Enantiomeric separation of mirtazapine and its metabolite in rat plasma by reverse polar ionic liquid chromatography using fluorescence and polarimetric detectors connected in series

A simple and rapid reverse polar ionic LC method was developed and validated for simultaneous separation and determination of mirtazapine, an antidepressant drug, and its main metabolite N-desmethyl mirtazapine using fluorescence and polarimetric detectors connected in series. The chromatographic separation was achieved on Chirobiotic V column packed with vancomycin as a stationary phase in an isocratic mode of elution of methanol:glacial acetic acid:anhydrous triethyl amine (100:0.2:0.1, v/v/v) as a mobile phase. The compounds were detected by their excitation at 290nm and emission at 370nm using fluorescence detector while the optical rotation (+/-) of the enantiomers was identified by polarimetric detector. The analytes were extracted from rat plasma by precipitation of proteins and the average yield was 88-111% for mirtazapine and 85-123% for N-desmethyl mirtazapine. The method was linear over the concentration range of 20-5000ng/mL. The method was successfully applied on rat plasma spiked with the enantiomers of mirtazapine and N-desmethyl mirtazapine.     

Polarimetric detection in high-performance liquid chromatography

Chiroptical detection for HPLC is particularly useful as a selective detection method for chiral molecules, and in enantiomeric purity determination with partial chiral separation or without chiral separation. The recent development of laser-based polarimeters with microdegree sensitivity has increased the applicability of optical rotation detection in HPLC. The detection limit of these instruments is submicrogram on-column for many chiral compounds in analytical HPLC. A variety of applications of the selective detection of optically active molecules are reviewed. The use of polarimetric detection with partial chiral separation is considered, both as an aid to method development and for enantiomeric purity determination. Finally applications to enantiomeric purity determination without chiral separation are reviewed, with the dual use of nonchirally selective and chiroptical detectors to determine the total amount and optical purity of the analyte. Determinations of chiral purity for samples of high enantiomeric excess are described, which with laser-based instrumentation may give accuracies of better than +/- 1% with sample loadings of 50 micrograms on an achiral column. Applications to the study of enantioselective reactions are also considered, with determination of enantiomeric excess in near-racemates to better than +/- 0.1%.     

Detection and identification of optical activity using polarimetry – applications to biophotonics,biomedicine and biochemistry

Polarized light that is reflected or transmitted through chiral specimens can be used to detect and identify biological and chemical materials including human tissue. The determination of the silent footprints of the chiral properties of the biological materials on scattered polarized light is the basis for these investigations. It is of primary importance to identify which combinations of the elements of the Mueller matrix for reflected or for transmitted light can be used to determine the optical activity of the biochemical materials. The optical activity of chiral materials is characterized by optical rotation and circular dichroism. The explicit analytical dependence of these specific elements of the Mueller matrix, upon the angles of incidence and scatter, upon the wavelength and upon the type of chirality has the potential to provide experimentalists with guidance in determining the optimum use of optical polarimetric scatterometers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Online monitoring of preferential crystallization of enantiomers

Polarimetry is used for continuous online monitoring of optical resolution by preferential crystallization. In combination with refractometry the liquid phase composition is determined, allowing one to follow the resolution progress quantitatively. The measurement techniques were calibrated up to relatively high solution concentrations and combined with the crystallizer. The resolution of DL-threonine was performed by preferential crystallization experiments in aqueous solution varying several process parameters like supersaturation, seed amount, initial enantiomeric excess, and scale. The resolution progress can be conveniently described by profiles of the optical rotation (polarimetric signal) and the crystallization pathway in the corresponding ternary phase diagram. The method outlined is applicable for dynamic process optimization and control purposes in "quasi-continuous" chiral separation processes.     

Optical rotary dispersion and the Keston polarimetric accessory: a simplified method for use and calibration.

A method is described for using and calibrating the Keston polarimetric attachment to the Beckman DU spectrophotometer which dispenses with the need to reset, at each wavelength, the symmetry of the analyzer positions about total extinction with the polarizer. The instrument is calibrated with sucrose solutions of concentrations from 2.5 to 40%, over a wave-length range of 250 to 675 nm. The constancy of calibration with a maximum error of 6.5% over this range is demonstrated. The smooth but slight dependency of the calibration constant on wavelength is shown and attributed to the optical properties of the Nicol prisms. Linear regression of the data allows for a high degree of accuracy in measuring the rotation of a sample, as a function of wavelength.     

