Simultaneous measurement of optical rotation dispersion and absorption spectra for chiral substances

Method for determination of optical rotatory dispersion curve by using a polarization axis finder

We report a new, to the best of our knowledge, measurement method of optical rotatory dispersion (ORD) based on a polarization axis finder (PAF) and digital image processing. The collimated quasi-monochromatic light is output by a monochromator and passes through a polarizer and the chiral samples, and a PAF is employed to modulate the light beam spatially to form an "hourglass" intensity pattern, which is utilized to determine the polarization direction of linearly polarized light. After being recorded by a digital camera, the modulated patterns with and without samples are analyzed by our specially designed digital image processing program to calculate the optical rotation (OR) angle of the chiral substance at a certain wavelength, and the ORD curve can be obtained by fitting a series of OR angles with different wavelengths. The ORD curves of two standard quartz plates with opposite chirality and a glucose solution with different concentrations are measured experimentally, and the fitted coefficients and measurement results agree well with their theoretical values, which indicates that the presented method is an effective and accurate way to determine the ORD of chiral substances conveniently.

Polarimetric Measurements of Surface Chirality Based on Linear and Nonlinear Light Scattering

A molecule, molecular aggregate, or protein that cannot be superimposed on its mirror image presents chirality. Most living systems are organized by chiral building blocks, such as amino acids, peptides, and carbohydrates, and any change in their molecular structure (i.e., handedness or helicity) alters the biochemical and pharmacological functions of the molecules, many of which take place at surfaces. Therefore, studying surface chirogenesis at the nanoscale is fundamentally important and derives various applications. For example, since proteins contain highly ordered secondary structures, the intrinsic chirality can be served as a signature to measure the dynamics of protein adsorption and protein conformational changes at biological surfaces. Furthermore, a better understanding of chiral recognition and separation at bio-nanointerfaces is helpful to standardize chiral drugs and monitor the synthesis of adsorbents with high precision. Thus, exploring the changes in surface chirality with polarized excitations would provide structural and biochemical information of the adsorbed molecules, which has led to the development of label-free and noninvasive measurement tools based on linear and nonlinear optical effects. In this review, the principles and selected applications of linear and nonlinear optical methods for quantifying surface chirality are introduced and compared, aiming to conceptualize new ideas to address critical issues in surface biochemistry.