Near-infrared Circular Dichroism Research Articles

Strong and Tunable Near-Infrared Circular Dichroism in Helical Tetrapyrrole Complexes.

The selective synthesis of nickel and copper complexes of 19-benzoyl-5,10,15-triphenyl-bilatrien-1-one (H2 TPBT) is reported, a molecule which crystallizes as a molecular helix of one-and-a-quarter which turns with a 5.7 Å radius and a 3.2 Å pitch, and all 26 participating atoms are sp2 -hybridized. UV/vis, ECD, ESR and cyclic voltammetry experiments reveal a strong interaction between metal and ligand and partial radical character when copper is coordinated instead of nickel. Strong ECD absorption in the 800 nm range is found which, using TD-DFT calculations as well as literature spectra, is shown to be highly tunable both by metal coordination and variation of the aryl groups in the TPBT periphery. The radical character of the ligand in Cu(TPBT) enables rapid interconversion between (M)- and (P)-enantiomers, possibly via intermittent breakage of a Cu-N bond. The 19-benzoyl group kinetically stabilizes enantiopure (M/P)-Ni(TPBT). The results are interpreted with regard to the application as circularly polarized light (CPL) detectors as well as to the chirality-induced spin-selectivity (CISS) effect which is currently lacking a concise theoretical model.

Near-infrared circular dichroism of the ytterbium DOTMA complex: an ab initio investigation.

The electronic structure and circular dichroism spectra of the ytterbium(III) complex [Yb(DOTMA)]- are calculated using complete and restricted active space self-consistent field wavefunction methods with the spin-orbit coupling treated by the state interaction approach. The influence of the dynamical correlation effect is then included via the 2nd order perturbation method. The experimental circular dichroism spectrum is well reproduced by calculations, both in terms of relative energy excitations and in terms of rotatory strength intensities. The results allow highlighting the mechanism that drives the chiroptical properties in Yb(III) complexes and reveal the importance of taking into account the 4f125d1 electronic configurations in the calculated wavefunctions to properly describe the chiroptical properties of the 4f-4f transitions.

Systematic Perturbations of Binuclear Non-heme Iron Sites: Structure and Dioxygen Reactivity of de Novo Due Ferri Proteins.

DFsc (single-chain due ferri) proteins allow for modeling binuclear non-heme iron enzymes with a similar fold. Three 4A → 4G variants of DFsc were studied to investigate the effects of (1) increasing the size of the substrate/solvent access channel (G4DFsc), (2) including an additional His residue in the first coordination sphere along with three additional helix-stabilizing mutations [3His-G4DFsc(Mut3)], and (3) the three helix-stabilizing mutations alone [G4DFsc(Mut3)] on the biferrous structures and their O2 reactivities. Near-infrared circular dichroism and magnetic circular dichroism (MCD) spectroscopy show that the 4A → 4G mutations increase coordination of the diiron site from 4-coordinate/5-coordinate to 5-coordinate/5-coordinate, likely reflecting increased solvent accessibility. While the three helix-stabilizing mutations [G4DFsc(Mut3)] do not affect the coordination number, addition of the third active site His residue [3His-G4DFsc(Mut3)] results in a 5-coordinate/6-coordinate site. Although all 4A→ 4G variants have significantly slower pseudo-first-order rates when reacting with excess O2 than DFsc (∼2 s(-1)), G4DFsc and 3His-G4DFsc(Mut3) have rates (∼0.02 and ∼0.04 s(-1)) faster than that of G4DFsc(Mut3) (∼0.002 s(-1)). These trends in the rate of O2 reactivity correlate with exchange coupling between the Fe(II) sites and suggest that the two-electron reduction of O2 occurs through end-on binding at one Fe(II) rather than through a peroxy-bridged intermediate. UV-vis absorption and MCD spectroscopies indicate that an Fe(III)Fe(III)-OH species first forms in all three variants but converts into an Fe(III)-μ-OH-Fe(III) species only in the 2-His forms, a process inhibited by the additional active site His ligand that coordinatively saturates one of the iron centers in 3His-G4DFsc(Mut3).