Quantification of trace levels of β-cyclodextrin and substitution patterns in hydroxypropyl-β-cyclodextrin using high-performance liquid chromatography with polarimetric detection

A high-performance liquid chromatographic (HPLC) method has been developed for separation and determination of components in hydroxypropyl-β-cyclodextrin (HP-β-CD). The method involves separation on an amino-bonded HPLC column using water–acetonitrile as a mobile phase with a polarimetric HPLC detector for quantification. It provides good selectivity and sensitivity and can also be used to compare different sources of HP-β-CD and to measure batch to batch variation. The similarity of the values of molar optical rotation for β-cyclodextrin (β-CD) and HP-β-CD suggests that a polarimetric HPLC detector may be used with a straightforward area normalization method, to quantify the proportion of β-CD in any HP-β-CD sample. Trace amounts of β-CD in HP-β-CD have been measured to a precision of 0.01%. © 1993 Wiley-Liss, Inc.     

Revisiting polarimetry near the isotropic point of an optically active,non‐enantiomorphous,molecular crystal

Accurate polarimetric measurements of the optical activity of crystals along low symmetry directions are facilitated by isotropic points, frequencies where dispersion curves of eigenrays cross and the linear birefringence disappears. We report here the optical properties and structure of achiral, uniaxial (point group D_2d) potassium trihydrogen di‐(cis‐4‐cyclohexene‐1,2‐dicarboxylate) dihydrate, whose isotropic point was previously detected (S. A. Kim, C. Grieswatch, H. Küppers, Zeit. Krist. 1993; 208:219–222) and exploited for a singular measurement of optical activity normal to the optic axis. The crystal structure associated with the aforementioned study was never published. We report it here, confirming the space group assignment I c2, along with the frequency dependence of the fundamental optical properties and the constitutive tensors by fitting optical dispersion relations to measured Mueller matrix spectra. k‐Space maps of circular birefringence and of the Mueller matrix near the isotropic wavelength are measured and simulated. The signs of optical rotation are correlated with the absolute crystallographic directions.     

Phasor approach of Mueller matrix optical scanning microscopy for biological tissue imaging

Mueller matrix microscopy is an advanced imaging technique providing a full characterization of the optical polarization fingerprint of a sample. The Lu-Chipman (LC) decomposition, a method based on the modeling of elementary polarimetric arrangements and matrix inversions, is the gold standard to extract each polarimetric component separately. However, this models the optical system as a small number of discrete optical elements and requires a priori knowledge of the order in which these elements occur. In stratified media or when the ordering is not known, the interpretation of the LC decomposition becomes difficult. In this work, we propose a new, to our knowledge, representation dedicated to the study of biological tissues that combines Mueller matrix microscopy with a phasor approach. We demonstrate that this method provides an easier and direct interpretation of the retardance images in any birefringent material without the use of mathematical assumptions regarding the structure of the sample and yields comparable contrast to the LC decomposition. By validating this approach through numerical simulations, we demonstrate that it is able to give access to localized structural information, resulting in a simple determination of the birefringent parameters at the microscopic level. We apply our novel, to our knowledge, method to typical biological tissues that are of interest in the field of biomedical diagnosis.     

Assessment of tissue polarimetric properties using Stokes polarimetric imaging with circularly polarized illumination

Tissue‐depolarization and linear‐retardance are the main polarization characteristics of interest for bulk tissue characterization, and are normally interpreted from Mueller polarimetry. Stokes polarimetry can be conducted using simpler instrumentation and in a shorter time. Here, we use Stokes polarimetric imaging with circularly polarized illumination to assess the circular‐depolarization and linear‐retardance properties of tissue. Results obtained were compared with Mueller polarimetry in transmission and reflection geometry, respectively. It is found that circular‐depolarization obtained from these 2 methods is very similar in both geometries, and that linear‐retardance is highly quantitatively similar for transmission geometry and qualitatively similar for reflection geometry. The majority of tissue circular‐depolarization and linear‐retardance image information (represented by local image contrast features) obtained from Mueller polarimetry is well preserved from Stokes polarimetry in both geometries. These findings can be referred to for further understanding tissue Stokes polarimetric data, and for further application of Stokes polarimetry under the circumstances where short acquisition time or low optical system complexity is a priority, such as polarimetric endoscopy and microscopy.      

