoacetyl-L -diaminobutyric acid) form colored complexes with ferric ions in water/dioxane solutions. These complexes are soluble at pH values lower than 2.8 and show their maximum absorption at 257 nm in the uv and at 478 nm in the visible region; whereas the ferric complex of the model compound n-hexylacetoacetamide exhibits absorptions centered at 258 and 536 nm, respectively. It is shown that in the complex of the model compound one metal ion is bound per acetoacetamide group, while in the complexes of the three polymers two β-ketoamides side chains are bound per ferric ion under the same solvent, pH, concentration, and ionic strength conditions. The binding constants of ferric ions to the three polymers, and the formation constant of the ferric complex of the model compound are also evaluated.     

Conformational change of poly( -lysine) induced by lipid vesicles of dilauroylphosphatidic acid

The effect of negatively charged dilauroylphosphatidic acid (DLPA) vesicles on the conformation of poly( -lysine) was investigated by circular dichroism measurements. DLPA vesicles induced a confomiational change Of poly( -lysine) from the random coil to β-structure in 5 mM Tes, pH 7.0. The fraction of induced β-structure (F_β) was determined via a procedure of curve fit the observed spectra to the reference spectra. F_β increased linearly with the molar ratio, r, of DLPA to lysine residues up to r 0.7, and reached a saturation value of 1 at r > 1. Within the range 0.7 r 1, precipitation occurred. The effect of dilution of the negative charge on vesicle membranes was examined by mixing DLPA with dilauroylphosphatidylcholine (DLPC). Although the β-structure Of poly -lysine) was also induced by mixed vesicles, the saturation value of F_β decreased with decreasing DLPA content in mixed vesicles. The variation in saturation value of F_β with the composition of mixed vesicles was interpreted in terms of the change in average distance between DLPA head groups in mixed vesicles.     

Laser-excited raman spectroscopy of biomolecules. XII. Thermally induced conformational changes in poly(L-glutamic acid)

The α-helical from of poly(L -glutamic acid) [α-poly(Glu)] gives rise to the same amide I and III lines as α-poly(γ-benzyl-L -glutamate) at 1652 and 1296 cm^?1, respectively. The latter is a superposition of the amide III line near 1290 cm^?1 and a line deu to vibrational made of CH₂ groups of the side chain near 1300 cm^?1. A line at 924 cm^?1 is tentatively identified as characteristics of α-poly(Glu). Both the β₁- and β₂- forms of poly(Glu) give rise to characteristic of β-amide. III frequencies that are similar because of their similar backbone structures. Differences in the conformations of their side chains and in the environments of the backbone are reflected in the region 800–1200 cm^?1 and in the amide I. A line at 1042 cm^?1 and a pair at 1021 and 1059 cm^?1 are tentatively assigned as characteristic of β₁-poly(Glu) and β₂-poly(Glu), respectively. The α-β₂ transition in poly(L -Glu⁷⁸L -Val²²) is shown by the appearance of all the β₂-characteristic lines in the thermally transformed sample. The same features observed in poly(L -Glu⁹⁵L -Val⁵) also indicate that the α-β₂ transition of poly(Glu) is facilitated by the presence of L -valine and that the content of L -valine is not critical for this purpose. Investigation of the Raman spectra of the calcium, strontium, barium and sodium slats of poly(Glu) shows that these salts, under the conditions of preparation used, all the have random-coil conformations.     

Solution conformations of some azetidine cyclic peptides

The ¹H-nmr spectra (270 MHz) of cyclic di- and tri-L -azatidine-2-carboxylic acid [cy-clo(L -Aze)₂ and cyclo(L -Aze)³] were determined in CDCl₃ and D₂O and computer simulated. The spectral results were compared with those obtained with cyclo (L -Pro)₂ and cyclo (L, -Pro)₃. In CDCl₃ and D₂O solution, the four membered ring of cyclo (L -Aze)₂ is puckered with the α-proton in a pseudo-axial position, and the ? angle is smaller in absolute value than ?60°, as found for cyclo (L -Pro)_2,. The puckering of the four-membered ring of cyclo(L, -Aze)₃ in CDCl₃ has the α-proton in a pseudo–equatorial position and ? angle larger in absolute value than ?60°, in agreement with cyclo(L -Pro)₃. In D₂O, cyclo(L -Aze)₃ was found to interconvert rapidly between different conformers. In the azetidine cyclic peptides studied, the range of values found for the ? angles was smaller than in the related proline cyclic peptides, indicating greater rigidity in the four-membered ring.     

