Studies on the conformation of amino acids. IX. Conformations of butyl, seryl, threonyl, cystennyl, and valyl residues in a dipeptide unit

Potential energies of conformation of a dipeptide unit with butyl, seryl, threonyl, eysteinyl, and valyl side groups have been computed by using classical energy expressions. The presence of a γ-atom introduces characteristic restrictions on the backbone rotational angles ? and ψ the γ-atom itself is restricted to three staggered positions about the C^α—C^β bond. The important results are that a γ-carbon in position I (χ¹ ? 60°) cannot be accommodated in the standard right-and left-handed α-helices, whereas a γ-oxygen or sulfur could easily be accommodated in the right-handed α-helix. Further, a γ-carbon or a heteroatom in position II (χ¹ ? 180°) does not favor a conformation ψ ? 180°, compared to two other positions. The valyl side group significantly reduces the allowed ? and ψ values and energetically prefers a β-conformation compared to right-or left-handed α-helical conformations. The less favorable α-helical conformation is possible only for γ (III, II) combination of the valyl residue. The observed ?, ψ, and χ¹ values of all the amino acid residues in the three protein molecules, lysozyme, myoglobin, and chymotrypsin are compared with the theoretical predictions and the agreement is excellent. The results bring out the important fact that even in large molecules, the conformation of local segments are predominantly governed by the short-range intramolecular interactions.     

Crystal structures of Boc-D- and L-Iva-L-Pro-OBZl: Unturned conformation of Aib-Pro sequence unaffected by replacement of Me with Et in Aib

The crystal structures of the isovaline (Iva) containing dipeptides, Boc-D -Iva-L -Pro-OBz l and Boc-L -Iva-L -Pro-OBz l, were determined by x-ray diffraction. The diastereomeric peptides were shown to adopt unturned conformations closely similar to each other (?_Iva 52°, ψ_Iva 46°, ?_Pro–65°, and ψ_Pro 143° for D -Iva-L -Pro sequence and ?_Iva 52°, ψIva 44°, ?_Pro ?63°, and ψ_pro 148° for L -Iva-L -Pro sequence). The Pro ring of each peptide was in C^γ-endo conformation. The unusually large ∠C_Iva-N_Pro-C values (131° in both peptides) were observed, that was due to steric repulsion between the δ-methylene of Pro and the alkyl side chain of Iva residue. These conformations were essentially the same as that of the corresponding α-aminoisobutyric acid (Aib)-containing peptide Boc-Aib-L -Pro-OBz l. The result has demonstrated that replacement of either one of the two methyl groups of the Aib residue in Boc-Aib-L -Pro-OBz l with an ethyl group does not cause any significant change in the unturned conformation of the dipeptide. © 1993 John Wiley & Sons, Inc.     

Conformational Flexibility of Choline Derivatives

CHOLINERGIC substrates have been found in a gauche conformation (G), skewed about the C_α–C_β bond of the (CH₃)₃N⁺–CH₂–CH₂–O cholinic fragment in a number of crystal structures^1–11. Sulphur and selenium isologues, on the other hand, with the (CH₃)₃N–CH₂–CH₂–(S,Se) group, are normally in the extended trans conformations (T)^10,12,13. In agreement with the assumption that the reduced spectrum of biological activity of many rigid analogues and C_α or C_β substituted derivatives of acetylcholine can be partially ascribed to the reduction in conformational flexibility^14–17, a theoretical investigation¹⁸ predicted the existence of four almost isoenergetic conformations TTTT, TGTT, TTGT and TGGT about the ψ₀, ψ₁, ψ₂ and ψ₃ internal rotation angles schematically represented in Fig. 1.     

Stereochemistry of nucleic acids and their constituents. IV. Allowed and preferred conformations of nucleosides,nucleoside mono-, di-, tri-, tetraphosphates,nucleic acids and polynucleotides

