Circular dichroism spectra of DNA hairpins studied by the green function method

The circular dichroism (CD) spectrum of a large biological molecule represented as an array of interacting chromophores is investigated. The configuration-averaged Green function formalism is developed to describe the CD and absorption spectra. The perturbation theory expansion is derived for absorption and CD spectra in the case of strong interaction of chromophores with their environment (solvent and/or internal dynamics) compared to their interaction with each other. We apply this formalism to study CD spectra of DNA hairpins.     

Dynamic perturbation effects upon the circular dichroism intensity induced in an aggregate of dye chromophores bound to biopolymers

The dynamic perturbation effects of polarizable monomer perturbers upon the circular dichroism intensity arising from absorption transitions of an arbitrary aggregate of dye chromophores bound to a large host polymer are formulated using the linear response theory in the decorrelation approximation, where the interchromophoric retardation phase factors are eliminated by a first-order Taylor expansion which is compatible with the use of the retarded helix selection rules in the long-wavelength approximation. A space-averaged and closed-form formulation of the non-conservative circular dichroism intensity which is perturbed by intensity with the outside perturber transitions is derived in the limit of the weak dynamic perturbation where perturber—perturber interactions are negligible. The relevant formulation is applied in order to investigate the intercalation model dependence of the non-conservative circular dichroism intensity induced at the visible absorption band of proflavine molecules intercalated in either poly(A—T) or poly(G—C).     

THE ATTACHMENTS OF PHYCOBILINS TO CYSTEINYL RESIDUES

The attachments of phycobilins to cysteinyl residues have been worked out through the reactions of phycoerytbrobilin dimethyl ester and phycocyanobilin dimethyl ester with cysteine methyl ester and reduced glutathione dimethyl ester respectively. A series of model compounds which carry the same conjugative skeletons as the chromophores in phycobillproteins have been purified anti identified. These compounds are characterized with strong fluorescence emission and circular dichroism effects which appeared weaker for the common bilinoid chromophores. Analyses of the circular dichroism effects of these compounds indicated that the major reaction products of phycobilins with cysteine methyl ester and reduced glutathione dimethyl ester carried the same stereochemical configuration as the chromophores in phvcobilinoroteins     

Stepwise evolution of the structure and electronic properties of DNA

The optical properties of a family of duplex DNA conjugates possessing from 1 to 11 A:T base pairs and covalently attached stilbene chromophores at both ends have been investigated. As few as from two to four base bairs are needed to establish base-pair absorption and circular dichroism spectra similar to those of longer oligomers. The stilbene chromophores exhibit exciton-coupled circular dichroism, which can be observed even with 11 intervening base pairs, a distance of ca. 40 A. The dependence of the sign and amplitude of these spectra reveals the ability of B-DNA to serve as a helical ruler.     

Sulfoxides as chiral complexation agents. Conformational enantiomer resolution and induced circular dichroism of bilirubins

Bilirubin-IX, the end product of porphyrin metabolism in mammals and the neurotoxic yellow-orange pigment of jaundice, exhibits a strong tendency to fold like a book into either of two interconverting enantiomeric conformations, which are further stabilized by intramolecular hydrogen bonding. Bilirubin exhibits optical activity in R-(-)-ethylmethylsulfoxide solvent, as seen by moderately strong bisignate circular dichroism Cotton effects (Δε^max₄₅₂=+10.9,Δε^max₄₀₄=-4.5), and in dichloromethane solution in the presence of 2M R-(+)-methyl-p-tolylsulfoxide its circular dichroism spectrum (Δε₄₆₃^max=+11.2,Δε₄₁₂^max=-7.1) is comparably strong. As observed earlier for chiral recognition of bilirubin by optically active amines and serum albumins, the optically active sulfoxide acts as a chiral complexation agent to induce an asymmetric transformation of bilirubin, whose bisignate circular dichroism spectra are characteristic of an exciton splitting arising from interaction of the two component dipyrrinone chromophores.     

Theory of electronic circular dichroism lineshapes

A microscopic, quantum field theory of lineshapes for electronic circular dichroism spectra is presented. A simple, model Hamiltonian for a single impurity in a crystal is considered. In this formalism, electron-photon coupling terms contribute directly to the magnetic transition dipole moment. Lineshape functions for absorbance and circular dichroism spectra are derived. Electronic circular dichroism spectra contain vibronic contributions which do not appear in absorbance spectra. This treatment does not require perturbation theory to obtain the vibrational contribution to the circular dichroism lineshape.     

