A sensitive method based on fluorescence-detected circular dichroism for protein local structure analysis

Fluorescence detected circular dichroism (FDCD) of a stereodynamic probe

The use of chiroptical techniques in combination with stereodynamic probes is becoming one of the leading strategies for chiral sensing. While in most of the reported studies circular dichroism (CD) is the adopted spectroscopic technique, examples regarding the use of vibrational CD (VCD), circularly polarized luminescence (CPL), and Raman optical activity (ROA) are emerging as innovative tools. In this communication, an anthracene-decorated tris(2-pyridylmethyl)amine zinc complex (TPMA) is reported for its capability to act as a chiral sensor using either CD or fluorescence detected circular dichroism (FDCD). The latter technique offers the unique possibility to determine the enantiomeric excess of a series of carboxylic acids at sensor concentrations down to 0.1 μM. Limitations and possibilities opened by the use of this methodology, in particular regarding the specificity of the probe in the presence of another contaminant, are discussed.

Strong fluorescence-detected two-photon circular dichroism of chiral gold nanoclusters

Progress in syntheses and understanding of the intriguing properties of chiral noble metal nanoclusters sparks interest to extend investigations of their chiroptical response to the nonlinear optics regime. We present a quantitative determination of two-photon circular dichroism of chiral gold nanoclusters with ATT and L- or D-Arg ligands (ATT = 6-aza-2-thiotymine and Arg = arginine). Introduction of arginine ligands enables the formation of two enantiomers of the nanoclusters, with strong chiroptical effects in both linear and nonlinear regime. We present two-photon absorption and luminescent properties measured in a wide range of wavelengths, with the two-photon absorption cross section reaching 1743 GM and two-photon brightness ∼1102 GM at 825 nm. We report strong, 245-fold enhancement of the two-photon circular dichroism of nanoclusters with respect to the one-photon absorption counterpart - the dissymmetry factor. The presence of multiple advantages of nanoclusters: high fluorescence quantum yield, strong nonlinear optical properties and well-controlled chirality is a powerful combination for applications of such clusters in multiphoton microscopy.

Fluorescence detected circular dichroism (FDCD) for supramolecular host-guest complexes

Fluorescence-detected circular dichroism (FDCD) spectroscopy is applied for the first time to supramolecular host-guest and host-protein systems and compared to the more known electronic circular dichroism (ECD). We find that FDCD can be an excellent choice for common supramolecular applications, e.g. for the detection and chirality sensing of chiral organic analytes, as well as for reaction monitoring. Our comprehensive investigations demonstrate that FDCD can be conducted in favorable circumstances at much lower concentrations than ECD measurements, even in chromophoric and auto-emissive biofluids such as blood serum, overcoming the sensitivity limitation of absorbance-based chiroptical spectroscopy. Besides, the combined use of FDCD and ECD can provide additional valuable information about the system, e.g. the chemical identity of an analyte or hidden aggregation phenomena. We believe that simultaneous FDCD- and ECD-based chiroptical characterization of emissive supramolecular systems will be of general benefit for characterizing fluorescent, chiral supramolecular systems due to the higher information content obtained by their combined use.

Surface plasmon resonance prism coupler for enhanced circular dichroism sensing

A novel method for enhanced circular dichroism (CD) detection is proposed based on a surface plasmon resonance (SPR) prism coupler and a polarization scanning ellipsometry technique. An analytical model is derived to extract the CD and degree of polarization (DOP) properties of optical samples with and without scattering effects, respectively. The validity of the analytical model is confirmed by means of numerical simulations. The simulation results show that the proposed detection method has a sensitivity of 10^-5~10^-6 RIU (refractive index unit) for refractive indices in the range of 1.32~1.36 and 1.3100~1.3118. The practical feasibility of the proposed method is demonstrated by the experimental results for the sensitivity of the CD with the chlorophyllin samples with/without scattering effect. It is shown that for both types of sample, the extracted CD value increases linearly with the chlorophyll concentration over the considered range. In general, the results obtained in this study show that the measured CD response is highly sensitive to the polarization scanning angle. Consequently, the potential of polarization scanning ellipsometry for high-resolution CD detection is confirmed.

Daunomycin-loaded superparamagnetic iron oxide nanoparticles: Preparation, magnetic targeting, cell cytotoxicity, and protein delivery research

The clinical use of daunomycin is restricted by dose-dependent toxicity and low specificity against cancer cells. In the present study, modified superparamagnetic iron oxide nanoparticles were employed to load daunomycin and the drug-loaded nanospheres exhibited satisfactory size and smart pH-responsive release. The cellular uptake efficiency, targeted cell accumulation, and cell cytotoxicity experimental results proved that the superparamagnetic iron oxide nanoparticle-loading process brings high drug targeting without decreasing the cytotoxicity of daunomycin. Moreover, a new concern for the evaluation of nanophase drug delivery's effects was considered, with monitoring the interactions between human serum albumin and the drug-loaded nanospheres. Results from the multispectroscopic techniques and molecular modeling calculation elucidate that the drug delivery has detectable deleterious effects on the frame conformation of protein, which may affect its physiological function.

Cationic porphyrin@SPION nanospheres as multifunctional anticancer therapeutics: magnetic targeting, photodynamic potential and bio-safety research

Porphyrin@SPION nanospheres are described as pH-controllable, multifunctional photosensitizations with delivery bio-safety.

Anisotropic elliptical dichroism and influence of imperfection of circular polarization upon anisotropic circular dichroism

In spite of the importance of anisotropic circular dichroism, in practice, it is difficult to get rid of the artifacts that arise from the imperfection of the circular polarization. Undesirable linear dichroism, interference of two orthogonal polarization states, and linear birefringence prevent us from making accurate measurements. We propose a theoretical method for evaluating the contributions of the first two, which are thought to be the main artifacts when specimens are not thick enough. Using the time-dependent perturbation theory and taking into account the direction of light propagation toward an orientationally fixed molecule, we formulated the transition probability of systems perturbed by arbitrarily polarized light and the absorption difference associated with two kinds of polarized light. We also formulated, as an extension of the dissymmetry factor of circular dichroism, a newly defined dissymmetry factor associated with two arbitrary polarization states. Furthermore, we considered a mixed-state of photon ensemble in which polarization states distribute at a certain width around a certain average. Although the purity of polarization and ellipticity does not correspond immediately, by considering the mixed state it is possible to treat them consistently. We used quantum statistical mechanics to describe the absorption difference for two kinds of photon ensembles and applied the consequent formula to examine the reported experimental results of single-molecule chiroptical responses under discussion in the recent past. The artifacts are theoretically suggested to be sensitive to the incident direction of elliptically polarized light and to the oriented systems, the ellipticity, and the orientation of ellipse. The mixed state has little, if any, effect when the polarization state distribution is narrow.

Chiroptical spectroscopic techniques based on fluorescence

Among the various chiroptical spectroscopic methods circular dichroism (CD), optical rotatory dispersion (ORD), Raman optical activity (ROA), the methods based on fluorescence have so far played a marginal role. Fluorescence detected circular dichroism and circularly polarized luminescence (CPL) were both introduced many years ago with important, but not so frequent, applications. In particular, CPL, which requires specialist hardware, has been restricted by the limited number of users. Recent interest, which has increased in the field of material science particularly, where the emission properties are more attractive and important than the absorption ones, has motivated new application fields and may change the situation in the future.