Characterization of tetrakis(thiadiazole)porphyrazine metal complexes by magnetic circular dichroism and magnetic circularly polarized luminescence

The magnetic circular dichroism (MCD) spectra of metal complexes of tetrakis(thiadiazole)porphyrazines ([TTDPzM] with M = 2H^I, Zn^II, Mg^II(H₂O), and Cd^II) have been recorded in dimethyl formamide solution. Together with the UV–Vis spectra, the MCD spectra provide useful information about the structure and electronic properties of the complexes. The experimental UV–Vis and MCD spectra compare pretty well with DFT calculations of two sorts, based either on the sum-over-states (SOS) approach or on the complex polarization propagator approach. They further corroborate the findings and interpretation of MCD spectra of porphyrazines based on the model of Michl for peripheral molecular orbitals. Magnetic circularly polarized luminescence (MCPL) spectra, quite uncommon in the literature, have been recorded for [TTDPzM] (M = 2H^I, Zn^II, Mg^II(H₂O)).     

Experimental and calculated circular dichroism spectra of monoaza[5]helicenes

Circular dichroism (CD) spectra have been measured in the range of 400-200 nm on CH₃OH solutions of both enantiomers for the almost complete series of monoaza[5]helicenes, namely the molecules where the hetero N atom occupies positions 1, 3, 4, 5, 6, and 7, respectively (the 2 isomer is missing due to difficulties in the synthesis). CD spectra recorded at controlled room temperature allow one to define precise racemization rates, that are nicely interpreted on the basis of DFT molecular orbital calculations. Time-dependent DFT methods provide us with calculated CD and UV spectra, that are compared with the corresponding experimental data. We discuss the role of the N atom in determining the height of the racemization barrier and in shaping the appearance of the CD spectra.     

Synthesis of poly(L-Tyr-L-Glu-L-Ala-Gly) and poly(L-Glu-L-Lys-L-Ala) and their circular dichroism spectra in aqueous solution

Two sequential polypeptides, poly(O-benzyl-L -Tyr-γ-benzyl-L -Glu-L -Ala-Gly) and poly(ε-benzyloxycarbonyl-L -Lys-L -Glu-L -Ala), were synthesized, the former by the pentachlorophenyl ester of the tetrapeptide monomer and the latter by the azide of the tripeptide monomer. After deprotection and dialysis, poly(L -Tyr-L -Glu-L -Ala-Gly) was obtained in 71% yield and had a molecular weight of 53,000. The circular dichroism spectra (CD) of the polymer at pH's 7.2, 10.5, and 11.8 and of oligomers and of the monomer at pH 7.2 indicated that the polymer exists in an α-helical conformation. After deprotection, poly(L -Lys-L -Glu-L -Ala) was obtained in 37% yield and had a molecular weight of 3000. The CD spectra of the polymer at pH 7.2 and 2.8, and of the monomer at pH 7.2, indicated that the polymer is in a randomly coiled configuration.     

3-[2-((2S)-2-cyano-pyrrolidin-1-yl)-2-oxo-ethylamino]-3-methyl-butyramide analogues as selective DPP-IV inhibitors for the treatment of type-II diabetes

Based on the structures of NVP-DPP728 (1) and NVP-LAF237 (Vildagliptin, 2), three series of DPP-IV inhibitors were synthesized by linking substituted anilines, benzylamines, and phenylethylamines to (2S)-cyanopyrrolidine through a linker. More than 20 compounds were evaluated for their in vitro DPP-IV inhibition and selectivity profile over DPP-II, DPP8, and FAP enzymes. Selected compounds 5f and 7i showed in vivo plasma DPP-IV inhibition and inhibited glucose excursion in OGTT after oral administration in Wistar rats. Compound 5f (DPP-IV IC50 = 116 nM) has the potential for development as antidiabetic agent.     

Absolute configuration determination of donor-acceptor [2.2]paracyclophanes by comparison of theoretical and experimental vibrational circular dichroism spectra

Vibrational circular dichroism (VCD) spectra were obtained for the assignments of the absolute configurations of diastereomeric 4,7-bis(methoxycarbonyl)-12,15-dimethoxy-[2.2]paracyclophanes (1 and 2), and density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level for all possible conformations of both 1 and 2 to give the theoretical VCD spectra. Comparisons of the experimental and theoretical VCD spectra obtained unambiguously established the absolute configurations of the dextrorotatory (+)-enantiomers as (4S(p);12S(p))-1 and (4S(p);12R(p))-2, respectively.     