A uniform notation and convention is suggested to describe the torsional angles in nucleic acids and their derivatives. The torsional angle χ, relating the stereochemistry of the base with respect to the sugar, shows more variation for the β-purine glycosides than for the β-pyrimidine glycosides. This variation is attributed to the fact that the β-purine derivatives may form intramolecular O(5′)-H…N(3) hydrogen bonding. The χ values for the α-purine and α-pyrimidine glycosides show preference for the –syn-clinal (or anti) conformation. The mode of puckering of the sugar also influences the χ value. The various possible conformations for the furanose ring are described by the torsional angles τ₀ τ₁, τ₂, τ₃, τ₄, about the five ring bonds. From an analysis of the torsional angles (ω, ?, ψ, ψ′, ?′, ω′) about the sugar phosphate bonds in the x-ray structures of the known nucleosides, nucleotides, phosphodiesters, nucleic acids, and related compounds, and from a consideration of molecular models, it is found that the possible conformations for the backbone of helical nucleic acids is strikingly limited. Most importantly, the preferred conformation of the nucleotide unit in poly nucleotides and nucleic acids turns out to be the same as that found for the nucleotide in the crystal structure. It is observed that base “stacking” is a consequence of the restricted backbone conformation. The torsional angles are illustrated in the form of conformational “wheels”. Interrelation between the torsion angles about successive pairs of sugar-phosphate bonds are presented in the form of conformational maps: ω,?; ?,ψ; ψ.ψ′; ψ′,?′; ?′,ω′; ω′,ω. The ω′,ω map shows the perferred conformations about the inter-nucleotide bonds of right- and left-handed helices and the possible conformations of phosphodiesters. The preferred conformation of the pyrophosphate and triphosphate is that in which the phosphate oxygens display a staggered arrangement when viewed along the P–P axis. A plausible structure and conformation for the ATPM^2? backbound complex is presented. This structure differs from that proposed by SzentGyorgi in that the metal (only transition metals are considered here) is not bound to the NH₂ nitrogen of adenine, but rather is simultaneously bound to N(7) of the ring and three phosphates (α, β, γ), or N(7) of the ring and two phosphates (β, γ). The remaining metal coordination may be satisfied by solvent–metal or enzyme–metal bonds.     

N.m.r. studies of the structures and bonding of some cobalt(II) and nickel(II) complexes of carboxylic acids

Some octahedral complexes of types ML₄T₂ and ML₂T₂, where M = Co^II, Ni^II, L = γ-picoline and T = C₂F₅CO₂, C₃F₇CO₂ and CHCl₂CO₂, have been studied by ¹H and ¹⁹F n.m.r. spectroscopy. The complexes exist in solution in both cis and trans isomeric forms, interconversion of the two forms being slow at ambient temperature in the case of the Ni^II complexes. The appreciable isomeric distinction shown in the ¹⁹F spectra of the acid ligands in contrast to the ¹H spectra of the γ-picoline ligands is attributed to a sizeable π-contact interaction of the acid ligands, particularly when they act as bidentate ligands.     

HYBRIDS AND INDUCED AMPHIPLOIDS OF ELYMUS CANADENSIS X AGROPYRON LIBANOTICUM

North American Elymus canadensis L., 2n = 28, and Asian Agropyron libanoticum Hack., 2n = 14, crossed with ease and yielded vigorous but sterile F₁ hybrids, 2n = 21. Chromosome pairing in the hybrids averaged 9.47^I, 5.38^II, and 0.26^III in 150 metaphase-I cells. One genome of E. canadensis is more or less homologous with the A. libanoticum genome. Treatment of the F₁ hybrids with colchicine produced 42-chromosome amphiploids, C₀, which were advanced through two seed generations, C₁ and C₂. More than half of the metaphase-I cells in the C₀ amphiploids contained 21^II; and average associations were 1.09^I, 20.16^II, 0.07^III, and 0.09^IV in 116 cells. Meiosis became increasingly irregular beyond metaphase-I; nevertheless, the C₀ amphiploids produced 68% stainable pollen and averaged 0.75 seed per spikelet. Multivalent frequencies increased in advanced generations, and the C₂ amphiploids averaged 1.11^I, 19.00^II, 0.23^III, and 0.55^IV in 100 metaphase-I cells. Meiosis was essentially regular in the C₁ and C₂ amphiploids beyond metaphase I, and the C₂ amphiploids averaged 73% stainable pollen and 2.28 seeds per spikelet. The amphiploids have an excellent chance of developing into a meiotically stable, fertile, new species. Forage characteristics of the amphiploids indicate that they have considerable economic potential as a forage grass.     