Characterization of broadband fiber optic line detectors for photoacoustic tomography

The frequency response of fiber optic line detectors is investigated in the presented paper. An analytical model based on oblique scattering of elastic waves is used to calculate the frequency dependent acousto‐optical transfer functions of bare glass optical and polymer optical fibers. From the transfer functions the transient response of fibers detectors to photoacoustically excited spherical sources is derived. Photoacoustic tomography is simulated by calculating the temporal response of arrays of fiber optic line detectors and subsequent image reconstruction. The results show that the choice of the fiber material is of significant importance and influences the quality of imaging. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies of the birefringence and birefringence dispersion of polypeptides and proteins

An apparatus has been constructed which permits the polarimetric observation of streaming solutions of macromolecules. The apparatus is a streaming birefringence device allowing the usual measurements of birefringence parallel to the cylinder axis but which in addition transmits light in the radial direction. Installation of the apparatus between the polarizer and analyzer of a Rudolph polarimeter makes possible the measurement of changes in optical rotation, dichroism and birefringence. The present work is concerned with the latter effect. The systems studied were α-helical polyglutamic acid, paramyosin, and collagen (ichthyocol). The combined measurements of radial and axial birefringence completely determines the refractive index ellipsoid of the streaming fluid. This result in turn permits the testing of the Peterlin-Stuart distribution function for streaming in a Couette device, apart from a proportionality constant. The comparison between theory and experiment is very satisfactory provided the system is reasonably homogeneous with regard to molecular length and is sufficiently dilute. On the other hand, it is concluded that the Peterlin-Stuart optical factor seriously overestimates the “form” birefringence in agreement with recent results and conclusions of Taylor and co-workers. The apparatus permits the study of the dispersion of the birefringence in the radial direction. The dispersion of collagen follows a one-term Sellmeier formula and is dominated by absorption bands in the neighborhood of 2000 A. On the other hand, the dispersion of the α-helical systems is complex and requires a multiterm Sellmeier formula. This contrast between the two kinds of polypeptidc helices is similar to results obtained with other optical techniques and is attributed to the splitting of absorption bands in the α-helix.     

Protocols for the analysis of theoretical optical rotations

The availability of sophisticated quantum mechanical methods for predicting molecular optical rotations has revolutionized the determination of molecular stereochemistry. However, the objectives and approaches used for the applications of optical rotation vary from one laboratory to the other. With the number of applications of optical rotation increasing at an unprecedented rate, it is considered necessary to discuss the protocols for the general use of optical rotations predicted with quantum mechanical methods. A summary of the quantum mechanical methods for predicting optical rotations and protocols for the use of predicted optical rotations are presented in this article.     

Tests of a linear model of visual-vestibular interaction using the technique of parameter estimation

The goal of this study was to test whether a superposition model of smooth-pursuit and vestibulo-ocular reflex (VOR) eye movements could account for the stability of gaze that subjects show as they view a stationary target, during head rotations at frequencies that correspond to natural movements. Horizontal smooth-pursuit and the VOR were tested using sinusoidal stimuli with frequencies in the range 1.0–3.5 Hz. During head rotation, subjects viewed a stationary target either directly or through an optical device that required eye movements to be approximately twice the amplitude of head movements in order to maintain foveal vision of the target. The gain of compensatory eye movements during viewing through the optical device was generally greater than during direct viewing or during attempted fixation of the remembered target location in darkness. This suggests that visual factors influence the response, even at high frequencies of head rotation. During viewing through the optical device, the gain of compensatory eye movements declined as a function of the frequency of head rotation (P < 0.001) but, at any particular frequency, there was no correlation with peak head velocity (P > 0.23), peak head acceleration (P > 0.22) or retinal slip speed (P > 0.22). The optimal values of parameters of smooth-pursuit and VOR components of a simple superposition model were estimated in the frequency domain, using the measured responses during head rotation, as each subject viewed the stationary target through the optical device. We then compared the model's prediction of smooth-pursuit gain and phase, at each frequency, with values obtained experimentally. Each subject's pursuit showed lower gain and greater phase lag than the model predicted. Smooth-pursuit performance did not improve significantly if the moving target was a 10 deg × 10 deg Amsler grid, or if sinusoidal oscillation of the target was superimposed on ramp motion. Further, subjects were still able to modulate the gain of compensatory eye movements during pseudo-random head perturbations, making improved predictor performance during visual-vestibular interactions unlikely. We conclude that the increase in gain of eye movements that compensate for head rotations when subjects view, rather than imagine, a stationary target cannot be adequately explained by superposition of VOR and smooth-pursuit signals. Instead, vision may affect VOR performance by determining the context of the behavior. Received: 16 June 1997 / Accepted: 5 December 1997     

Computation of optical motion by movement detectors

The first part of this paper deals with a system-theoretical approach for the decomposition of multi-input systems into the sum of simpler systems. This approach is applied here to analyse the algorithm which represents the computations underlying the extraction of motion information from the optical environment by biological movement detectors. The second part concentrates on a specific model for motion computation known to be realized by the visual system of insects and of man. These detectors provide the visual system with information on both the velocity and structural properties of a moving pattern. In the third part of this article the properties of two-dimensional arrays of movement detectors are analyzed and their relations to meaningful physiological responses are discussed.     