Circular Dichroism Techniques: Biomolecular and Nanostructural Analyses‐ A Review

This paper reviews the best known techniques using circular dichroism spectroscopy such as conventional circular dichroism (i.e. electronic circular dichroism), magnetic circular dichroisms (magnetic vibrational circular dichroism, x-ray magnetic circular dichroism), fluorescence detected circular dichroism, near-infrared circular dichroism, vibrational circular dichroism, Fourier transform infrared circular dichroism, high pressure liquid chromatography circular dichroism, stopped-flow circular dichroism, and synchrotron radiation circular dichroism. Also, we have described here the most important applications of circular dichroism spectroscopy in structural biochemistry and nanoscience.

Special issue “Advances in Chiroptical Methods”

Special issue “Advances in Chiroptical Methods”

Solution study of chiral 1,2-diol Yb 3+ chelates through near-IR circular dichroism

Ytterbium is efficiently chelated by 1,2-diols in a variety of very different solvents. We found a set of solution equilibria among three complexes of minimum formula [Yb·(diol)], [Yb·(diol) 2] and [Yb·(diol) 3]. By extensive near-Infrared circular dichroism (NIR CD) investigation, we determined fingerprint CD spectra of the di- and tri-chelated species and the relative formation constants. Since NIR CD is sensitive only to chiral Yb-species, NIR CD can be very useful for studying complex mixtures, such as those used in enantioselective catalysis and involving a chiral non-racemic diol as an auxiliary and an ytterbium salt. The chiroptical spectra show completely conserved features through very different diol structures, demonstrating the identity of the coordination polyhedron around Yb(III) and giving rise to a new empirical method for assigning 1,2-diols absolute configuration.

Kinetic and spectroscopic studies of N694C lipoxygenase: a probe of the substrate activation mechanism of a nonheme ferric enzyme.

Lipoxygenases (LOs) comprise a class of substrate activating mononuclear nonheme iron enzymes which catalyze the hydroperoxidation of unsaturated fatty acids. A commonly proposed mechanism for LO catalysis involves H-atom abstraction by an FeIII-OH- site, best described as a proton coupled electron transfer (PCET) process, followed by direct reaction of O2 with the resulting substrate radical to yield product. An alternative mechanism that has also been discussed involves the abstraction of a proton from the substrate by the FeIII-OH leading to a sigma-organoiron intermediate, where the subsequent sigma bond insertion of dioxygen into the C-Fe bond completes the reaction. H-atom abstraction is favored by a high E(o) of the FeII/FeIII couple and high pK(a) of water bound to the ferrous state, while an organoiron mechanism would be favored by a low E(o) (to keep the site oxidized) and a high pK(a) of water bound to the ferric state (to deprotonate the substrate). A first coordination sphere mutant of soybean LO (N694C) has been prepared and characterized by near-infrared circular dichroism (CD) and variable-temperature, variable-field (VTVH) magnetic circular dichroism (MCD) spectroscopies (FeII site), as well as UV/vis absorption, UV/vis CD, and electron paramagnetic resonance (EPR) spectroscopies (FeIII site). These studies suggest that N694C has a lowered E degrees of the FeII/FeIII couple and a raised pKa of water bound to the ferric site relative to wild type soybean lipoxygenase-1 (WT sLO-1) which would favor the organoiron mechanism. However, the observation in N694C of a significant deuterium isotope effect, anaerobic reduction of iron by substrate, and a substantial decrease in k(cat) (approximately 3000-fold) support H-atom abstraction as the relevant substrate-activation mechanism in sLO-1.