Nature of the Ln-Co magnetic interactions in compounds [Ln2Co3(C5H4NPO3)6] · 4H2O with open-framework structures

The reactions of Ln(NO₃)₃ · xH₂O, CoSO₄ · 7H₂O or ZnSO₄ · 6H₂O and 2-pyridylphosphonic acid under hydrothermal conditions result in heterometallic phosphonate compounds with formula [Ln₂M₃(C₅H₄NPO₃)₆] · 4H₂O (Ln₂M₃; M = Co^II or Zn^II; Ln = La^III, Ce^III, Pr^III, Nd^III, Sm^III, Eu^III, Gd^III, Tb^III, Dy^III). These compounds are isostructural and crystallize in a chiral cubic space group I2₁3. Each structure contains the {LnO₉} polyhedra and {MN₂O₄} octahedra which are connected by edge-sharing to form an inorganic open-framework structure with a 3-connected 10-gon (10, 3) topology. The nature of Ln^III-Co^II magnetic interactions in Ln₂Co₃ is investigated by a comparison with their Ln^III-Zn^II analogues. It is found that the Ln^III-Co^II interaction is weak antiferromagnetic for Ln = Ce and ferromagnetic for Ln = Sm, Gd, Tb and Dy. In the cases of Ln = Pr, Nd and Eu, no significant magnetic interaction is observed.     

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.     

Nuclear magnetic resonance study of the rate of electron transfer between cytochrome c and iron hexacyanides

The rates of electron exchange between ferricytochrome c (C^III)3 and ferrocytochrome c (C^II) were observed as a function of the concentrations of ferrihexacyanide (Fe^III) and ferrohexacyanide (Fe^II) by monitoring the line widths of several proton resonances of the protein. Addition of Fe^II to C^III homogeneously increased the line widths of the two downfield paramagnetically shifted heme methyl proton resonances to a maximal value. This was interpreted as indicating the formation of a stoichiometric complex, C^III·Fe^II, in the over-all reaction:

$\text{C}{}^{\mathrm{III}}\text{+}\text{Fe}{}^{\mathrm{II}}\text{?}\text{k}{}_{\mathrm{?1}}\text{k}{}_1\text{C}{}^{\mathrm{III}}\text{·}\text{Fe}{}^{\mathrm{II}}\text{?}\text{k}{}_{\mathrm{?2}}\text{k}{}_2\text{C}{}^{\mathrm{II}}\text{·}\text{Fe}{}^{\mathrm{III}}\text{?}\text{k}{}_{\mathrm{?3}}\text{k}{}_3\text{C}{}^{\mathrm{III}}\text{+}\text{Fe}{}^{\mathrm{II}}$

Values for $\text{k}{}_1\text{k}{}_{\mathrm{?1}}\text{= 0.4 × 10}{}^3\text{m}{}^{\mathrm{?1}}\text{and}\text{k}{}_2\text{= 208}\text{s}{}^{\mathrm{?1}}$, respectively, were calculated from the maximal change in line width observed at pH 7.0 and 25 °C. Changes in the line width of C^III in the presence of Fe^II and either KCl or Fe^III suggest that complexation is principally ionic, that Fe^III and Fe^II compete for a common site. Addition of saturating concentrations of Fe^III to C^III produced only minor changes in the nuclear magnetic resonance spectrum of C^III suggesting that complexation occurs on the protein surface.Addition of Fe^III to C^II in the presence of excess Fe^II (to retain most of the protein as C^II) increased the line width of the methyl protons of ligated methionine 80. A value for k_?2 ≈ 2.08 × 10⁴ s^?1 was calculated from the dependence of linewidth on the concentration of Fe^II at 24 °C. These rates are shown to be consistent with the over-all rates of reduction and oxidation previously determined by stopped flow measurements, indicating that k₂ and k_?2 were rate limiting. From the temperature dependence the enthalpies of activation are 7.9 and 15.2 kcal/mol for k₂ and k_?2, respectively.     

Endothelial cell phagokinesis in response to specific metal ions

Salts of Cu^{I, II}, Ni^II, Sn^II, In^III and a sub-fraction of the cupoprotein ceruloplasmin induced phagokinesis of cultured aortal endothelial cells. A variant aortal endothelial cell line was highly sensitive; cells travelled up to 1000 μm in 24 h in response to 2× 10^?6 M SnCl₂. Other metal ions tested (Zn^II, Co^II, Mn^II, Cr^II, Fe^III, Al^III, Sb^III and Mo^II) were not active. The motility response of endothelial cells to Cu ions in vitro is proposed as a model system for studying early events in neovascularization and as a sensitive assay for detecting angiogenic activity in fractions from cells and tissues.     

