17O and 14N n.m.r. studies of the Co (II) interaction with cyclo(Ala*-Ala) in aqueous solution

The complexation of cyclo(Ala*-Ala) with the cobaltous ions in aqueous solution was investigated by 17O and 14N n.m.r. spectroscopy. The 17O and 14N transverse relaxation time (T2p) and chemical shift (delta omega a) of cyclo(Ala*-Ala) were measured as a function of the temperature at pH = 7.03 +/- 0.02, and pH = 6.45 +/- 0.02, and as a function of pH at room temperature. No effects of pH on the transverse relaxation time and chemical shift were observed. Complementary 17O studies of the solvent water molecules were also carried out. The hyperfine coupling constant and the entropy and enthalpy of activation for the exchange of cyclo(Ala*-Ala) and water molecules between the coordinated and noncoordinated states were determined by least-square fit of theoretical equation for the chemical shift delta omega a to experimental data. The hyperfine coupling constant of the peptide bound oxygen was determined to be (-1.6 +/- 0.1) X 10(5) Hz and the entropy and enthalpy (32.0 +/- 3.0) kJ/mol and (-12.0 +/- 1.0) e.u, respectively. Information obtained from 17O n.m.r. study allows some inferences concerning the probable coordination sphere of the cobaltous ion. There are three types of complexes: Co(H2O)6(2+), CoL X 5H2O and CoL2 X 4H2O, with relative concentrations 19.9%, 2.9%, and 77.2%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

17O-NMR studies of the conformational and dynamic properties of enkephalins in aqueous and organic solutions using selectively labeled analogues

The synthesis of Leu-enkephalin selectively 17O-enriched in Gly2 and Gly3 is reported. The 17O-nmr chemical shifts of [17O-Gly2, Leu5]- and [17O-Gly3, Leu5]-enkephalins in H2O are almost identical and independent of the pH. Since hydrogen bonding is the dominant factor governing the chemical shifts of the peptide oxygen, it can be concluded that the hydration state of both oxygens is identical and independent of the pH. The 17O chemical shifts of the [17O-Leu5]-enkephalin terminal carboxyl group at pH approximately 1.9 and 5.6 are very different in H2O but very similar in CH3CN/DMSO (4:1) solution. This suggests that the protonation state of the carboxyl group at both pH values in CH3CN/DMSO solution is the same and consequently that Leu-enkephalin exists in the neutral form at pH approximately 5.6. In this organic mixed solvent system both Gly2 and Gly3 oxygen resonances exhibit a significant shift to high frequency by the same extent (delta delta approximately 30 ppm). It is concluded that both peptide oxygens are not hydrogen bonded to an appreciable extent and that no specific 2----5 hydrogen bonding exists to an appreciable extent. This conclusion is in agreement with the energy of activation for molecular rotation, as determined from T1 measurements, which was found to be almost identical for both [17O-Gly2, Leu5]- and [17O-Gly3, Leu5]-enkephalins in CH3CN/DMSO (4:1) mixed solvent.     

Synthesis of 17O isotope labeled Leu-enkephalin and 17O n.m.r

Oxygen-17 isotope was introduced into the alpha-carboxyl group of glycine, 1-phenylalanine, 1-leucine and 1-tyrosine by acid catalyzed exchange of 17O from H2O(17) or by acid hydrolysis of respective amino acid methyl esters in H2O(17). Quantitative enrichment of glycine was achieved by acid hydrolysis of amino acetonitrile in H2O(17). For alpha-amino protection in amino acids t-butoxycarbonyl (Boc) group was employed for 17O labeled enkephalin synthesis. Five analogues of Leu-enkephalins (I-V) labeled with 17O at different amino acid residues were synthesized by solid phase method. 17O n.m.r. spectra were measured at 24.4 and 67.8 MHz for Leu-enkephalins 17O labeled at Gly2 and Phe4 positions. A downfield shift was observed for 17O labeled Gly2 Leu-enkephalin upon heating. This shift is indicative of the rupture of intramolecular hydrogen bonds. The preliminary results confirm the hypothesis that an intramolecular hydrogen bond exists between the carbonyl group of Gly2 and NH group of Leu5.     

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.     

