Observation of protein-derived (BSA) oxygen-centered radicals by EPR spin-trapping techniques

Electron paramagnetic resonance (EPR) spin-trapping experiments, employing the novel spin-trap DEPMPO, provide evidence for the formation of protein-peroxyl radicals from the reaction of bovine serum albumin (BSA) or lysozyme with HO · in the presence of O 2 . Spin-trapping leads to the detection of anisotropic spectra of partially immobilized protein-peroxyl spin-adducts; positive identification is based on a novel spectrum simulation approach (through which broadened anisotropic spectra are simulated and compared with experiment) and by comparison of results with those obtained when MeO 2 · is trapped and the adduct frozen in a solid matrix.     

Fluorescence enhancement of europium (III) perchlorate by 1,10‐phenanthroline on the 1‐(naphthalen‐2‐yl)‐2‐(phenylsulthio)ethanone complex and luminescence mechanism

A novel ligand, 1‐(naphthalen‐2‐yl)‐2‐(phenylsulthio)ethanone was synthesized using a new method and its two europium (Eu) (III) complexes were synthesized. The compounds were characterized by elemental analysis, coordination titration analysis, molar conductivity, infrared, thermo gravimetric analyzer‐differential scanning calorimetry (TGA‐DSC), ¹H NMR and UV spectra. The composition was suggested as EuL₅ · (ClO₄)₃ · 2H₂O and EuL₄ · phen(ClO₄)₃ · 2H₂O (L = C₁₀H₇COCH₂SOC₆H₅). The fluorescence spectra showed that the Eu(III) displayed strong characteristic metal‐centered fluorescence in the solid state. The ternary rare earth complex showed stronger fluorescence intensity than the binary rare earth complex in such material. The strongest characteristic fluorescence emission intensity of the ternary system was 1.49 times as strong as that of the binary system. The phosphorescence spectra were also discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Circular dichroism calculations for double-stranded polynucleotides of repeating sequence

Optical property calculations are presented for poly(A·U), poly[(A-U)·(A-U)], poly(G·C), and poly[(G-C)·(G-C)] in RNA, B-DNA, and C-DNA conformations. An all-order classical coupled oscillator polarizability theory was used, and an effective dielectric constant of 2 was assumed. The calculated CD spectra were found to be sensitive to both geometry and sequence. Agreement with the measured CD spectra of poly(A·U), poly(G·C), and poly(dG·dC) is very good. Calculations for other sequences and geometries are less satisfactory and are particularly poor for poly[(G-C)·(G-C)] in RNA geometry and poly(A·T) in B-DNA geometry. Attempts to improve agreement with measured spectra by varying monomer properties have been only partially successful for these calculations, but they illustrate the types of changes that may prove to be necessary. Calculations using other published X-ray coordinates for certain deoxypolynucleotides of simple sequence, some of which are quite different from B-DNA coordinates, did not result in better agreement with measured spectra. Finally, the dependence of the calculated CD on chain length is examined. Results show that non-nearest neighbor interactions can be important when runs of 3 or more identical base pairs appear in a given sequence.     

Luminescence enhancement of terbium(III) perchlorate by 2,2′‐dipyridyl on bis(benzylsulfinyl)methane complex and luminescence mechanism

A novel ternary complex, Tb₂L₄·L′·(ClO₄)₆·8H₂O, has been synthesized using bis(benzylsulfinyl)methane as the first ligand L and 2,2′‐dipyridyl as the second ligand L′. The ternary complex was characterized by element analysis, molar conductivity, coordination titration analysis, infrared, thermogravimetric‐differential scanning calorimetric and ultraviolet spectra. The results indicated that the composition of the complex was Tb₂L₄·L′·(ClO₄)₆·8H₂O (L = C₆H₅CH₂SOCH₂SOCH₂C₆H₅; L′ = Dipy). Fourier transform infrared results revealed that the perchlorate group was bonded with the Tb(III) ion by the oxygen atom, and the coordination was bidentate. The fluorescent spectra illustrated that the complex displayed characteristic fluorescence in the solid state. After the introduction of the second ligand, 2,2‐dipyridyl, the relative emission intensity and fluorescence lifetime of the ternary complex Tb₂L₄·L′·(ClO₄)₆·8H₂O were enhanced compared to the binary complex TbL_2.5(ClO₄)₃·3H₂O. This indicated that the presence of both organic ligand bis(benzylsulfinyl)methane and the second ligand 2,2‐dipyridyl could sensitize the fluorescence intensity of Tb(III) ion, and introduction of the 2,2‐dipyridyl group resulted in an enhancement of the fluorescence of the Tb(III) ternary rare earth complex. The strongest characteristic fluorescence emission intensity of the ternary complex was 9.36 times that of the binary complex. The phosphorescence spectra and fluorescence lifetime of the complex were also measured. Copyright © 2014 John Wiley & Sons, Ltd.     