Nicotianamine chelates both FeIII and FeII. Implications for metal transport in plants

Nicotianamine (NA) occurs in all plants and chelates metal cations, including Fe^II, but reportedly not Fe^III. However, a comparison of the Fe^II and Zn^II affinity constants of NA and various Fe^III-chelating aminocarboxylates suggested that NA should chelate Fe^III. High-voltage electrophoresis of the FeNA complex formed in the presence of Fe^III showed that the complex had a net charge of 0, consistent with the hexadentate chelation of Fe^III. Measurement of the affinity constant for Fe^III yielded a value of 10^20.6, which is greater than that for the association of NA with Fe^II (10^12.8). However, capillary electrophoresis showed that in the presence of Fe^II and Fe^III, NA preferentially chelates Fe^II, indicating that the Fe^IINA complex is kinetically stable under aerobic conditions. Furthermore, Fe complexes of NA are relatively poor Fenton reagents, as measured by their ability to mediate H₂O₂-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2′-deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2′-deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed.     

Studies on copper(II) complexes of o-quinone monooximes. 4. Interaction between aquobis(1,2-naphthoquinone 1-oximato)copper(II) and lanthanide(III) ions. New heteropolynuclear complexes containing CuII and LnIII

By reacting aquobis(1,2-naphthoquinone 1-oximato)copper(II) [Cu(nqo)₂·H₂O] with lanthanide chlorides, new heteropolynuclear complexes containing both Cu^II and Ln^III (Ln^III = La^III, Nd^III) were obtained. The compounds have been characterized by elemental and thermogravimetric analysis, electron microprobe analysis, and electronic and vibrational spectral data. A different Cu^II complex, containing nqo ligands and ionic perchlorate but no lanthanide ions, was obtained by reaction of Cu(nqo)₂·H₂O with lanthanide perchlorates.     

Conformational studies of N-acetyl-N′-methylamide derivatives of α-aminobutyric acid,norvaline, and valine. I. Preferred conformations in solution as studied by 1H-nmr spectroscopy

The ¹H-nmr studies were extensively carried out to elucidate preferred conformations of dipeptides CH₃C*O—X—NHCH₃, with X = Abu, nVal, and Val in various solvents. The vicinal ¹H—¹H coupling constants for the NH—C^αH moiety and those around the C^α—C^β bond in the articulated side chain provided the information regarding the average conformation of these molecules. The results indicate that transformation of skeletal conformations takes place in solution among conformers having similar dihedral angles, θ, in the Karplus expression.     

A new class of clear mutants from coliphage 434 hy not complementing CII and C3 mutants for lysogenization

Summary Clear mutants which differ from regular C _I , C _II , CIIIand y mutants have been isolated from phage 434 hy. These mutants resemble C _I mutants in plaque and spot phenotype but efficiently complement C _I mutants for lysogenization. Like C _II mutants, they do not complement authentic C _II mutants for lysogenization but in contrast to C _II mutants they also fail to complement C _III mutants. They map between the lambda-434 non-homology region and Co ₁ (aC _II mutant). On account of this map position adjacent to C _II the mutants of the new type are called C _IIa . They arise from phage 434 hy with a frequency comparable to that of C _I and C _II mutants. Such mutants are also obtained from phage lambda but apparently not from phage b₅. C _IIa mutants would not fit into a picture of three independently acting cistrons C _I , C_II, and C _III . The hypothesis is presented that C _IIa and C _II mutants are in the same structural gene. Two possibilities are discussed that would account for the complementation patterns: 1. C _IIa mutants may block the expression of gene C _III in cis position; or 2. the products of genes C _II and C _III function through an oligomeric complex they form.     

Syntheses, structures and antimicrobial activities of various metal complexes of hinokitiol

