Circular dichroism early studies

A. Evidence from Circular Dichroism Studies Early CD Studies on Polypeptides... 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

The spin Hamiltonian formalism, which is also needed to interpret, for example, electron paramagnetic resonance or magnetic circular dichroism spectra see Magnetic Circular Dichroism (MCD) Spectroscopy), was first applied to the interpretation of magnetic Mossbauer spectra by Wickmann, Klein and Shirley and was implemented into a computer program by Miinck et al. in the early 1970s. For most studies of mononuclear iron centers with electron spin quantum number S, the following electronic Hamiltonian is used ... 

The demonstration of the optical activity of octahedral complexes was important in confirming Alfred Werner s intuitive ideas about coordination chemistry. Early work involved the resolution of complexes characterized by optical rotations. Modem instmments for optical rotatory dispersion were developed first, but circular dichroism (CD) spectra proved to be more useful. CD has been a powerful tool for detailed studies of the stereochemistry of octahedral complexes. Contributions to rotational strength of chelate ring conformational, configurational, and vicinal contributions are additive. Chiral metal complexes are now used in enantioselective synthesis of chiral pharmaceuticals. 

In the line of these studies, we investigated rod-like PEC particles using stiff cationic a-helical PEL as the templating PEL component to be complexed by a flexible polyanion [16]. On the one hand, the a-helix of PLL might be induced by media parameters like pH or certain salt ions, as used for the fabrication of anisotropic related PEM. On the other hand, early work by Shinoda et al. [89] based on circular dichroism (CD) spectroscopy reported induction of the a-helical conformation of PLL by certain polyanions. PAC was used, which was claimed to form a stoichiometric left-handed superhelix around the right-handed a-helix of PLL. However, this polyanion/PLL templating effect on the intermacromolecular order was not studied further on the supramolecular, nanoscopic, or even colloid level. 

X-Ray difiiaction studies of natural (Fuller etal., 965) and synthetic (Leslie et al., 1980) DNA fibers have shown that depending on its base sequence, water content, and counterion, DNA can adopt at least four diflfeient conformations A, B, C, and D forms. Since the early 1970s it has become evident through CD spectroscopy (Pohl and Jovin, 1972 Ivanov et al., 1973) that DNA secondary structure also varies in solution. However, owing to the limited resolution of X-ray fiber diflBraction, the DNA models thus derived were sequence-averaged and each had a uniform backbone conformation with a mononucleotide repeat unit (Fig. 3). Circular dichroism also has the drawback that detailed structures of DNA in solution cannot be... 

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