Magneto-optic spectroscopy of a protein tetramer binding two exciton-coupled chlorophylls

In vitro chlorophyll (Chl) aggregates have often served as models for in vivo forms of long-wavelength Chl. However, the interaction of protein-bound Chl molecules is typically different than that occurring in solvent-based self-aggregates. We have chosen a water-soluble Chl-binding protein (WSCP) from cauliflower in order to help characterize the spectroscopic properties of Chl in a single well-defined native environment and also to study the pigment-pigment (exciton) interactions present in assemblies of this protein. WSCP forms tetrameric units upon binding two Chl molecules. We present the absorption, circular dichroism (CD), magnetic circular dichroism (MCD), and emission spectra at 1.7 K of recombinant WSCP tetramers containing either Chl a or Chl d. The spectroscopic characteristics provide evidence for significant exciton interaction between equivalent Chl molecules. Our simple exciton analysis allows an estimate of the molecular geometry of the dimer, which is predicted to have an "open sandwich"-type structure. We find that the ratio of the magnetic circular dichroism to absorption, deltaA/A, is substantially increased (approximately 60%) for Chl a in this system compared to its value in solution. This increase is in marked contrast to substantial reductions (>50%) of deltaA/A seen in solvent-based Chl aggregates and in photosynthetic reaction centers. Current theoretical models are unable to account for such large variations in the MCD to absorption ratio for Chl. We propose that spectroscopic studies of WSCP mutants will enable a fundamental understanding of Chl-Chl and Chl-protein interactions.     

Induced circular dichroism of cyclodextrin inclusion complexes: examining the cavity with a bilateral probe

The induced circular dichroism (ICD) of 1@7-Cy gives rise to an unusual spectrum which can be explained by a positive and a negative contribution of two identical chromophores. This finding is in agreement with rules predicting the ICD of chiral supramolecular systems.     

Exciton chirality method in vibrational circular dichroism

The interaction of two IR chromophores yields a strong vibrational circular dichroism couplet whose sign reflects the absolute configuration of the molecule. We present a method to determine absolute configuration of a chiral molecule based on this couplet without need of theoretical calculation. Not only can this method analyze various molecules whose absolute configuration is difficult to determine by other spectroscopic methods, but also it can significantly enhance VCD signals.     

Donor-acceptor polymers: a conjugated oligo(p-phenylene vinylene) main chain with dangling perylene bisimides

Two new donor-acceptor copolymers that consist of an enantiomerically pure oligo(p-phenylene vinylene) main chain with dangling perylene bisimides have been synthesized by using a Suzuki cross-coupling polymerization. Absorption and circular dichroism spectroscopy revealed that the transition dipole moments of the donor in the main chain and the dangling acceptor moieties of the copolymers are coupled and in a helical orientation in solution, even at elevated temperatures. A strong fluorescence quenching of both chromophores indicates an efficient photoinduced charge transfer after photoexcitation of either donor or acceptor. The formation and recombination kinetics of the charge-separated state were investigated in detail with femtosecond and near-steady-state photoinduced absorption spectroscopy. The charge-separated state forms within 1 ps after excitation, and recombination occurs with a time constant of 45-60 ps, both in solution and in the solid state. These optical characteristics indicate a short distance and appreciable interaction between the electron-rich donor chain and the dangling electron-poor acceptor chromophores.     

光学活性金属卟啉的磁手性效应研究

磁光学活性与自然光学活性均可用介质对左右圆偏振光的吸收之差来表示 .但自然光学活性和磁光学活性的物理机制是截然不同的 ,前者源于镜象不能互相重叠介质的非定域光学响应 ( nonlocaloptical response) ,而后者则是由于介质的时间反演对称性 ( time- reversal symmetry)被磁场打破所致 .理论分析表明 ,当介质的两种光学活性同时存在时 ,将会出现一个新的附加光学效应 ,这种磁光学活性与自然光学活性之间的交叉效应称为磁手性效应 ( magneto- chiral effect)或磁手二色性 ( magneto-chiral dichroism) [1,2 ] .磁手性效应通常很弱 ,直到 1…     

The chiroptical properties of a new liquid-crystalline polymer: (trityl)(pentyl)cellulose

The chiroptical properties of a new liquid crystalline polymer, trityl pentyl cellulose (TPeC), are discussed. This polymer forms lyotropic liquid-crystalline phases in tetrahydrofuran and chloroform. The cholesteric twist sense is right handed in both solvents. The dilute solution optical activity of the phenyl chromophores, measured by circular dichroism (CD), showed no evidence of the exciton splitting expected for a helical arrangement of chromophores. The sign and magnitude of the CD signals were found to change considerably upon mesophase formation.     

DNA molecular photonics

Synthetic DNA conjugates in which one or both ends of a short duplex is capped by a stilbene chromophore have been prepared and characterized crystallographically. Selective excitation of the chromophore can be used to initiate electron transfer processes in which a nucleobase serves as either an electron donor or an electron acceptor. These processes include hole- and electron injection and hole migration. The dynamics of these processes and its dependence on distance, driving force, and base sequence have been investigated by means of femtosecond time-resolved spectroscopy. Duplexes with identical chromophores at both ends have been used to study both the dynamics of electron transfer processes and exciton coupling between the two chromophores by means of circular dichroism spectroscopy. Duplexes with different chromophores can also be used to study distance dependence of both electron transfer and exciton coupling.     