Metal-oxygen bonding complexes (M = Mg^II, Mn^II, Ni^II, Mo^VI, W^VI, Pd^II, Sb^III, Bi^III, Fe^III, Ti^IV, K^I, Ba^II, Zr^IV and Hf^IV) with a hinokitiol (Hhino; 2-hydroxy-4-isopropylcyclohepta-2,4,6-trienone or β-thujaplicin) ligand, which has two unequivalent oxygen donor atoms, were synthesized and characterized by elemental analysis, TG/DTA, FT-IR and solution (¹H and ¹³C) NMR spectroscopy. Single-crystal X-ray structure analysis revealed various molecular structures for the complexes, which were classified into several families of family, i.e. type A [M^II(hino)₂(L)]₂ (M = Mg^II, Mn^II, Ni^II; L = EtOH or MeOH), with a dimeric structure consisting of one bridging hino⁻ anion, one chelating hino⁻ anion and one alcohol or water molecule, type B, with the octahedral, cis-dioxo, bis-chelate complexes cis-[M^VIO₂(hino)₂] (M = Mo^VI, W^VI), type C, with square planar complex [M^II(hino)₂] (M = Pd^II), type D, with tris-chelate, 7-coordinate complexes with one inert electron pair [M^III(hino)₃] (M = Sb^III, Bi^III), type D′, with the bis-chelate, pseudo-6-coordinate complexes with one inert electron pair [M^III(hino)₂X] (M = Sb^III, X = Br), type E, with tris-chelate, 6-coordinate complexes with Δ and Λ isomers [M^III(hino)₃] (M = Fe^III), type E′ of bis-chelate, 6-coordinate complex [M^IV(hino)₂X₂] (M = Ti^IV, X = Cl), type F, with water-soluble alkali metal salts [M^I(hino)] (M = K^I), and type H, with tetrakis-chelate, 8-coordinate complexes [M^IV(hino)₄](M = Zr^IV, Hf^IV). These structural features were compared with those of metal complexes with a related ligand, tropolone (Htrop). The antimicrobial activities of these complexes, evaluated in terms of minimum inhibitory concentration (MIC; μg mL⁻¹) in two systems, were compared to elucidate the relationship between structure and antimicrobial activity.     

Molecular orbital studies on the conformation of phospholipids. II. Preferred conformations of hydrocarbon chains and molecular organization in biomembranes.

The preferred conformations of the nonpolar β and γ (hydrocarbon) chains in phospholipids have been derived using EHT and CNDO calculations. These calculations indicate that the possible conformations of phospholipids are highly restricted. The calculations find support from X-ray diffraction studies and NMR measurements on model compounds. When considering conformations relevant to structures in cell membranes, a further selection is possible because of the fact that in aqueous solutions hydrophobic interactions stabilize an arrangement where the hydrocarbon chains (β and γ) are stacked almost parallel to one another, leading to a bilayer structure. The various models for β and γ-chains which satisfy this condition have been compared and it has been shown that of these only four are favoured by energy considerations. These arrangements differ from one another in the orientation of the β-chain and γ-chains in the interior of the bilayer structure. A low energy pathway connects these conformations and thus the molecule can easily flip from one stable bilayer arrangement to another. The possible conformations of the polar group (α) are likewise restricted. The proposed model provides explanations to a number of dynamic and static properties of phospholipids, in particular to the observed NMR coupling constants, ¹H and ¹³C relaxation times, studies based on ESR spin labels and the observed X-ray diffraction results on model compounds.     

Investigating proline puckering states in diproline segments in proteins

In the current study, the puckering states of the Proline ring occurring in diproline segments (^LPro‐^LPro) in proteins has been investigated with a segregation made on the basis of cis and trans states for the Pro‐Pro peptide bond and the conformational states for the diproline segment to investigate the effects of conformation of the diproline segment on the corresponding puckering state of the Proline ring in the segment if any. The value of the endocyclic ring torsional angles of the pyrrolidine ring has been used for calculating and visualizing various puckering states using a proposed new sign convention (+/?) nomenclature. The results have been compared to that obtained in a previous study on peptides from this group. In this study, quite interestingly, the Planar (G) conformation that was present in 14.3% of the cases in peptides, appears to be nearly a rare conformation in the case of proteins (1.9%). The present study indicates that the (C^γ‐exo/C^γ‐exo), (C^γ‐exo/Twisted C^γ‐exo‐C^β‐endo) and (Twisted C^γ‐endo‐C^β‐exo/Twisted C^γ‐endo‐C^β‐exo) categories are the most preferred combinations. For Proline rings in proteins, the states C^γ‐exo, Twisted C^γ‐exo‐C^β‐endo and Twisted C^γ‐endo‐C^β‐exo are the most preferred states. Within diproline segments, the pyrrolidine ring conformations do not show a strong co‐relation to the backbone conformation in which they are observed. It is likely that five‐membered rings have a considerable plasticity of structure and are readily deformed to accommodate a variety of energetically preferred backbone conformations. © 2013 Wiley Periodicals, Inc. Biopolymers 99: 605–610, 2013.     