Synthesis and conformation of cyclic hexapeptide cyclo(Pro-Sar-Sar)2

A cyclic hexapeptide, cyclo(Pro-Sar-Sar)₂, which consists of N-substituted amino acids only was synthesized, and its solution conformation was investigated by n.m.r. spectroscopy. Seven different C₂-symmetric conformations were detected, which were distinguishable from each other on the n.m.r. time scale. This is due to the cis/trans isomerization of N-substituted peptide bonds. Allowed C₂-symmetric conformations were computed on the basis of a hard-sphere model. Some conformations detected in n.m.r. spectra were not allowed in the calculation. This disagreement suggests that some asymmetric conformations with regard to the single bond rotation are averaged out due to a rapid rotation on the n.m.r. time scale. These points indicate that the molecule of cyclo (Pro-Sar-Sar)₂ is very flexible     

Conformation and complexation with metal ions of cyclic hexapeptides: cyclo (L-Leu-L-Phe-L-Pro)2 and cyclo [L-Cys(Acm)-L-Phe-L-Pro]2

Cyclic hexapeptides, cyclo (L-Leu-L-Phe-L-Pro)2 and cyclo[L-Cys(Acm)-L-Phe-L-Pro]2, in which Acm represents an acetoamide-methyl group, were synthesized, and the conformation and complexation with metal ions were investigated. Cooperation of the carbonyl groups of the Cys(Acm) side chains with those of the cyclic skeleton in complexation was especially examined. Cyclo(L-Leu-L-Phe-L-Pro)2, which possesses no functional groups on side chains, was taken as the reference compound. 13C- and two-dimensional n.m.r. measurements revealed that cyclo(L-Leu-L-Phe-L-Pro)2 and cyclo[L-Cys(Acm)-L-Phe-L-Pro]2 took a C2-symmetric conformation containing cis L-Phe-L-Pro bonds in chloroform and acetonitrile. Both cyclic hexapeptides were found to complex selectively with Ba2+ and Ca2+ in acetonitrile. On complexation the conformation of either cyclic hexapeptide changed into a similar one. However, the binding constant of cyclo[L-Cys(Acm)-L-Phe-L-Pro]2 was higher than that of cyclo(L-Leu-L-Phe-L-Pro)2. The n.m.r. measurements showed that the amide carbonyl groups of Cys(Acm) side chains as well as those of cyclic skeleton in cyclo[L-Cys(Acm)-L-Phe-L-Pro]2 cooperatively bound the cations.     

Solution conformations of two flexible cyclic pentapeptides: cyclo(Gly-Pro-D-Phe-Gly-Ala) and cyclo(Gly-Pro-D-Phe-Gly-Val).

In an effort to explore the residue preferences in three-residue reverse turns (so-called gamma-turns), two cyclic pentapeptides--cyclo(Gly1-Pro2-D-Phe3-Gly4-Ala5) (I) and cyclo(Gly1-Pro2-D-Phe3-Gly4-Val5) (II)--have been synthesized and analyzed by nmr. It was anticipated that the Gly-Pro-D-Phe-Gly portions of these molecules would favor a beta-turn conformation, leaving the remainder of the molecule to adopt a gamma turn, as seen in several previously studied model cyclic pentapeptides. The nmr data for both peptides in CDCl3 (5% DMSO-d6) and in neat DMSO-d6 indicate that the most populated conformation contains a distorted beta turn around Pro2-D-Phe3, which includes a gamma turn around D-Phe3. The distortion in the beta turn does not impede the formation of an inverse gamma turn around residue 5, and indeed, this conformation is observed in both peptides. Both the alanine and the bulkier valine residues are therefore found to be compatible with an inverse gamma turn. Molecular dynamics simulations on the title peptides are reported in the following paper. These simulations indicate that there is conformational flexibility around the D-Phe3-Gly4 peptide bond, which enables the formation of the gamma turn around D-Phe3. The third paper in this series explores the impact of a micellar environment on conformational equilibria in II.     

