3 ΔI/I). Hence linear- and circular-dichroism measurements usually can be performed on the same experimental setup. Indeed, most dichrographs, designed for sensitive CD measurements, offer the accessory for LD measurements. In these instruments, the high-frequency modulation and demodulation techniques are very important in warranting high signal to noise ratios, which in turn make very weak signals, 10−4–10−5 OD in magnitude, measurable. Unlike CD, LD—for “good” samples, exhibiting strong, 10–20% dichroism—can be EX 527 chemical structure measured with the aid of spectrophotometers and passive polarization optical elements. (Care must be taken to avoid possible artefacts due to, e.g., polarization selective monochromators

or detectors. A simple test is: LD must reverse sign if rotated by 90º around the direction of propagation of the measuring beam.) Linear dichroism In order to obtain a non-zero LD signal in a macroscopic sample, the particles must

be aligned because in random samples, the difference between the absorbance with the two orthogonally polarized beams averages to zero, i.e., the LD vanishes even if the samples possess intrinsically anisotropic molecular architectures. Evidently, the magnitude of the LD depends on the efficiency of the alignment of the sample, and ultimately on the selection of the method of orientation. Methods of orientation of membranes and particles The first rule is that there is no single good technique; rather, different methods are suited for different samples and purposes. For whole chloroplasts and entire thylakoid membranes, LCZ696 cost a magnetic field of about 0.5 T (Tesla) provides a very good, nearly saturating degree of alignment. It aligns the membranes with their planes preferentially perpendicular to the field, thus offering convenient edge-aligned position of the membranes (Fig. 1). (With this alignment, A 1 and A 2, respectively, are the absorbances of the polarized light parallel

and perpendicular to the membrane plane, i.e., LD = A ‖ − A ⊥; for the face-aligned position, the propagation of the measuring beam being perpendicular to the membrane plane, A 1 = A 2.) Moreover, this technique ASK1 poses no limitation on the reaction medium; also, the aligned state can readily be trapped at low temperatures (or in gel). Field strengths of 0.5–1 T can readily be obtained between two alloy magnets, and thus the alignment can be performed in the sample compartment. Magnetic alignment can also be used for lamellar aggregates of Light-Harvesting Chl a/Chl b Complex II (LHCII), which may require somewhat higher fields for saturation. These magnetic alignments are based on sizeable diamagnetic anisotropies of the sample, which arise due to ordered arrays of molecules or particles possessing well defined, but individually very small OSI-027 diamagnetisms.