The inhibitory effect of some new synthesized xanthates on mushroom tyrosinase activities

Three iso-alkyldithiocarbonates (xanthates), as sodium salts, C₃H₇OCS₂Na (I), C₄H₉OCS₂Na (II) and C₅H₁₁OCS₂Na (III), were synthesized, by the reaction between CS₂ with the corresponding iso-alcohol in the presence of NaOH, and examined for inhibition of both cresolase and catecholase activities of mushroom tyrosinase (MT) from a commercial source of Agricus bisporus. 4-[(4-methylbenzo)azo]-1,2-benzendiol (MeBACat) and 4-[(4-methylphenyl)azo]-phenol (MePAPh) were used as synthetic substrates for the enzyme for the catecholase and cresolase reactions, respectively. Lineweaver-Burk plots showed different patterns of mixed and competitive inhibition for the three xanthates and also for cresolase and catecholase activities of MT. For cresolase activity, I and II showed a mixed inhibition pattern but III showed a competitive inhibition pattern. For catecholase activity, I showed mixed inhibition but II and III showed competitive inhibition. These new synthesized compounds are potent inhibitors of MT with K_i values of 9.8, 7.2 and 6.1 μM for cresolase inhibitory activity, and also 12.9, 21.8 and 42.2 μM for catecholase inhibitory activity for I, II and III, respectively. They showed a greater inhibitory potency towards the cresolase activity of MT. Both substrate and inhibitor can be bound to the enzyme with negative cooperativity between the binding sites (α>1) and this negative cooperativity increases with increasing length of the aliphatic tail in these compounds in both cresolase and catecholase activities. The cresolase inhibition is related to the chelating of the copper ions at the active site by a negative head group (S^? ) of the anion xanthate, which leads to similar values of K_i for all three xanthates. Different K_i values for catecholase inhibition are related to different interactions of the aliphatic chains of I, II and III with hydrophobic pockets in the active site of the enzyme.     

Conformations of cyclic peptide/calcium complexes in solution

Synthetic cyclic octapeptides of general structure cyclo[Glu(γOBzl)-Sar-Gly-(N-R)Gly]₂ (R = n-hexyl and cyclohexyl) transport calcium ions selectively across organic phases and phospholipid membranes. We have now used proton nmr spectroscopy (360 MHz) to study the solution conformation(s) of their calcium complexes. When Ca(ClO₄)₂ was added to solutions of these peptides in CDCl₃, nmr spectra of the resulting calcium complexes were characteristic of a single C₂-symmetric conformer. From a Karplus-Bystrov analysis of vicinal coupling constants in both the peptide backbone and Glu side chain (treated as an ABCC′MX spin system), in conjuction with model-building studies, a structure was proposed in which the calcium ion is bound in an octahedral-type complex by the four (coplanar) carbonyl groups of the (all-trans) Glu-Sar and Gly-(N-R)Gly peptide bonds. Occurrence of preferred rotamers about Glu side chain C^α–C^β bonds indicated that restricted rotation in peptide side chains arises upon calcium binding.     

Diffusive conductance of rice (Oryza sativa) leaves during gradually induced soil water stress

The diffusive conductance (C_s) of rice (Oryza sativa cvs Jaya and Bala) leaves was measured during a soil drying cycle from flooding to decreasing soil water potential (φ_s) in a controlled-environment chamber. Plants were grown continuously under 5 cm submergence up to 69 days after transplanting and thereafter were subjected to gradual soil drying for a period of 17 days in the vegetative growth stage. In both the cultivars, the values of C_s were generally more on adaxial than abaxial leaf surfaces. This response of stomata during the period of soil drying was independent of leaf rolling. Further, the slopes of the curves (C_s, vs φ_s) also did not differ significantly (P= 0·05). The total C_s, of both cultivars during flooding was almost equal (0·60 cm s^-1) but at the end of the soil drying cycle, the values of total C_s, were 0·11 cm s^-l at ψ_s of -1·3 MPa and 0·08 cm s^-1 at ψ_s, of -0·8 MPa in cvs Jaya and Bala, respectively. For total C_s, slopes differed significantly (P = 0·05). A close relationship between total C_s, and ψ_s, in both cultivars (C_s, = 0·58-0·40 ψ_s, for cv. Jaya and C_s= 0·46-0·56 ψ_s, for cv. Bala) indicated that stomata were sensitive to increasing soil water deficit.