The analysis of circular dichroism spectra of natural DNAs using spectral components from synthetic DNAs

We have tested 21 different basis sets of synthetic DNA circular dichroism spectra and have slected one for use in spectral analyses of natural DNAs. This “standard” set consists of spectra of eight polymers: poly[d(A-A-T)·d(A-T-T)], poly[d(A-G-G)·d(C-C-T)], poly[d(A-T)·d(A-T)], poly[d(G-C)·d(G-C)], poly[d(A-G)·d(C-T)], poly[d(A-C)·d(G-T)], poly[d(A-T-C)·d(G-A-T)], and poly[d(A-G-C)·d(G-C-T)]. This basis set, applied according to the first-neighbor polymer procedure of Gray and Tinoco, allows a more uniformly accurate spectral analysis of six natural complex DNAs and eight (A+T)-rich satellite DNAs for base composition and first-neighbor frequencies than was previously possible. We find that spectra of poly[d(A)·d(T)] and/or poly[d(A-C-T-)·d(A-G-T)] are not generally required for good analysis results but we show in this and the following paper that these spectra are needed for the most accurate analyses of some satellite DNAs.     

Anthocyanin,flavonol copigments,and pH responsible for larkspur flower color

The anthocyanin and flavonol glycosides in Larkspur flowers (cv. Dark Blue Supreme) are delphinidin 3-di(p-hydroxybenzoyl)glucosylglucoside, kaempferol 3-robinobioside-7-rhamnoside (robinin), kaempferol 3-rutinoside, kaempferol 7-rhamnoside, and kaempferol 3-(caffeylgalactosylxyloside)-7-rhamnoside. As young flowers age the pH of epidermal tissue increases from 5·5 to 6·6 and the color of many of the cells changes from moderate reddish-purple to light purplish-blue. Many of the older cells also contain blue crystals. Visible absorption spectra of moderate reddish-purple and light purplish-blue cells were simulated with a solution of the anthocyanin (10⁻² M) plus robinin (5 × 10⁻³ M) at pH 5·6 and 7·1, respectively. Changes in the absorption spectra of living tissue with heating or cooling and of concentrated solutions of the anthocyanin with dilution or moderate heat, indicate that in the natural state the pigment is present in an associated form.     

Circular dichroism of adenine and thymine containing synthetic polynucleotides

Circular dichroism (CD) spectra of poly(dA), poly(dT), poly(dA)·poly(dT), and poly[d(A-T)]·poly[d(T-A)] have been measured as a function of temperature. From these data difference spectra have been calculated by subtracting the spectrum measured at low temperature from the spectra measured at higher temperatures. The CD difference spectra obtained upon melting of the two double-stranded polymers are very similar. From a comparison of these difference spectra with calculated ones it is shown that optical transitions near 272 nm (on A) and 288 nm (most probably on T) are present. The premelting changes of the CD spectrum of poly[d(A-t)]·poly[d(T-A)] are due to a change in conformation in which the secondary structure goes from a C- to B-type spectrum by increasing the A-type nature of the polymer. Such a change is not observed for poly(dA)·poly(dT). Instead, a transition between two different B-type geometries occurs.     