Crystal structures of (2R,4R)-2-(polyhydroxyalkyl)-1,3-thiazolidine-4-carboxylic acids: condensation products of l-cysteine with d-hexoses

We report herein the first crystal structures of (4-carboxy-1,3-thiazolidin-2-yl)pentitols [2-(polyhydroxyalkyl)thiazolidine-4-carboxylic acids], condensation products of l-cysteine with d-galactose and d-mannose: 2-(d-galacto-pentahydroxypentyl)thiazolidine-4-carboxylic acid hydrate, Gal-Cys·H₂O (1), and 2-(d-manno-pentahydroxypentyl)thiazolidine-4-carboxylic acid hydrate, Man-Cys·H₂O (2). In 1 and 2 the compounds crystallize as zwitterions, with the carboxylic groups deprotonated and the thiazolidine N atoms protonated. The sugar moiety and carboxylate group are in a cis configuration relative to the thiazolidinium ring, which adopts different conformation: twisted (T) on C^β–S in 1, and S-puckered envelope (E) in 2. The carbon chain of the galactosyl/mannosyl moiety remains in an extended zig-zag conformation. The orientation of the sugar O2 atom with respect to the thiazolidinium S and N atoms is trans–gauche in 1 and gauche–gauche in 2. The molecular conformation is stabilized by the intramolecular N–H?O_Cys contacts in both 1 and 2 and by the additional N–H?O_Man interaction in 2. The crystal packing of orthorhombic 1 and monoclinic 2 is determined mainly by N/O/C–H?O hydrogen bonds forming ribbons linked to each other by direct and water-mediated O/C–H?O/S contacts.     

Raman spectroscopic study of poly (β-benzyl-L-aspartate) and sequential polypeptides

Poly-β-benzyl-L -aspartate (poly[Asp(OBzl)]) forms either a lefthanded α-helix, β-sheet, ω-helix, or random coil under appropriate conditions. In this paper the Raman spectra of the above poly[Asp(OBzl)] conformations are compared. The Raman active amide I line shifts from 1663 cm^?1 to 1679 cm^?1 upon thermal conversion of poly[Asp(OBzl)] from the α-helical to β-sheet conformation while an intense line appearing at 890 cm^?1 in the spectrum of the α-helix decreases in intensity. The 890 cm^?1 line also displays weak intensity when the polymer is dissolved in chloroform–dichloroacetic acid solution and therefore is converted to the random coil. This line probably arises from a skeletal vibration and is expected to be conformationally sensitive. Similar behavior in the intensity of skeletal vibrations is discussed for other polypeptides undergoing conformational transitions. The Raman spectra of two cross-β-sheet copolypeptides, poly(Ala-Gly) and poly(Ser-Gly), are examined. These sequential polypeptides are model compounds for the crystalline regions of Bombyx mori silk fibroin which forms an extensive β-sheet structure. The amide I, III, and skeletal vibrations appeared in the Raman spectra of these polypeptides at the frequencies and intensities associated with β-sheet homopolypeptides. Since the sequential copolypeptides are intermediate in complexity between the homopolypeptides and the proteins, these results indicate that Raman structure–frequency correlations obtained from homopolypeptide studies can now be applied to protein spectra with greater confidence. The perturbation scheme developed by Krimm and Abe for explaining the frequency splitting of the amide I vibrations in β-sheet polyglycine is applied to poly(L -valine), poly-(Ala-Gly), poly(Ser-Gly), and poly[Asp(OBzl)]. The value of the “unperturbed” frequency, V₀, for poly[Asp(OBzl)] was significantly greater than the corresponding values for the other polypeptides. A structural origin for this difference may be displacement of adjacent hydrogen-bonded chains relative to the standard β-sheet conformation.     

13C and 1H NMR studies of helix-coil transition of poly(β-benzyl-L-aspartate) and poly(γ-benzyl-L-glutamate): Behavior in nonprotonating solvent mixtures,and origin of solvent-induced chemical shift

From the results of ¹³C-nmr measurement of poly(β-benzyl-L -aspartate) and its model compounds in dimethyl sulphoxide/deuterated chloroform mixtures, it was found that the side chain of poly(β-benzyl-L -aspartate) is solvated by dimethyl sulphoxide in the region more than dimethyl sulphoxide 20% (v/v), where the backbone maintains the α-helix. The chemical shift differences in the benzyl group carbons of poly(γ-benzyl-L -glutamate) (trifluoroacetic acid/deuterated chloroform) accompanied by the helix-coil transition, originate from the interaction between the ester group of the side chain and trifluoroacetic acid. The chemical shift difference in the ester carbon is similar. On the other hand, the chemical shift differences of the side-chain carbons in the alkyl portion (C^β, C^γ) originate not only from the interaction between the ester group of the side chain and trifluoroacetic acid, but also from some other unknown factors. The chemical shift differences of the side-chain carbons of poly(β-benzyl-L -aspartate) originate from the interaction between the ester group of the side chain and trifluoroacetic acid.     

Structure of the single-stranded polyribonucleotide poly(2'-O-methylcytidylic acid)

X-ray diffraction studies have been made on oriented polycrystalline fibres of poly(2′-O-methylcytidylic acid). A form observed at 66% relative humidity has orthorhombic (P2₁2₁2₁) symmetry and unit cell dimensions a = 1.58 nm, b = 2.16 nm, c = 1.89 nm (fibre repeat). In this form the molecules are singlestranded, 6-fold (6₁) helices with a conformation very similar to that observed for polycytidylic acid. Evidently methylation at O-2′ does not necessarily cause a change in the pucker of the sugar ring. Nor are the intermolecular hydroxyl-hydroxyl hydrogen bonds observed in the polycytidylic acid rhombohedral crystal structure necessary to maintain the 6-fold helical symmetry. However, they may be necessary to maintain the symmetry at very high relative humidities where the polycytidylic acid structure is conserved but poly(2′-O-methylcytidylic acid) undergoes a transition to a form in which the rotation per residue is reduced from 60 ° to a value near 50 °.