Oxygen-17 n.m.r. of peptides

Peptide-17O chemical shifts of linear dipeptides with and without protecting groups in H2O, CH3OH, CH2Cl2, CHCl3, CCl4, CH3CN and DMSO were between 256-350 ppm downfield from external water. Increasing solvent H-bond donating ability correlated with shifts to higher field. The 17O resonance of several cyclic dipeptides appeared at higher field relative to comparable linear dipeptides (303-317 p.p.m. vs. 327-337 p.p.m.). Separate signals were simultaneously observed by 13C and 17O n.m.r. for cis and trans N-tert.-butyl-formamide in binary mixtures with H2O, (CH3)2CO, and CCl4. The differences in the 17O nuclear screening of the amide isomers and most probably for cis and trans peptides were independent of contributions from H-bonding at the amide or peptide linkage, apparently reflecting differences between geometric isomers in electron distribution and through space effects. Peptide-17O of Gly-Ala, Gly-Leu and Gly-Glu in aqueous solution experienced upfield shifts of 6-12 p.p.m. and 12-16 p.p.m. upon deprotonation of the C-terminal COOH and of the N-terminal NH3+ groups respectively. These observations were rationalized in terms of the attendant changes in substituent effects, especially on the pi electron donating ability of the N atom at the peptide linkage and increased partial negative charge on the peptide oxygen. Temperature studies of peptide-17O of Gly-Ala between pH 1.5-9.0 revealed a chemical shift coefficient of 0.08 p.p.m./degree K and similar behavior of T1 and T2 relaxation times. Ea for molecular rotation was 5 kcal/mol between 301-331 degrees K. Rotational correlation times, tau c, were within the range expected from the Stokes-Einstein relation.     

Ring conformation of cyclic octapeptide cyclo (L-Pro-Sar)4 in the crystalline state

A single-crystal X-ray diffraction analysis has been made of the structure of the cyclic octapeptide cyclo(L-Pro-Sar)4. The material [C32H48O8N8 X (21/4) H2O X (1/2) CH3OH, Mr = 799.43] crystallizes in the monoclinic space group C2 with cell dimensions a = 14.544 (3), b = 11.902 (2), c = 14.064 (3), and beta = 122.26 (2) degrees (lambda = 1.54178 A, T = 293 K). The final R value for the 1980 observed reflections is 0.079. The ring conformation has the peptide bond sequences of cis-cis-trans-trans-cis-cis-trans-trans (Pro-Sar-Pro peptide bond linkages are cis-cis- or trans-trans). The pyrrolidine rings in the four proline residues take an envelope form in which the gamma-carbon atom deviates from the plane of the remaining four atoms in the ring.     

15N n.m.r. spectroscopy: 36. 1H n.m.r. and 15N n.m.r.spectroscopic investigation on the protonation of polypeptides

¹⁵N n.m.r. (9.12 MHz) spectra of acetamide, polyglycine, poly([l-alanine) and poly(l-leucine) were measured in various acidic solvents. These solvents include dichloroacetic acid (DCA), trifluoroacetic acid (TFA), methane sulphonic acid (MSA) and fluorosulphonic acid (FSA). Full protonation of both amides and polypeptides causes downfield shifts of 17–20 ppm. Furthermore, the concentration dependence of the chemical shift was measured. In solvents which cause partial protonation, decreasing concentration of amide groups may cause downfield shifts up to 8.5 ppm, while in the case of full protonation or in the absence of protonation no concentration dependence is observable. The protonation of peptide groups induces H/D-exchange of the αC proton which was monitored by ¹H n.m.r. spectroscopy. The mechanism of this H/D-exchange is discussed.     

Conformations of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3.

A 270-MHz 1H nuclear magnetic resonance investigation of an ion-binding cyclic peptide analogue of valinomycin, cyclo(L-Val-Gly-Gly-L-Pro)3, and its cation complexes is reported. In CD2Cl2 and CDCl3, the peptide is proposed to occur in a C3-symmetric conformer with the N--H's of all six glycine residues intramolecularly hydrogen bonded. This conformation is different from the familiar valinomycin bracelet structure and lacks any "cavity". Cations do not bind, or bind only weakly, to the peptide in these solvents. Uncomplexed cyclo(L-Val-Gly-Gly-L-Pro)3 in acetonitrile appears to be averaging among several conformations with no evidence found for any preferred intramolecular hydrogen bonds. The strong 1:1 complexes of cyclo(L-Val-Gly-Gly-L-Pro)3 with K+ ANd Ba2+ in acetonitrile are structurally analogous to the bracelet conformation of valinomycin and involve the N--H's of the Val residues and of the Gly's preceding Pro in intramolecular hydrogen bonding. Tl+ was also found to form strong 1:1 complexes with the dodecapeptide.     

Crystal and molecular structure of cyclo(L-prolyl-glycyl)3. A cyclic hexapeptide with a cis peptide bond

The crystal structure of cyclo(L-Pro-Gly)3 was solved using X-ray crystallographic techniques. The backbone of the peptide is asymmetric and is made up of five trans peptide units and one cis peptide. There is a hydrogen bonded water bridge that links the carbonyl oxygens, O1 and O4. The molecules exist as dimers in the crystal lattice. The two molecules of the dimer are related by crystallographic twofold symmetry and are linked by two N-H ... O hydrogen bonds. The crystals are trigonal, space group P3(2)12 with a = 11.379(3), c = 32.93(1) and z = 6. The structure was solved by multisolution methods and refined by least squares technique to an R of 0.083.     