Sequence dependence of the circular dichroism of synthetic double-stranded RNAs

We have synthesized and studied the CD spectra of five new double-stranded RNA polymers: poly[r(A-G)·r(C-U)], poly[r(A-U-C)·r(G-A-U)], poly[r(A-C-U)·r(A-G-U)], poly[r(A-A-C)·r(G-U-U)], and poly[r(A-C-C)·r(G-G-U)]. Together with previously published spectra of seven other RNA sequences, the spectra of these new sequences provide a library sufficient to approximate the spectra of all other RNA sequences by first-neighbor formulas and, in addition, give four spectra with which we may test the validity of first-neighbor approximations. (1) We find that the spectra of RNA sequence isomers are very different, but that the spectra essentially do obey first-neighbor relationships. (2) We have derived tentative first-neighbor assignments of negative bands at about 295 and 210 nm in the CD spectra. (3) A test of spectral independence shows that among the 12 polymer spectra there are at least seven significant independent spectral shapes, one less than the eight needed to give the most accurate spectral analysis of an unknown RNA sequence for its first-neighbor frequencies. (4) Spectra are calculated for RNAs of random base composition, approximating natural RNAs having complex sequences. (5) A T-matrix of spectral components assigned to the first-neighbor base pairs is derived from 10 of the spectra. This matrix allows an estimation of the CD spectrum of any other known RNA sequence or an analysis of the spectrum of an unknown sequence for its distribution of first-neighbor base-pair frequencies. (6) Test analyses of two of the synthetic polymers and of two natural RNAs set a probable limit on the accuracy of first-neighbor frequency determinations using this T-matrix. (7) Finally, we summarize in an appendix the melting temperatures for all the RNA and corresponding DNA sequences; it appears that the T_m values of both DNAs and RNAs approximately obey first-neighbor relationships.     

Sequence-dependence of the CD of synthetic double-stranded RNAs containing inosinate instead of guanylate subunits

The CD spectra and melting profiles have been measured for nine synthetic double-stranded RNAs containing I · C instead of G · C base pairs: poly[r(I) · r(C)], poly[r(I-C) · r(I-C)], poly[r(A-I-C) · r(I-C-U)], poly[r(A-C) · r(I-U)], poly[r(A-I) · r(C-U)], poly[r(A-C-C) · r(I-I-U)], poly[r(A-A-C) · r(I-U-U)], poly[r(A-C-U) · r(A-I-U)], and poly[r(A-U-C) · r(I-A-U)]. CD spectra have not previously been reported for the latter six of these polymers. The substitution of inosinate for guanylate led to recognizable CD differences, with all but two of the polymers having two resolved positive bands above 230 nm. Also, the I-containing RNAs differed from their G-containing counterparts in the almost complete absence of negative CD bands at long wavelengths and in the reduction of negative CD bands near 210 nm. First-neighbor comparisons showed that the CD spectra of the I-containing RNAs were consistent with the nearest-neighbor sequences of the polymers, as previously shown for G-containing RNAs (D. M. Gray, J.-J. Liu, R. L. Ratliff, and F. S. Allen, Biopolymers (1981) 20 , 1337–1382). Moreover, two of the first-neighbor comparisons involved spectra of poly[r(A) · r(U)] and poly[r(I) · r(C)], polymers known to be in the A family of conformations in fibers (S. Arnott, D. W. L. Hukins, S. D. Dover, W. Fuller, and A. Hodgson, (1973) J. Mol. Biol. 81 , 107–122). Thus, differences in the CD spectra of I- and G-containing RNAs could be simply explained as resulting from differences in the hypoxanthine and guanine chromophores, without invoking differences in conformation. Finally, melting temperatures of the I-containing RNAs were found to vary much less with base composition than do the melting temperatures of G-containing RNAs, since A · U base pairs are closer to I · C than to G · C base pairs in stability.     

活性氧诱发人类11号染色体基因突变

对体外产生的和内源性刺激产生的活性氧 (ROS)诱发人类 11号染色体 (Hchr 11)基因突变规律及其突变谱进行研究 .体外羟自由基 (·OH)用过氧化氢 (H2 O2 )与Fe2 + 反应产生 ,并用化学发光(CL)进行相对定量分析 ;内源性ROS用佛波醇酯 (PMA)刺激人外周血白细胞产生 ,并用CL和特异性抗氧化物检测和鉴定 ;用包含单条Hchr 11的人 中国仓鼠卵巢细胞 (AL)为靶 ,经CD59表面抗原抗体筛选突变细胞克隆 ,研究ROS诱发的Hchr 11基因突变 ;突变克隆细胞DNA用Hchr 11上 5种标志基因引物进行多重PCR分析 ,结合琼脂糖凝胶电泳绘制基因突变谱 .结果表明 ,体外ROS可诱发Hchr 11基因突变 ,且·OH诱发基因突变的能力明显强于H2 O2 ,两者的突变谱也存在明显差异 ;PMA可刺激人外周血白细胞产生大量的多种ROS ,并诱发Hchr 11基因突变 ,突变谱综合了H2 O2 和·OH的所有特征 ;一些抗氧化物对内源性产生的ROS诱发Hchr 11基因突变有明显抑制作用 .提示体外和内源性ROS可诱发Hchr 11基因突变 ,不同的活性氧分子诱发的基因突变可能具有特异性     