Effects of Ca2+ and Zn2+ on trifluoperazine-S100 proteins interactions: induced circular dichroism and fluorescence spectra

Interactions of trifluoperazine (TFP) with S100 proteins, EF-hand type Ca2+-binding proteins, in the presence of Ca2+ and Zn2+ were studied with induced circular dichroism (CD) and fluorescence spectra. The positive CD bands of TFP were induced at around 265 nm by adding either S100a or S100a0 protein in the presence of Ca2+. No CD band of TFP was, however, induced by adding S100b protein in the presence of Ca2+. Addition of Zn2+ to the TFP/S100 protein solutions did not induce any CD band at all. The fluorescence intensity of 2-p-toluidinylnaphthalene 6-sulfonate (TNS) bound to S100a or S100a0 protein decreased by adding TFP in the presence of Ca2+, while that bound to S100b protein decreased by adding TFP in the presence of Zn2+, indicating that TFP binds to S100a protein and S100a0 protein in a Ca2+-dependent manner and to S100b protein in a Zn2+-dependent manner. From these results together with other experimental findings it was suggested that (1) TFP binds to S100a protein and S100a0 protein in the presence of Ca2+, with half-saturation points of 18 and 3 microM, respectively, (2) TFP binds to S100b protein only in the presence of Zn2+, (3) alpha-subunit of S100 protein binds to TFP specifically in a Ca2+-dependent manner and beta-subunit in a Zn2+-dependent manner.     

The structure of tri-proline in water probed by polarized Raman, Fourier transform infrared, vibrational circular dichroism, and electric ultraviolet circular dichroism spectroscopy

Tripeptidesserve as model systems for understanding the so-called random-coil state of peptides and proteins. While it is well known that polyproline or proline-rich polypeptides adopt the very regular polyproline-II (PPII) or left-handed 3(1)-helix conformation, it was thus far not clear whether this is also the predominant structure adopted by proline-containing tripeptides. To clarify this issue, we have investigated the amide I' band profile in the ir, isotropic, and anisotropic Raman, and vibrational circular dichroism (VCD) spectrum of cationic and zwitterionic tri-proline in D(2)O. The data were analyzed by modifying a recently developed algorithm, which allows one to obtain the central dihedral angles of tripeptides from the amide I' band intensities (R. Schweitzer-Stenner, Biophysical Journal, 2002, Vol. 83, pp. 523-532). Our analysis revealed that the peptide adopts a nearly canonical PPII structure in water with psi and phi values in the range of 175 degrees -165 degrees and -70 degrees -(-80 degrees ), respectively. This is fully confirmed by the respective electronic ultraviolet-CD spectra. Our result indicates that the strong PPII propensity of trans proline results from local interactions between the pyrrolidine ring and the backbone and is not due to any long-range interactions.     

Resonance Raman and magnetic circular dichroism studies of reduced [2Fe-2S] proteins.

The structural and electronic properties of the [2Fe-2S] clusters in reduced putidaredoxin, Spinacea oleracea ferredoxin, and Clostridium pasteurianum [2Fe-2S] ferredoxin have been investigated by resonance Raman and variable temperature magnetic circular dichroism spectroscopies. Both techniques are shown to provide diagnostic fingerprints for identifying [2Fe-2S]+ clusters in more complex multicomponent metalloenzymes. The Fe-S stretching modes of oxidized and reduced putidaredoxin are assigned via 34S and D2O isotope shifts and previous normal mode calculations for adrenodoxin (Han, S., Czernuszewicz, R. S., Kimura, T., Adams, M. W. W., and Spiro, T. G. (1989) J. Am. Chem. Soc. 111, 3505-3511). The close similarity in the resonance Raman spectra of reduced [2Fe-2S] centers, in terms of both the vibrational frequencies and enhancement profiles of the Fe-S stretching modes, permits these assignments to be generalized to all clusters of this type. Modes primarily involving Fe(III)-S(Cys) stretching are identified in all three reduced [2Fe-2S] proteins, and the frequencies are rationalized in terms of the conformation of the cysteine residues ligating the Fe(III) site of the localized valence reduced cluster. D2O isotope shifts indicate few, if any, amide NH-S hydrogen bond interactions involving the cysteines ligating the Fe(III) site. Preliminary resonance Raman excitation profiles suggest assignments for the complex pattern of electronic bands that comprise the low temperature magnetic circular dichroism spectra of the reduced proteins. S----Fe(III) and Fe(II)----S charge transfer, Fe d-d, and Fe(II)----Fe(III) intervalence bands are identified.     