Structural recognition of an ICAM-1 peptide by its receptor on the surface of T cells: conformational studies of cyclo (1, 12)-Pen-Pro-Arg-Gly-Gly-Ser-Val-Leu-Val-Thr-Gly-Cys-OH.

The purpose of this study is to elucidate the solution conformation of cyclic peptide 1 (cIBR), cyclo (1, 12)-Pen1-Pro2-Arg3-Gly4-Gly5-Ser6-Val7-Leu8-V al9-Thr10-Gly11-Cys12-OH, using NMR, circular dichroism (CD) and molecular dynamics (MD) simulation experiments. cIBR peptide (1), which is derived from the sequence of intercellular adhesion molecule-1 (ICAM-1, CD54), inhibits homotypic T-cell adhesion in vitro. The peptide hinders T-cell adhesion by inhibiting the leukocyte function-associated antigen-1 (LFA-1, CD11a/CD18) interaction with ICAM-1. Furthermore, Molt-3 T cells bind and internalize this peptide via cell surface receptors such as LFA-1. Peptide internalization by the LFA-1 receptor is one possible mechanism of inhibition of T-cell adhesion. The recognition of the peptide by LFA-1 is due to its sequence and conformation; therefore, this study can provide a better understanding for the conformational requirement of peptide-receptor interactions. The solution structure of 1 was determined using NMR, CD and MD simulation in aqueous solution. NMR showed a major and a minor conformer due to the presence of cis/trans isomerization at the X-Pro peptide bond. Because the contribution of the minor conformer is very small, this work is focused only on the major conformer. In solution, the major conformer shows a trans-configuration at the Pen1-Pro2 peptide bond as determined by HMQC NMR. The major conformer shows possible beta-turns at Pro2-Arg3-Gly4-Gly5, Gly5-Ser6-Val7-Leu8, and Val9-Thr10-Gly11-Cys12. The first beta-turn is supported by the ROE connectivities between the NH of Gly4 and the NH of Gly5. The connectivities between the NH of Ser6 and the NH of Val7, followed by the interaction between the amide protons of Val7 and Leu8, support the presence of the second beta-turn. Furthermore, the presence of a beta-turn at Val9-Thr10-Gly11-Cys12 is supported by the NH-NH connectivities between Thr10 and Gly11 and between Gly11 and Cys12. The propensity to form a type I beta-turn structure is also supported by CD spectral analysis. The cIBR peptide (1) shows structural similarity at residues Pro2 to Val7 with the same sequence in the X-ray structure of D1-domain of ICAM-1. The conformation of Pro2 to Val7 in this peptide may be important for its binding selectivity to the LFA-1 receptor.     

Antiparallel pleated beta-sheets observed in crystal structures of N,N-bis(trichloroacetyl) and N,N-bis(m-bromobenzoyl) gramicidin S.

Despite intensive efforts, the structures of gramicidin S (GS) [cyclo(-Val-Orn-Leu-d-Phe-Pro-)(2)] and its analogues have not been elucidated by the X-ray diffraction method, except for the GS-urea complex (Hull et al., Nature 275, 206-207, 1978; Tishchenko et al., Acta Cryst. D53, 151-159, 1997). We focused on the acetylation of GS to obtain suitable crystals for X-ray diffraction. The amino groups of Orn residues were capped with trichloroacetic and m-bromobenzoic acids. Both trichloroacetyl and m-bromobenzoyl GSs (TcGS and BzGS, respectively) are hydrophobic and their properties are similar to those of acetyl-GS (AcGS). Although it is well known that AcGS yields hexagonal crystals, TcGS and BzGS yield monoclinic and orthorhombic crystals in aqueous dimethylformamide solution, respectively. Their cell volumes were approximately one-fourth or one-eighth of the hexagonal cell volume. The crystal structures of TcGS and BzGS were determined as the first examples of acetylated GS analogues: TcGS, C(64)H(90)N(12)O(12)Cl(6). 3(C(3)H(7)NO), M(r) = 1651.47, monoclinic, P2(1), a = 15.4366(6) A, b = 18.5312(4) A, c = 16.4774(6) A, beta = 14.160(2) degrees, V = 4300.6(2) A(3), Z = 2; and BzGS, C(64)H(98)N(12)O(12)Br(2). 1.54(H(2)O), M(r) = 1535.21, orthorhombic, P2(1)2(1)2(1), a = 16.748(10) A, b = 18.834(5) A, c = 28.558(10) A, V = 9008(7) A(3), Z = 4. Both these peptide molecules formed an antiparallel pleated beta-sheet, and pseudo twofold symmetries existed in the repeated sequence. beta-Turns formed at the fragments of d-Phe-Pro were classified into type II' based on their characteristics. The peptide conformations of TcGS and BzGS were similar to each other, and these structural features agreed with those of structures proposed by the previous studies.     