Studies of the circular dichroism spectra of dissymmetric Schiff-bases by means of the exciton chirality method

The circular dichroism spectra of some dissymmetrical Schiff-bases derived from condensation of 2 mol salicylaldehyde with 1 mol of a chiral diamine are analyzed in terms of a simple conformational model taking the angle between the aromatic chromophores as a variable. The rotatory strengths of the π→π* transitions are calculated with AM1 molecular orbitals in combination with the dipole-coupling approximation. Calculated CD curves are found to be in reasonable agreement with the experimental CD spectra. Differences in the CD spectra of the Schiff-bases are discussed in relation to variation of the theoretical CD curve of ethylenebis(salicylideneimine) as a function of conformation.     

Circularly Polarized Light Probes Excited-State Delocalization in Rectangular Ladder-type Pentaphenyl Helices

Ladder-type pentaphenyl chromophores have a rigid, planar π-system and show bright fluorescence featuring pronounced vibrational structure. Such moieties are ideal for studying interchromophoric interactions and delocalization of electronic excitations. We report the synthesis of helical polymers with a rigid square structure based on spiro-linked ladder-type pentaphenyl units. The variation of circular dichroism with increasing chain length provides direct evidence for delocalization of electronic excitations over at least 10 monomeric units. The change in the degree of circular polarization of the fluorescence across the vibronic side bands shows that vibrational motion can localize the excitation dynamically to almost one single unit through breakdown of the Born-Oppenheimer approximation. The dynamic conversion between delocalized and localized excited states provides a new paradigm for interpreting circular dichroism in helical polymers such as proteins and polynucleic acids.     

Method to determine enantiomeric excess of glucose by nonchiral high-performance liquid chromatography using circular dichroism detection

Using nonchiral high-performance liquid chromatography (HPLC) coupled with a circular dichroism (CD) detector, we developed a new method to determine the enantiomeric excess (e.e.) of glucose derivatized with chromophores (p-bromobenzoate). This method is suitable for microscale analysis of glycoside and saccharide. As an application of this method, the enantiomeric excess of the component glucose of a glucan was assayed.     

Konformation und Circulardichroismus der Sesquiterpene vom Trichothecan-Typ und deren makrocyclischen Estern

The circular dichroism of keto derivatives with trichothecane skeleton and some of their rearrangement products has been measured. The sign (and sometimes even the magnitude) of all COTTON effects can be explained by applying the respective rules for the cyclohexanone, cyclopentanone, enone and ene-ester chromophores. Thus conformations could also be determined in those cases which did not follow unambiguously from conformational analysis. The conformation of the muconic acid moiety of the macrocyclic ring of verrucarin A is the same in dioxane solution as in the crystalline state.     

FLUORESCENCE AND CIRCULAR DICHROISM STUDIES ON THE PHYCOERYTHROCYANINS FROM THE CYANOBACTERIUM

Two phycoerythrocyanin (PEC) fractions have been obtained from the phycobilisomes of the cyanobac-terium Westiellopsis prolifica ARM 365. They have been characterized by absorption, fluorescence and circular dichroism spectroscopy. One of them is spectroscopically similar to a PEC trimer known from other organisms. Whereas efficient energy transfer from its violin (α-84) to the cyanin (β-84, 155) chromophores is efficient in the trimer (αβ it is impeded after dissociation to the monomer (α,β). A second fraction of PEC which we earlier termed PEC(X) (Maruthi Sai et al., Photochem. Photobiol. 55 ,119–124, 1992), exhibited the spectral properties similar to that of the α-subunit of PEC from Mastigocladus laminosus. With this highly photoactive fraction, the circular dichroism spectra of the violobilin chromophore in both photoreversible states were obtained.     

Determination of the stereochemistry of condensation products between α,β-diamines and α,γ-dioxo compounds by circular dichroism, 1H NMR and x-ray diffraction

The absolute configuration of molecules containing two chromophores of the type -NH-CC-CO bridged by -?-?- have been studied in solution by UV absorption and circular dichroism. The spectra have been simulated by means of component curves shaped as skew gaussians. The data have been interpreted by means of exciton theory for coupled chromophores to give information about the relative orientation of the chromophores. The result is compared with the result of an X-ray structure determination of N,N'-bis(4-oxo-2-penten-2-yl)-R-1,2-diaminopropane.     

Novel chiral pyromellitdiimide (1,2,4,5-benzenetetracarboxydiimide) dimers and trimers: exploring their structure,electronic transitions,and exciton coupling

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200-260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems.