1995).) The squared length of the transition dipole moment is proportional to the extinction coefficient of the molecule for the given absorbance band. The specific transition dipole moment for the given transition determines not only the strength of the absorption but also the A-1155463 chemical structure ability of the molecule to interact with polarized light, and sets the conditions for intermolecular interactions as well. For linearly

polarized light, the absorbance is proportional to the square of the scalar product of the electric vector (E) of the light and the transition dipole vector (μ), i.e., the absorbance is proportional to E 2 μ2 cos2 α, where α is the angle between the two vectors. This is the basis of all LD selleck compound measurements. In circularly polarized light spectroscopy, i.e., for CD, the interaction between the light and the sample also depends, albeit often in a complex Sapanisertib nmr manner, on the orientations of the transition dipole moments of the molecules that compose the structure. Linearly and circularly polarized light: LD and CD measurements For linearly polarized light (often called plane-polarized light), the electric vector E (“the light vector”)

oscillates sinusoidally in a direction (plane) which is called the polarization direction (plane). For circularly polarized light, the magnitude of E remains constant, but it traces out a helix as a function of time. In accordance with the convention used in CD spectroscopy, in the right and the left circularly polarized light beams,

when viewed by an observer looking toward the light source, the end-point of E rotates clockwise and counterclockwise, respectively. (See supplemental Movie 1.) On using the principle of superposition, it can easily be shown that circularly and linearly polarized light beams can be represented as the sum of two orthogonal linearly polarized beams, in which the amplitudes are equal and the phases are shifted exactly by a quarter or a half of the wavelength, respectively (supplemental movie 1). This principle can be used for producing Tacrolimus (FK506) orthogonal linearly (e.g., vertically and horizontally) or circularly (left- and right-handed) polarized beams. In most commercially available dichrographs and home-built setups, this is done by using a photoelastic modulator (PEM) that operates at high frequency, typically at 50 kHz. In this way, the polarization state of the measuring beam is modulated sinusoidally. In order to measure the dichroism of the sample, the signal of the detector is demodulated by a proper circuit, usually an AC amplifier locked at the frequency and phase of the polarization modulation. This yields a difference, or differential polarization (DP) signal, ΔI.