Specific binding to adrenal particulate fraction of cyclo(histidyl-proline), a TRH metabolite

Histidyl-proline diketopiperazine (cyclo(His-Pro), a metabolite of the neuropeptide thyrotropin releasing hormone, has been shown to possess intrinsic biological activities. The binding of this peptide to various tissue particulate preparations was investigated. While the peptide showed no apparent binding to particulate fractions derived from brain, pituitary, and some other tissues, binding to adrenal and liver was demonstrated. The binding of cyclo(His-Pro) to bovine adrenal cortical particles was further characterized. Binding at equilibrium was greater at 4 degrees C than at 37 degrees C. The binding was dependent on tissue concentration, showed a pH optimum between 7 and 8, and was inactivated by treatment of the particulate fraction with trypsin or by boiling. The interaction of cyclo(His-Pro) with the tissue was not associated with any metabolism of the peptide. Kinetic studies of association of cyclo(His-Pro) with adrenal cortical particles indicated a single class of binding sites with a KD of approximately 900 nM and a maximum number of sites of 92 pmoles/mg protein. The binding was stereospecific and the histidine moiety of the peptide was the major determinant of the binding. A variety of catechols, serotonin and histamine competed with cyclo(His-Pro) for binding with IC50's ranging from 17-450 muM. Cyclo(His-Pro) did not affect monoamine oxidase or adenylate cyclase activity in adrenal cortical particulate preparations.     

Conformations of cyclic octapeptides. 4. Diastereomers of cyclo(Gly-Pro-Phe-Ala-Asn-Ala-Val-Ser)

Stereoisomers of cyclo(Gly-Pro-Phe-Ala-Asn-Ala-Val-Ser) were synthesized. NMR studies of their solution conformations, focusing on peptide N-H solvent exposure, were made. These indicated that a single proline residue in the cyclic octapeptide ring is insufficient constraint to stabilize the backbone conformations that were previously established for cyclo(Gly-Pro-Phe-Ala)2.     

Peptide conformation. 48. Conformation and biological activity of proline containing cyclic retro-analogues of somatostatin

Six cyclic retro-analogues of the peptide hormone somatostatin have been synthesized using the solid phase technique. The peptides cyclo(-Xaa1-Phe2-Thr3-Lys4-Ybb5-Phe6-) and cyclo(-Phe1-Xaa2-Thr3-Lys4-Ybb5-Phe6-) with Xaa = D- or L-Pro and Ybb = D- or L-Trp were cyclized via the azide method. The conformations of the cyclic hexapeptides in DMSO-d6 solution were determined by a number of homo- and heteronuclear two-dimensional n.m.r.-techniques including 2D rotating frame NOE-spectroscopy. Two-step coherence transfers, ROE and chemical exchange, are observed for the first time in ROESY spectra. The backbone conformation of the all-trans cyclopeptides consists of a beta-turn containing the Pro residue in the position i + 1. These retro-analogues of somatostatin exhibit a high activity in the inhibition of cholate and phalloidin uptake by liver cells (cytoprotective effect); however, the hormonal activities of the natural hormone are completely suppressed. The constitutional and conformational requirements for the cytoprotective activity are discussed.     

Two-dimensional proton NMR studies on poly(VPGVG) and its cyclic conformational correlate, cyclo(VPGVG)3