Binding of CC-1065 to poly- and oligonucleotides

The binding of the antitumor agent CC-1065 to a variety of poly- and oligonucleotides was studied by electronic absorption, CD, and resistance to removal by Sephadex column chromatography. Competitive binding experiments between CC-1065 and netropsin were carried out with calf-thymus DNA, poly(dI-dC) · poly(dI-dC), poly(dI) · poly(dC), poly(rA) · poly(dT), poly(dA- dC) · poly(dG-dT), and poly(dA) · 2poly(dT). CC-1065 binds to polynucleotides by three mechanisms. In the first, CC-1065 binds only weakly, as judged by the induction of zero or very weak CD spectra and low resistance to extraction of drug from the polynucleotide by Sephadex chromatography. In the second and third mechanisms, CC-1065 binds strongly, as judged by the induction of two distinct, intense CD spectra and high resistance to extraction of drug from the polynucleotide, by Sephadex chromatography in both cases. The species bound by the second mechanism converts to that bound by the third mechanism with varying kinetics, which depend both on the base-pair sequence and composition of the polynucleotide. Competitive binding experiments with netropsin show that CC-1065 binds strongly in the minor groove of DNA by the second and third mechanisms of binding. Netropsin can displace CC-1065 that is bound by the second mechanism but not that bound by the third mechanism. CC-1065 binds preferentially to B-form duplex DNA and weakly (by the first binding mechanism) or not at all to RNA, DNA, and RNA–DNA polynucleotides which adopt the A-form conformation or to single-strand DNA. This correlation of strong binding of CC-1065 to B-form duplex DNA is consistent with x-ray data, which suggest an anomalous structure for poly(dI) · poly(rC), as compared with poly(rI) · poly(dC) (A-form) and poly(dI) · poly(dC) (B-form). The binding data indicate that poly(rA) · poly(dU) takes the B-form secondary structure like poly(rA) · poly(dT). Triple-stranded poly(dA) · 2poly(dT) and poly(dA) · 2poly(dU), which are considered to adopt the A-form conformation, bind CC-1065 strongly. Netropsin, which also shows a binding preference for B-form polynucleotides, also binds to poly(dA) · 2poly(dT) and occupies the same binding site as CC-1065. These binding studies are consistent with results of x-ray studies, which suggest that A-form triplex DNA retains some structural features of B-form DNA that are not present in A-form duplex DNA; i.e., the axial rise per nucleotide and the base tilt. Triple-stranded poly(dA) · 2poly(rU) does not bind CC-1065 strongly but has nearly the same conformation as poly(dA) · 2poly(dT) based on x-ray analysis. This suggests that the 2′-OH group of the poly(rU) strands interferes with CC-1065 binding to this polynucleotide. The same type of interference may occur for other RNA and DNA–RNA polynucleotides that bind CC-1065 weakly.     

Infrared study of G · C complex formation in template-dependent oligo(G) synthesis

G · C complex formation was studied by infrared spectroscopy for a system that has been shown by Inoue & Orgel (1982) to give efficient, template-dependent synthesis of oligo(G). Guanosine-5′-phosphor-2-methylimiazolide (2-MeImpG) exhibits rapid formation with poly(C) of a G · C double helix at pD ~ 8 and of a C · G · CH⁺ triple helix at pD ~ 6.5 in the presence of Na⁺. Significant oligo(G) synthesis does not occur under these conditions. In the presence of synthetically effective concentrations of Mg²⁺ G · C complex formation is much slower but eventually goes to completion. The rate of complex formation parallels that of chemical synthesis. Infrared spectra and melting curves confirm that oligo(G) of high molecular weight is formed in high yield. The bulk of the G · C complex at any given time during the reaction is composed of G residues that have already been polymerized and not of the monomer 2-MeImpG. Evidence indicates that synthesis proceeds primarily at growing points at the ends of the G · C helical regions and not randomly on a fully occupied template.     