Investigation of the Effects of Different Carotenoids on the Absorption and CD Signals of Light Harvesting 1 Complexes

Absorption and circular dichroism (CD) spectra of light-harvesting (LH)1 complexes from the purple bacteria Rhodobacter (Rba.) sphaeroides and Rhodospirillum (Rsp.) rubrum are presented. The complexes exhibit very low intensity, highly nonconservative, near-infrared (NIR) CD spectra. Absorption and CD spectra from several mutant and reconstituted LH1 complexes, with the carotenoid neurosporene and the precursor phytoene replacing the wild-type (WT) carotenoids, are also examined. The experiments show that the position of the carotenoid bands as well as the bacteriochlorophyll (BChl)/carotenoid ratio affect the NIR CD spectra: bluer bands and larger ratios make the NIR CD signal more conservative. Modeling results that support this finding are presented. This study, combined with the theoretical approach of the companion paper, where modeling of such complexes is presented and discussed in detail, provide a complete explanation of the origin of the nonconservative NIR CD spectra of LH1 and B820.

Explaining the Visible and Near-Infrared Circular Dichroism Spectra of Light-Harvesting 1 Complexes from Purple Bacteria: A Modeling Study

In this work, we investigate the origin and characteristics of the circular dichroism (CD) spectrum of various light-harvesting 1 (LH1) complexes. The near-infrared (NIR) CD signal of these core antennae is strongly nonconservative, and the nature of this nonconservativity is under examination in this paper. So far, on the basis of the high-resolution structures of LH2, we have been able to model the absorption and CD spectra in the bacteriochlorophyll (BChl) Q(Y) and Q(X) regions of LH2 (Georgakopoulou et al., Biophys. J. 2002, 82, 2184-2197), as well as in the carotenoid region (Georgakopoulou et al., Biophys. J. 2004, 87, 3010-3022). We proceed by applying the same modeling method in order to reproduce the LH1 spectra. We assume a ring of dimers in a perfect circular arrangement with 16-fold symmetry, and account for all excitonic interactions within the ring. Because LH1 complexes exhibit Q(Y) and Q(X) CD signals of very low intensity, higher transitions can easily affect these regions. Therefore, we expand the model and take into account also the Soret and carotenoid transitions. We can now understand the shape of the absorption and CD spectra and contemplate the structure of the LH1 complex. The latter is similar to LH2 in that it is a very symmetric ring dominated by excitonic interactions. The larger number of symmetry and the bigger diameter of LH1, combined with small rotations of the BChl transition dipole moments, are responsible for the display of CD signals that are very low in intensity. The interaction of the Q(Y) with the carotenoid transitions results in complete loss of the conservativity. Interaction energies between all the pigments in the ring are calculated, and their values are in good accordance with what is reported in the literature.

Structural and chiroptical properties of the two coordination isomers of YbDOTA-type complexes.

Studies of the structural, physical, and chemical properties of the lanthanide(III) complexes of DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) and related ligands are often complicated by the presence of two coordination isomers in solution. Since these coordination isomers are in exchange and cannot be separated, many techniques offer information only on the weighted average of the two isomers. Lanthanide ion complexes formed with the ligands S(RRRR)NO2BnDOTMA and S(SSSS)NO2BnDOTMA preferentially adopt only one of the two common coordination isomers in solution, so the ytterbium complexes of these ligands offer a unique opportunity to study the near-infrared circular dichroism (NIR-CD) characteristics of each coordination geometry in isolation. The spectra acquired support many of the conclusions and assumptions of previous studies and demonstrate that this spectroscopy is particularly sensitive to the distortion of the coordination polyhedron. This will have particularly relevant consequences when studying achiral YbDOTA-like systems forming labile adducts with (chiral) hosts and receptors.

The Reaction Center H Subunit Is Not Required for High Levels of Light-Harvesting Complex 1 in Rhodospirillum rubrum Mutants