Two-dimensional nuclear Overhauser enhancement (2D NOESY) data are reported for the polypentapeptide of elastin, poly(VPGVG), and the cyclopentadecapeptide, cyclo(VPGVG)3. In both, the repeating type II Pro2-Gly3 beta-turn can be derived from the NOE data, providing confirmation of many previous studies. In addition, other through-space connectivities are detailed that also compare favorably with previously determined crystal and solution structures for cyclo(VPGVG)3. Also, near identical data for the cyclopentadecapeptide and the polypentapeptide demonstrate the cyclic conformation-linear (helical) conformational correlate relationship between the two molecules. The 2D NOESY experiment is seen to be an effective means of establishing the presence or absence of a conformational relationship between a cyclic repeating sequence and its higher molecular weight linear counterpart. This is an approach of substantial practical value when developing the conformation of sequential polypeptides and when attempting to identify the presence of the conformation of a repeating peptide sequence within a more complex primary structure. Having established the basic conformational relationship between a cyclic conformation and its linear helical counterpart, cross peaks present in the linear helical structure that are not present in the cyclic conformational correlate can provide information on the interactions between adjacent turns of the helix. In this connection, a Val gamma CH3 in equilibrium Pro beta CH2 interaction is reported that can be the basis for determining the number of pentamers per turn of helix once it is determined whether it is dominantly the Val1 or Val4 gamma CH3 that is interacting with the Pro2 beta CH2.     

14,15N, 13C, 57Fe, and 1,2H Q-band ENDOR study of Fe-S proteins with clusters that have endogenous sulfur ligands.

The benefits of performing ENDOR experiments at higher microwave frequency are demonstrated in a Q-band (35 GHz) ENDOR investigation of a number of proteins with [nFe-mS] clusters, n = 2, 3, 4. Each protein displays several resonances in the frequency range of 0-20 MHz. In all instances, features are seen near v approximately 13 and 8 MHz that can be assigned, respectively, to "distant ENDOR" from 13C in natural-abundance (1.1%) and from 14N (the delta m1 = +/- 2 transitions); the nuclei involved in this phenomenon are remote from and have negligible hyperfine couplings to the cluster. In addition, a number of proteins show local 13C ENDOR signals with resolved hyperfine interactions; these are assigned to the beta carbons of cysteines bound to the cluster [A(13C) approximately 1.0 MHz]. Five proteins show resolved, local delta m1 = +/- 2 ENDOR signals from 14N with an isotropic hyperfine coupling, 0.4 less than or equal to A(14N) less than or equal to 1.0, similar to those seen in ESEEM studies; these most likely are associated with N-H...S hydrogen bonds to the cluster. Anabaena ferredoxin further shows a signal corresponding to A(14N) approximately 4 MHz. Quadrupole coupling constants are derived for both local and distant 14N signals. The interpretation of the data is supported by studies on 15N- and 13C-enriched ferredoxin (Fd) from Anabaena 7120, where the 15N signals can be clearly correlated with the corresponding 14N signals and where the 13C signals are strongly enhanced. Thus, the observation of 14N delta m1 = +/- 2 signals at Q-band provides a new technique for examining weak interactions with a cluster.(ABSTRACT TRUNCATED AT 250 WORDS)     

Molecular structure of cyclo[-(D-Val-L-Hyi-L-Val-D-Hyi)2-] revealed by x-ray analysis.

The crystal structure of a synthetic analogue of valinomycin, cyclo[-(D-Val-L-Hyi-L-Val-D-Hyi)2-] (octa-meso-valinomycin) (I) (C40H68N4O12.1.5.C4H8O2, M(r) = 937.01 + 88.10), has been determined. Crystals grown from dioxane are monoclinic, space group P2(1)/a, with cell parameters a = 21.487 (8), b = 16.836 (5), c = 16.089 (4) A, beta = 111.70 (4), and Z = 4. The atomic coordinates for nonhydrogen atoms were refined in the anisotropic thermal motion approximation. H atom positions were included in the structure factor calculations at their geometrically expected positions. Values of the standard and weighted R factors after refinement are 0.11 and 0.13, respectively. The conformation of the depsipeptide crystallized from dioxane is different from that crystallized from chloroform (II). The molecule adopts a rectangular shape with two type IV beta-turns containing a hydrogen bond and possesses pseudorotational symmetry. The side chains are located on the molecular periphery. The orientation of the carbonyl groups of the molecule is not conducive for efficient metal-ion coordination and in the observed conformation cannot behave as an ionophore. In the crystal the molecules form infinite chains parallel to the c axis, and are stabilized by two intermolecular hydrogen bonds that are shorter and have better geometry than the intramolecular hydrogen bonds. A phi/psi plot for dodecadepsipeptides with a (DLLD)3 sequence has well-defined areas for Val and Hyi residues only in cases when the crystals have been grown from nonpolar or medium-polar solvents. The phi/psi plot for octadepsipeptides crystallized from chloroform (II) shows this behavior also.(ABSTRACT TRUNCATED AT 250 WORDS)