Analysis of an RNA Pseudoknot Structure by CD Spectroscopy

Abstract

The RNA PK5 (GCGAUUUCUGACCGCUUUUUUGUCAG) forms a pseudoknotted structure at low temperatures and a hairpin containing an A · C opposition at higher temperatures (J. Mol. Biol. 214, 455–470 (1990)). CD and absorption spectra of PK5 were measured at several temperatures. A basis set of spectra were fit to the spectra of PK5 using a method that can provide estimates of the numbers of A · U, G · C, and G · U base pairs as well as the number of each of 11 nearest-neighbor base pairs in an RNA (Biopolymers 31, 373–384 (1991)). The fits were close, indicating that PK5 retained the A conformation in the pseudoknot structure and that the fitting technique is not hindered by pseudoknots or A · C oppositions. The results from the analysis were consistent with the pseudoknotted structure at low temperatures and with the hairpin structure at higher temperatures. We concluded that the method of spectral analysis should be useful for determining the secondary structures of other RNAs containing pseudoknots and A · C oppositions.     

Ring-current shifts in the 300 MHz nuclear magnetic resonance spectra of six purified transfer RNA molecules

We present the 300 MHz high-resolution proton nuclear magnetic resonance spectra of the ring NH hydrogen-bonded protons of six purified tRNAs. Good agreement was obtained between the observed spectra and those computed on the assumption of the suitable cloverleaf models. In the computation it is assumed that the hydrogen-bonded ring NH in each type of base pair has an intrinsic position with respect to 2,2-dimethyl-2-silapentane-5-sulfonate, i.e. in A·U it is at ?14·8 parts per million, in G·C at ?13·7 parts per million and in A·Ψat ?13·5 parts per million. The shifts of these resonances from these positions are calculated by including ring current fields from the nearest neighbors. The agreement is very good, adding support to our earlier findings that there is no evidence for additional Watson-Crick base pairs detected beyond those in the cloverleaf. In general, resolved resonances are fitted by the computed spectra to within ±0·2 part per million showing that there is no need for any additional physical mechanism to explain the nuclear magnetic resonance positions. Hence, the nuclear magnetic resonance spectra can be interpreted in terms of the structure of their neighbors and in a few important cases this has been particularly valuable in understanding the structure beyond the end of a helical region. In the tRNA^Glu_E.coli′ for example, the positions of the resonances in A·U no. 7 and A·U no. 49 at the interior ends of the acceptor and -T-Ψ-C- stems, respectively, strongly suggest that these two stems are in a continuous helix. Other structural effects at the ends of the helical regions are also suggested by the nuclear magnetic resonance spectra.     

Interaction of poly-5-bromouridylic acid. I. Formation of helical complexes with various adenine and 2-aminoadenine derivatives

Complexes of poly(BU) with various adenine derivatives were investigated by circular dichroism (CD) and absorption spectroscopy. A 1:2 stoichiometry was indicated on CD mixing curves for typical complexes of 9-substituted adenine and 2-aminoadenine derivatives with poly(BU). The CD spectrum of adenosine·2poly(BU) is characterized by well-resolved bands in the range of 210–350 nm. Other adenine derivative–poly(BU) complexes also afford similar CD spectra, while 2-aminoadenine derivative–poly(BU) complexes give quite different spectra. Attempts to assign representative CD spectra were made using the transition of helical poly(BU) and the respective purine polynucleotides. The similarity of the CD spectra suggests that poly(A)·2poly(BU) and adenine derivative–poly(BU) complexes are nearly identical in structure except for the ribose–phosphate linkage. The fact that the uv isosbestic point of adenosine·2poly(BU) falls in close proximity to that of the corresponding polymer complex also supports this conclusion. In the formation of stable helices, the ribose moiety is dispensable in the “strand” of purine. The T_m of 9-methyladenine·2poly(BU) is somewhat higher than that of adenosine·2poly(BU) under equivalent conditions. The T_m difference with the monomer–poly(U) system was found to be about 20°C in 0.4M NaCl–0.02M Na–cacodylate–5 × 10^?4M EDTA (pH 7.0). Further, it was noted that the monomer–poly(BU) complexes are formed even when the T_m is lower than that of self-folded poly(BU).     