Interaction of (-)-epigallocatechin-3-gallate with human serum albumin: fluorescence, fourier transform infrared, circular dichroism, and docking studies

(-)-Epigallocatechin-3-gallate (EGCG), the major constituent of green tea has been reported to prevent many diseases by virtue of its antioxidant properties. The binding of EGCG with human serum albumin (HSA) has been investigated for the first time by using fluorescence, circular dichroism (CD), Fourier transform infrared (FTIR) spectroscopy, and protein-ligand docking. We observed a quenching of fluorescence of HSA in the presence of EGCG. The binding parameters were determined by a Scatchard plot and the results were found to be consistent with those obtained from a modified Stern-Volmer equation. From the thermodynamic parameters calculated according to the van't Hoff equation, the enthalpy change deltaH degrees and entropy change deltaS degrees were found to be -22.59 and 16.23 J/mol K, respectively. These values suggest that apart from an initial hydrophobic association, the complex is held together by van der Waals interactions and hydrogen bonding. Data obtained by fluorescence spectroscopy, CD, and FTIR experiments along with the docking studies suggest that EGCG binds to residues located in subdomains IIa and IIIa of HSA. Specific interactions are observed with residues Trp 214, Arg 218, Gln 221, Asn 295 and Asp 451. We have also looked at changes in the accessible surface area of the interacting residues on binding EGCG for a better understanding of the interaction.     

Tetrapyrrole biosynthesis in Anacystis nidulans; incorporation of [1-13C]-, [2-13C]-, [1,2-13C]- and [2-13C, 2-2H3]acetate

Labelling experiments with [2-¹³C]- and [1,2-¹³C]acetate showed that both photopigments of Anacystis nidulans, chlorophyll a and phycocyanobilin, share a common biosynthetic pathway from glutamate. The fate of deuterium during these biosynthetic events was studied using [2-¹³C, 2-²H₃]acetate as a precursor and determining the labelling pattern by ¹³C NMR spectroscopy with simultaneous [¹H, ²H]-broadband decoupling. The loss of ²H (ca 20%) from the precursor occurred at an early stage during the tricarboxylic acid cycle. After formation of glutamate there was no further loss of ²H in the assembly of the cyclic tetrapyrrole intermediates or during decarboxylation and modification of the side-chains. Thus the labelling data support a divergence in the pathway to cyclic and linear tetrapyrroles after protoporphyrin IX.     

Circular dichroism and circular polarization of luminescence of reduced nicotinamide adenine dinucleotide in solution and bound to dehydrogenases.

The CD (circular dichroism) and CPL (circular polarization of luminescence) spectra of NADPH in aqueous solution were studied and found to be markedly different. The spectra were not affected by cleavage of the coenzyme molecule with phosphodiesterase. The differences are thus not due to the existence of extended and folded conformations of NADPH and it is concluded that they originate in excited state conformational changes of the nicotinamide--ribose fragment. Opposite signs of both the CD and CPL spectra were observed for NADH bound to horse liver alcohol dehydrogenase and to beef heart lactate dehydrogenase indicating structural differences between the nicotinamide binding sites. The binding of substrate analogues to enzyme--coenzyme complexes did not affect the CD spectra and hence no significant conformational changes are induced upon formation of the ternary complexes. No changes in the CPL spectrum of NADH bound to lactate dehydrogenase were observed upon adding oxalate to form the ternary complex. Marked differences were found between the CPL spectra of binary and ternary complexes with liver alcohol dehydrogenase, while the CD spectra of these complexes were identical. It is concluded that a conformational change of the excited NADH molecule occurs in the binary but not in the ternary complex involving LADH, thus indicating an increased rigidity of the latter complex.     

Differences in circular dichroism and fluorescence between the DNA complexes formed with 3-amino- and 3,8- diaminophenanthridinium derivatives

The conformation of DNA complexes formed with various 3-amino- and 3,8-diamino-phenanthridinium derivatives were examined by CD and fluorescence methods. The CD of these complexes is characterized by major bands in the 300–350-nm and the 400–550-nm regions. The CD properties of the complexes formed with diaminophenanthridinium derivatives suggest that the structure of such complexes is well represented by the intercalation complex formed between DNA and ethidium bromide. The substantial and regular increases in ellipticities near 308 nm that occur with increasing DNA-bound diaminophenanthridinium to DNA phosphate ratios (r) may result from direct interactions between molecules intercalated in neighbouring binding sites. In contrast, the changes in the shape of the CD of DNA complexes of monoaminophenanthridinium derivatives with r and the much lower maximum ellipticities attained suggest that near-neighbor interactions among intercalated monoaminophenanthridinium derivatives occur much less efficiently than in the corresponding diamino complexes, if at all. Although alternative explanations for the differences in the optical properties between the mono- and diaminophenanthridinium complexes of DNA may be offered, such results seem to indicate that complexes formed with monoaminophenanthridinium derivatives are characterized by a conformation which is quite distinct and different from that of the DNA–diaminophenanthridinium complexes. This conclusion is further supported by the considerable increase in fluorescence that accompanies the binding of the diaminophenanthridinium derivatives to DNA as compared to the minor increases, which occur upon the binding of the monoaminophenanthridinium compounds. The importance of conformation as a factor influencing template, function, especially with respect to the RNA polymerase-catalyzed synthesis of RNA, is now well appreciated. Therefore, methods which could provide information readily about changes in the conformation of a template, i.e., as a result of dye intercalation, are expected to facilitate our understanding of the effects of conformational change on the function and activity of templates.     