Evaluation of optical motion information by movement detectors

Summary The paper is dealing in its first part with a system-theoretical approach for the decomposition of multi-input systems into the sum of simpler systems. By this approach the algorithm for the computations underlying the extraction of motion information from the optical environment by biological movement detectors is analysed. In the second part it concentrates on a specific model for motion computation known to be realized by the visual system of insects and of man. These motion detectors provide the visual system with information on both, velocity and structural properties of a moving pattern. The last part of the paper deals with the functional properties of two-dimensional arrays of movement detectors. They are analyzed and their relations to meaningful physiological responses are discussed.     

Solution conformation of maltose from optical rotation: A procedure for evaluating carbohydrate force fields

As one means of evaluating the various force fields that are being developed for the molecular modeling of carbohydrate structures, the statistically averaged optical rotation of β-maltose can be calculated using the energy surface to be tested in combination with the detailed conformation dependence of β-maltose optical rotation presented here. Comparison with the observed rotation will indicate the quality of the force field, especially with respect to the parameterization of solvent effects. © 1992 John Wiley & Sons, Inc.     

Assignment of the absolute configuration of [n]-ladderanes by TD-DFT optical rotation calculations

In this study, we report theoretical specific rotation values for a series of cis-/trans-alkylated-[5]-ladderanes and cis-/trans-methylated-[n]-ladderanes. Using time-dependent density functional response theory optical rotation calculations, we can assign (+) and (-) optical rotation signs to trans-(S)-alkyl-[5]-ladderane and trans-(R)-alkyl-[5]-ladderane configurations, respectively. In order to qualitatively validate our absolute configuration predictions, we computed optical rotation values at three different levels of theory--B3LYP, RI-BP86, and Hartree-Fock--using the aug-cc-pVDZ basis set. We observe a novel rung-parity-controlled oscillatory optical rotatory phenomenon in our computations, which, to the best of our knowledge, has never been reported or observed before. Furthermore, this preliminary analysis of optical rotation properties in this class of compounds should facilitate the correct absolute stereochemical assignment of natural and synthetic ladderanes, such as the trans-isomer of pentacyclic C(20)-fatty acid methyl ester (pentacycloanammoxic methyl ester), without the need for derivatization, in particular for cases where NMR or X-ray crystal structures are not readily available.     

Specific anion effects on the optical rotation of glucose and serine

Optical activity is directly related to molecular conformation through the anisotropic polarizabilities of molecules and the refractive index of materials. L-amino acids and D-sugars are characteristic essential bioactive molecules. Since molecular recognition and enzyme activity are related to the conformation of substrates, the relevance of optical activity to biological processes is evident. Specific ion, or Hofmeister, effects that occur with electrolytes at moderately high concentrations modify the behavior of interfaces, molecular forces between membranes, of bulk solutions, of enzymes, and even of DNA. Such effects are universal. Here we report a study on the change in optical rotation induced by some sodium salts for the enantiomers of serine and glucose in water solution. The optical rotation is shown to depend on the kind of anion and on the salt concentration. To obtain further insights into the mechanism behind the phenomenon, Fourier transform infrared (FTIR) spectral studies of serine and glucose solutions in electrolytes were also carried out. The results suggest that it is the differences in interactions of anions at specific chemical sites of the solutes that are responsible for the effects. These forces depend strongly on anion polarizability in water. Such specific ion preferential interactions can affect conformation and internal field, and result in significant changes in optical rotation.     

Biological verification of heavy ion treatment planning

Chinese hamster ovary (CHO) cells and thermoluminescent detectors (TLD-700) were used for physical and biological verification of heavy ion treatment planning. Experiments were performed in a cylindrical water phantom, in some cases with lung and bone equivalent material in front of the target volume. The results confirm the possibility of using thermoluminescent detectors for a quantitative verification of dose distributions. CHO cells can be used at least for qualitative dose verification. Received: 15 December 1997 / Accepted in revised form: 6 February 1998