The Structure of Triple Helical Poly(U)·Poly(A)·Poly(U) Studied by Raman Spectroscopy

Abstract

Using Raman spectroscopy, we examined the ribose-phosphate backbone conformation, the hydrogen bonding interactions, and the stacking of the bases of the poly(U)·poly(A) ·poly(U) triple helix. We compared the Raman spectra of poly(U)·poly(A)·poly(U) in H₂O and D₂O with those obtained for single-stranded poly(A) and poly(U) and for double-stranded poly(A)·poly(U). The presence of a Raman band at 863 cm^?1 indicated that the backbone conformations of the two poly(U) chains are different in the triple helix. The sugar conformation of the poly(U) chain held to the poly(A) by Watson-Crick base pairing is C3′ endo; that of the second poly(U) chain may be C2′ endo. Raman hypochromism of the bands associated with base vibrations demonstrated that uracil residues stack to the same extent in double helical poly(A)·poly(U) and in the triple-stranded structure. An increase in the Raman hypochromism of the bands associated with adenine bases indicated that the stacking of adenine residues is greater in the triple helix than in the double helical form. Our data further suggest that the environment of the carbonyls of the uracil residues is different for the different strands.     

Solvent Effects in the Spin Trapping Of Lipid Oxyl Radicals

Studies documenting spin trapping of lipid radicals in defined model systems have shown some surprising solvent effects with the spin trap DMPO. In aqueous reactions comparing the reduction of H₂O₂ and methyl linoleate hydroperoxide (MLOOH) by Fe^z+, hydroxyl (HO·) and lipid alkoxyl (LO·) radicals produce identical four-line spectra with line intensities 1:2:2:1. Both types of radicals react with commonly-used HO· scavengers, e.g. with ethanol to produce ·C(CH₃)HOH and with dirnethylsulfoxide (DMSO)togive ·CH₃. However, DMSO radicals (either ·CH₃or ·OOCH₃) react further with lipids, and when radicals are trapped in these MLOOH systems, multiple adducts are evident. When acetonitrile is added to the aqueous reaction systems in increasing concentrations, ·CH₂CN radicals resulting from HO· attack on acetonitrile are evident, even with trace quantities of that solvent. In contrast, little, if any, reaction of LO· with acetonitrile occurs, even in 100% acetonitrile. A single four-line signal persists in the lipid systems as long as any water is present, although the relative intensity of the two center lines decreases as solvent-induced changes gradually dissociate the nitrogen and β-hydrogen splitting constants. Extraction of the aqueous-phase adducts into ethyl acetate shows clearly that the identical four-line spectra in the H₂0₂ and MLOOH systems arise from different radical species in this study, but the lack of stability of the adducts to phase transfer may limit the use of this technique for routine adduct identification in more complex systems. These results indicate that the four-line 1:2:2:1. a_N = a_H = 14.9G spectrum from DMPO cannot automatically be assigned to the HO· adduct in reaction systems where lipid is present, even when the expected spin adducts from ethanol or DMSO appear confirmatory for HO-. Conclusive distinction between HO· and LO· ultimately will require use of ¹³C-labelled DMPO or HPLC-MS separation and specific identification of adducts when DMPO is used as the spin trap.     

Heterocyclic amine adducts of bis(2-thiopyridine N-oxide)nickel(II)

Heterocyclic amine adducts of bis(2-thiopyridine N-oxide)nickel(II), Ni(OS)₂·B (B = pyridine, piperidine, pyrazine and 3,4-lutidine) and Ni(OS)₂·2B (B = 3-picoline, 4-picoline and 3,4-lutidine). In addition, a stable 1:1 adduct was formed with piperazine as well as a thermally unstable 2:1 adduct. The thermal decomposition enthalpies of these adducts and their infrared and electronic spectra are discussed.     