N'-[2-(2-thiophene)ethyl]-N'-[2-(5-bromopyridyl)] thiourea as a potent inhibitor of NNI-resistant and multidrug-resistant human immunodeficiency virus-1

The thiophene-ethyl thiourea (TET) compound N′-[2-(2-thiophene)ethyl]-N′-[2-(5-bromopyridyl)]-thiourea (compound HI-443) was five times more potent than trovirdine, 1250 times more potent than nevirapine, 100 times more potent than delavirdine, 75 times more potent than MKC-442, and 50 times more potent than AZT against the multidrug resistant HIV-1 strain RT-MDR with a V106A mutation. HI-443 was almost as potent against the NNI-resistant HIV-1 strain A17 with a Y181C mutation as it was against HTLV_IIIB. The activity of HI-443 against A17 was ten times more potent than that of trovirdine, 2083 times more potent than that of nevirapine, and 1042 times more potent than that of delavirdine. HI-443 inhibited the replication of the NNI-resistant HIV-1 strain A17 variant with Y181C plus K103N mutations in RT with an IC₅₀ value of 3.3 μM, whereas the IC₅₀ values of trovirdine, nevirapine, and delavirdine were all >100 μM. These findings establish the novel thiophene containing thiourea compound HI-443 as a novel NNI with potent antiviral activity against NNI-sensitive, NNI-resistant and multidrug-resistant strains of HIV-1.     

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.     

Synthesis of [1,2-3H2]cholecalciferol and metabolism of [4-14C,1,2-3H2]- and [4-14C,1-3H]-cholecalciferol in rachitic rats and chicks

[1,2-(3)H(2)]Cholecalciferol has been synthesized with a specific radioactivity of 508mCi/mmol by using tristriphenylphosphinerhodium chloride, the homogeneous hydrogen catalyst. With doses of 125ng (5i.u.) of [4-(14)C,1-(3)H(2)]cholecalciferol the tissue distribution in rachitic rats of cholecalciferol and its metabolites (25-hydroxycholecalciferol and peak P material) was similar to that found in chicken with 500ng doses of the double-labelled vitamin. The only exceptions were rat kidney, with a very high concentration of vitamin D, and rat blood, with a higher proportion of peak P material, containing a substance formed from vitamin D with the loss of hydrogen from C-1. Substance P formed from [4-(14)C,1,2-(3)H(2)]cholecalciferol retained 36% of (3)H, the amount expected from its distribution between C-1 and C-2, the (3)H at C-1 being lost. 25-Hydroxycholecalciferol does not seem to have any specific intracellular localization within the intestine of rachitic chicks. The (3)H-deficient substance P was present in the intestine and bone 1h after a dose of vitamin D and 30min after 25-hydroxycholecalciferol. There was very little 25-hydroxycholecalciferol in intestine at any time-interval, but bone and blood continued to take it up over the 8h experimental period. It is suggested that the intestinal (3)H-deficient substance P originates from outside this tissue. The polar metabolite found in blood and which has retained its (3)H at C-1 is not a precursor of the intestinal (3)H-deficient substance P.     

Exploring potentialities and limitations of stapled o‐oligo(phenyleneethynylene)s (o‐OPEs) as efficient circularly polarized luminescence emitters

In this paper, we have studied the chiroptical properties of a family of o‐oligo(phenyleneethynylene) (o‐OPE) derivatives with different steric hindrance. Experimental results show high dissymmetry factors (g_abs and g_lum up to 1.1 × 10^?2) and very similar electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) for all the derivatives that make this basic o‐OPE scaffold a robust pure organic emitter. Vibrational circular dichroism spectra are used to characterize conformational properties in solution. Density functional theory and time‐dependent density functional theory calculations support experimental results also proving that ECD and CPL are almost exclusively linked to helical moiety and not to size or conformation of substituents. As chiroptical properties of these emitters are independent of substituents, this OPE scaffold can be used as basic skeleton for the design of sensing probes with high CPL efficiencies.