The gene (puhA) encoding the H subunit of the reaction center (RC) was deleted by site-directed interposon mutagenesis by using a kanamycin resistance cassette lacking transcriptional terminators to eliminate polar effects in both the wild-type strain Rhodospirillum rubrum S1 and the carotenoid-less strain R. rubrum G9. The puhA interposon mutants were incapable of photoheterotrophic growth but grew normally under aerobic chemoheterotrophic conditions. Absorption spectroscopy and sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed that the RCs were absent. In minimal medium and also in modified medium containing succinate and fructose, the light-harvesting 1 complex (LH1) levels of the S1-derived mutants were about 70 to 100% of the wild-type levels in the same media. The correct assembly of LH1 in the membrane and the pigment-pigment interaction were confirmed by near-infrared circular dichroism spectroscopy. LH1 formation was almost absent when the carotenoid-less G9-derived puhA mutants were grown in standard minimal medium, suggesting that carotenoids may stabilize LH1. In the fructose-containing medium, however, the LH1 levels of the G9 mutants were 70 to 100% of the parental strain levels. Electron micrographs of thin sections of R. rubrum revealed photosynthetic membranes in all mutants grown in succinate-fructose medium. These studies indicate that the H subunit of the RC is necessary neither for maximal formation of LH1 nor for photosynthetic membrane formation but is essential for functional RC assembly.

Stereochemistry and Near-Infrared Circular Dichroism of a Chiral Yb Complex

The near-infrared CD spectrum of ytterbium complexed by DOTMA, [DOTMA = (1R,4R,7R, 10R)-α,α‘,α‘ ‘,α‘ ‘‘-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid)] is presented. This is the first report of a near-infrared CD spectrum in solution of the f−f transitions of ytterbium in a complex whose solution structure and dynamics are completely defined. The spectrum shows several well-resolved transitions around 980 nm with very high dissymmetry factors. In water and in methanol, there are two diastereomeric conformations of YbDOTMA in equilibrium, Λ(δδδδ) or n and Λ(λλλλ) or p, being the latter always in a large excess. The relative proportion depends on temperature and solvent composition, but the CD spectrum is rather insensitive to these parameters, which demonstrates that neglecting the minor form contribution is an acceptable approximation. On lowering the temperature, the presence of hot bands is highlighted, which leads to a tentative assignment of the transitions. The absorption count...

The structural role of the carotenoid in the bacterial light-harvesting protein 2 (LH2) of Rhodonbacter capsulatus. A Fourier transform Raman spectroscopy and circular dichroism study

In previous work (Zurdo J, Fernández-Cabrera C and Ramírez JM (1993) Biochem J 290: 531-537), it had been shown that selective extraction of the carotenoid from the light-harvesting protein 2 (LH2) of Rhodobacter capsulatus induced the dissociation of 800-nm absorbing bacteriochlorophyll (Bchl), a 10-nm red shift of 854-nm Bchl, and a decrease of the stability of the protein in detergent solution. In the present study, the Fourier transform Raman and near-infrared circular dichroism spectra of native and carotenoid-depleted LH2 membrane preparations were compared. It was found that while the coupled carbonyls of 854-nm Bchl remained specifically H-bonded to the peptides after carotenoid extraction, the optical activity of the near-infrared electronic transition was significantly altered. Given the excitonic origin of such optical activity, our data suggest that carotenoid extraction elicits a rearrengement of the chromophore cluster and of the associated polypeptide subunits. This implies a significant role of the carotenoid in maintaining the native quaternary structure of the protein, which would be consistent with the observed dissociation of 800-nm Bchl and the loss of solubilized LH2 stability that result from carotenoid removal. There is no evidence for a similar role of the carotenoid in the LH1 protein.

Near-infrared circular dichroism and magnetic circular dichroism and x-ray absorption spectral comparison of the non-heme ferrous active sites of plant and mammalian 15-lipoxygenases

Lipoxygenases (LOs) are non-heme iron enzymes which catalyze the reaction of dioxygen with cis,cis- 1,4-pentadiene-containing fatty acids to form hydroperoxide products, which in mammals are the precursors to the inflammation- and immunity-mediating compounds lipoxins and leukotrienes. Recent X-ray crystal structures of ferrous soybean lipoxygenase-1 (SLO-I) offer two different descriptions of the active site: one four-coordinate and one five- or six-coordinate. Near-infrared (NfIR) circular and magnetic circular dichroism (CD/MCD) and variable- temperature, variable-field (VTVH) MCD have been used to study SLO-I in solution which is found to exist as a 40/60% mixture of five- and six-coordinate forms, respectively. Addition of linoleate substrate or alcohols shifts the mixture to the purely six-coordinate form. NIR CD/MCD studies of two mammalian LOs, rabbit reticulocyte and recombinant human 15-LOs, show that these exist as pure six-coordinate forms. X-ray absorption Fe K-edge and pre-edge data also show that the mammalian 15-LOs and SLO-I in glycerol are six-coordinate. This is consistent with the extended X-ray absorption fine structure (EXAFS) results of SLO-I in glycerol which show the iron active site to have 5 ± 1 N/0 at ~2.16 A. VTVH MCD data on the six-coordinate sites show that the mammalian and soybean enzymes have very different ground-state splittings, indicative of differences in bonding interactions with the ligand set. These differences in ferrous site coordination in solution and ground-state splittings are attributed to the substitution of a stronger histidine ligand in the mammalian 15-LOs for an asparagine in SLO-I.

