(iii) In chirally organized systems, e.g., in the so-called psi-type aggregates, such as DNA aggregates, condensed chromatins, and viruses, very intense CD signals have been observed, with non-conservative, anomalously shaped bands, which are accompanied by long tails Vactosertib cell line outside the absorbance originating from differential scattering
of the sample (Keller and Bustamante 1986; Tinoco et al. 1987). Hierarchically organized systems, such as granal thylakoid membranes, or lamellar aggregates of LHCII (Simidjev et al. 1997), contain all the three different types of signals; they are superimposed on each other (Fig. 3). Fig. 3 Circular-dichroism PHA-848125 spectra exhibited by the thylakoid pigments at different levels of organization. The pigment concentrations (adjusted to 20 μg Chl(a + b)/ml) are identical in the three samples: the acetonic (80%) extract—yielding intrinsic CD (for easier comparison, the signal is multiplied by a factor 5), pea thylakoid membranes suspended in low salt hypotonic medium (30 mM Tricine pH 7.8, 10 mM KCl, 2 mM EDTA)—dominated by the sum of the excitonic bands, and
the same membranes suspended in isotonic medium in the presence of Mg ions (the medium above is supplemented with 330 mM sorbitol and 5 mM MgCl2). (V. Barzda, M. Szabó and G. Garab, unpublished.) Intrinsic find more CD of photosynthetic pigment molecules In monomeric solutions, chlorophylls and carotenoids exhibit very weak CD signals: for 1 absorbance unit, in the range of some 10−5 intensities. In general, molecules with planar and rather symmetric structures (such as (B)Chls) and those as rods (such as carotenoids) result in weak rotational strengths (R), which are a measure of the CD intensity (R is proportional to the scalar product of the electric and magnetic dipole moments). In most photosynthetic systems, the contributions from these intrinsic CD signals can safely be ignored or corrected, based on the absorbance band structure and the CD in the pigment solutions (cf. Fig. 3—intrinsic CD, in acetonic solution). It Loperamide is also possible, however, that the protein environment induces some twisting of, for instance, carotenoids or the open
ring tetrapyrrole chromophores (phycobilins) in phycobilisomes of cyanobacteria. This effect can complicate the interpretation of CD spectra, since it is hard to make quantitative estimates of its corresponding spectral shape and size. Fortunately, the conjugated ring systems of (B)Chls are not easily twisted, and for those molecules, both the intrinsic and the induced effects can be ignored. An exception has been found in a Chl a/Chl c antenna, where a strong CD band, having the same band structure as the absorbance, has been detected in a long-wavelength absorbing Chl a molecule (Büchel and Garab 1997). This CD band is most probably induced by distortion of the porphyrin ring by a charged aromatic amino acid residue (cf. Pearlstein 1991).