Conformational properties of poly[d(G-T)].poly[d(C-A)] and poly[d(A-T)] in low- and high-salt solutions: NMR and laser Raman analysis

³¹P- and ¹H-nmr and laser Raman spectra have been obtained for poly[d(G-T)]·[d(C-A)] and poly[d(A-T)] as a function of both temperature and salt. The ³¹P spectrum of poly[d(G-T)]·[d(C-A)] appears as a quadruplet whose resonances undergo separation upon addition of CsCl to 5.5M. ¹H-nmr measurements are assigned and reported as a function of temperature and CsCl concentration. One dimensional nuclear Overhauser effect (NOE) difference spectra are also reported for poly[d(G-T)]·[d(C-A)] at low salt. NOE enhancements between the H8 protons of the purines and the C5 protons of the pyrimidines, (H and CH₃) and between the base and H-2′,2″ protons indicate a right-handed B-DNA conformation for this polymer. The NOE patterns for the TH3 and GH1 protons in H₂O indicate a Watson–Crick hydrogen-bonding scheme. At high CsCl concentrations there are upfield shifts for selected sugar protons and the AH2 proton. In addition, laser Raman spectra for poly[d(A-T)] and poly[d(G-T)]·[d(C-A)] indicate B-type conformations in low and high CsCl, with predominantly C2′-endo sugar conformations for both polymers. Also, changes in base-ring vibrations indicate that Cs⁺ binds to O2 of thymine and possibly N3 of adenine in poly[d(G-T)]·[d(C-A)] but not in poly[d(A-T)]. Further, ¹H measurements are reported for poly[d(A-T)] as a function of temperature in high CsCl concentrations. On going to high CsCl there are selective upfield shifts, with the most dramatic being observed for TH1′. At high temperature some of the protons undergo severe changes in linewidths. Those protons that undergo the largest upfield shifts also undergo the most dramatic changes in linewidths. In particular TH1′, TCH₃, AH1′, AH2, and TH6 all undergo large changes in linewidths, whereas AH8 and all the H-2′,2″ protons remain essentially constant. The maximum linewidth occurs at the same temperature for all protons (65°C). This transition does not occur for d(G-T)·d(C-A) at 65°C or at any other temperature studied. These changes are cooperative in nature and can be rationalized as a temperature-induced equilibrium between bound and unbound Cs⁺, with duplex and single-stranded DNA. NOE measurements for poly[d(A-T)] indicate that at high Cs⁺ the polymer is in a right-handed B-conformation. Assignments and NOE effects for the low-salt ¹H spectra of poly[d(A-T)] agree with those of Assa-Munt and Kearns [(1984) Biochemistry 23 , 791–796] and provide a basis for analysis of the high Cs⁺ spectra. These results indicate that both polymers adopt a B-type conformation in both low and high salt. However, a significant variation is the ability of the phosphate backbone to adopt a repeat dependent upon the base sequence. This feature is common to poly[d(G-T)]·[d(C-A)], poly[d(A-T)], and some other pyr–pur polymers [J. S. Cohen, J. B. Wouten & C. L Chatterjee (1981) Biochemistry 20 , 3049–3055] but not poly[d(G-C)].     

Structural correlations between trans- and cis-bis(diphenylphosphino)ethene,bis(diphenylphosphino)methane and their chlorogold(I) complexes

The crystal structures of trans- and cis-bis(diphenylphosphino) ethene (1, 2) have been determined by single crystal X-ray diffraction. The conformation of these free ligands is compared with structural data available in the literature for the corresponding 1:2 complexes with gold(I) chloride (4, 5). In the cis-ligand 2 the conformation of the Ph₂P-groups is such, that the molecule approaches non-crystallographic C_s symmetry with the lone pairs at phosphorus pointing towards each other. Upon addition of AuCl, rotation of one Ph₂P group around the PC bond by approximately 60° leads to a structure for 5 which allows an intramolecular Au···Au contact of 3.05(1)Å. The trans-ligand 1 undergoes little structural change upon adduct formation, but intermolecular Au···Au contacts of 3.043(1) Å are secured through aggregation. The synthesis, properties and ¹⁹⁷Au Mössbauer spectra of 1:1 and 1:2 complexes of 1 and 2 with AuCl are summarized with reference to a recent controversy in the literature.The crystal structure of bis(diphenylphosphino)- methane (3) has also been determined and the results compared with those published previously for the 1:2 complex with AuCl (7, crystallographic C₂ symmetry, Au···Au distance 3.351(2) Å). There is very little change of the ligand conformation upon coordination.