Large intramolecular energy flow in vibrational overtone spectra of cyclohexene-3,3,6,6-d4

The vibrational structure of CH stretching states in gas-phase cyclohexene-3,3,6,6-d 4 was studied using FTIR spectroscopy in the range 1200-11500 cm -1 and intracavity dye laser photoacoustic spectrometry in the range 12900-16000 cm -1 . The structure was modeled using an effective vibrational Hamiltonian which describes the Fermi resonance couplings of the CH stretching states with suitable low-frequency vibrations. Some conclusions are made on the possible ways of intramolecular vibrational redistribution of the energy (IVR) on the two methylenic groups for wavenumbers below 11 000 cm -1 in connection with the existence of near-infrared circular dichroism in monoterpenes

Vibrational circular dichroism as a criterion for local-mode versus normal-mode behavior. Near-infrared circular dichroism spectra of some monoterpenes

Vibrational circular dichroism as a criterion for local-mode versus normal-mode behavior. Near-infrared circular dichroism spectra of some monoterpenes

A Microprocessor Controlled Near-Infrared Circular Dichroism Spectrophotometer

A near-infrared circular dichrometer was constructed to cover the wavelength range 0.5 to 2.1 μm. The instrument employed a commercial photoelastic modulator with an infrasil quartz optical element to provide quarter-wave modulation throughout the range of the instrument. An indium antimonide [In(Sb)] doped photovoltaic detector was used at Λ > 1 μm, and a near-IR sensitive photomultiplier detector was used at Λ < 1 μm. Phase- and frequency-sensitive detection was employed and an automatic normalization circuit was used to extract the circular dichroism from the detector signal. The level of artifactual signals was reduced by the use of a passive depolarizing element. The instrument was interfaced with a microprocessor system which provided experimental control and signal enhancement capability. Data were transferred to a minicomputer system which provided bulk storage and both numeric and graphic hard copy output. The instrument, with its computer control capabilities, represents a significant improvement over previously existing instruments operating in this wavelength range in its facility for digital signal enhancement and data analysis.

A Near-Infrared Circular Dichroism and Magnetic Circular Dichroism Instrument

We report a high sensitivity, near-infrared circular dichroism (CD) instrument. CD is measured by the standard modulation method; phase modulation is provided by an Infrasil quartz photoelastic modulator. The spectral range 0.8–3.0 μ is spanned. Incorporation of a superconducting magnet allows magnetic circular dichroism (MCD) measurements to be made. The natural CD of nickel and samarium tartrate complexes and the MCD of a single crystal of Co/MgO at 12 K and 40.6 kG are reported. Performance of the instrument is comparable to visible-ultraviolet CD instruments.

Visible and near-infrared circular dichroism of some blue copper proteins

Circular dichroism spectra between 300 and 1100 nm are presented for stellacyanin, Rhus vernicifera laccase, Polyporus versicolor laccase and human ceruloplasmin. The number of bands in this region varies between four and six for the different proteins. In addition to these bands some unresolved ellipticity is present between 350 to 400 nm. It is shown that all these bands are associated with the blue chromophore. The effect of azide on ceruloplasmin is studied with optical and circular dichroism spectroscopy. The results are not fully interpreted but it is concluded that the decrease of the blue colour cannot be explained simply by reduction of copper.