Circular dichroism measurement units

The method is less powerful in characterizing proteins lacking tryptophan, for reasons mentioned earlier (see Support Protocol), and does not apply at all to the rare proteins that lack tyrosine as well. Near-UV circular dichroism measurements (unit 7.6) can still be of considerable value for proteins containing tyrosine and, in both cases, both far-UV circular dichroism and urea-gradient gel electrophoresis (unit 7.4) provide powerful contributions to characterization. 

Utilization of the single hydrogen bond between pyridine and benzoic acids in SLCP s has been a source of inspiration for other groups in the development of main-chain supramolecular polymers based on diacids and dipyridines.53-56 Supramolecular rod-coil polymers have been developed by assembly of 4,4 -bipyridines and telechelic polypropylene oxide with benzoic acid end-groups, which show highly ordered liquid crystalline phases.57 The use of tartaric acid derivatives in combination with bipyridine units resulted in the formation of hydrogen-bonded, chiral main-chain LCP s, as has been shown by circular dichroism measurements, optical microscopy, and X-ray data.58,59... 

Incorporation of chiral units into polymers generates optically active polymers.27 Two types of optically active polymers could be obtained according to where the chiral units reside optically active polymers with chirality derived from chiral side chains and optically active polymers with chirality derived from tire chiral main chain. The circular dichroism (CD) measurement of 32, an optically active polymer with chiral side chains, showed that the chiral substituents have induced main-chain chirality. The induced main-chain chirality disappeared at higher temperature and appeared upon cooling. This type of chiral conjugated polymer is potentially useful in reversing optical recording28 ... 

Secondary structural predictions about NPAs, and direct biophysical measurements, have demonstrated that the NPAs are rich in a-helix, with no p-structure either predicted from secondary structure prediction algorithms, or detected by circular dichroism (Kennedy et al, 1995b). In this they are the antithesis of the similarly sized cLBPs and lipocalins. The predictions are that each individual NPA unit protein will fold into four main regions of helix, and it has been speculated that the tertiary structure is as a four-bundle helix protein, similar to other invertebrate carrier proteins (Sheriff et al., 1987). 

X-Band ESR spectra were obtained at 77 and 293 K with a JES-FE-3X spectrometer. The g-values were determined relative to Li-TCNQ(g = 2.0026) and the magnetic fields were calibrated by the splitting of Mn(II) in MgO(AH3 4 = 86.9 G). Magnetic circular dichroism (MCD) measurements, using a 11.7-kG magnet, were carried out on a Jasco J-20 spectro-polarimeter and are expressed in terms of molecular ellipticity, [0] = 2.303 (4500/7r) ( - r), with units of (deg cm2)/d mol. 

Spectroscopic Studies. All measurements were made at 25° in 0.05M or 0.1M potassium phosphate buffer, pH 7.0. Each spectrum was recorded with several samples, which generally differed in concentration, and was scanned two or three times. Circular dichroism is expressed as molar ellipticity [9] in units of degrees cm2 dmole 1. 

Circular dichroism is measured using a dichrograph, which modulates a monochromatic beam of light between left-handed and right-handed circular polarization. A difference between and k produces an intensity modulation that can be amplified and displayed. A separate measurement of Ct nd r would be much more exacting because the differences between Sicl and rc7 are usually very small. (The optical densities differ by I0 -10 units.)... 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

Modern CD instrumentation measures the difference in absorbance of right-add left-handed circularly polarised light directly, so CD spectra are often reported as As, with units the same as for s itself. Sometimes, however, for historical reasons, the circular dichroism of a sample is recorded as ellipticity. The origin of the term lies in the fact that if one of the circularly polarised beams emerging from the sample is attenuated relative to the other, on recombination the two circularly polarised beams form elliptically polarised light, not plane polarised light. The ellipticity 0, is tan (Z / ) where b is the minor and a the major axis of the ellipse. The conventional molar ellipticity [9]... 

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

The electronic spectra of the five-membered ring compounds have been intensively studied by the experimental and theoretical works. These molecules are fundamental units in many important biological systems. Furthermore, their excitation spectra are benchmark examples for theoretical studies of molecular excited states [51,55-58]. For furan and thiophene, various types of excitation spectra were measured the vacuum ultraviolet (VUV) spectrum, electron energy-loss (EEL) spectrum and magnetic circular dichroism (MCD) spectrum. The SAC-Cl method offered consistent interpretations of these electronic spectra [51-53]. 

In order to address the question of whether or not the specific side chain conformation influences the main chain conformation, cylindrical brush polymers with poly-L-lysine (PLL) side chains were synthesized [88], Here, we will discussed in some detail the cylindrical brush sample CB-PLL55, which comprises 960 main chain repeat units and an average of 55 lysine repeat units in one side chain sample. Typically, PLL forms a random coil in aqueous solution In aqueous 0.5 M NaC104 solution, however, it is known to adopt one of the most prominent structures of polypeptides an a-helix (see also Sect 6.1) [89]. Upon reduction of the charge density by changing the pH to >9.8 and/or increasing the temperature or by addition of surfactant, a p-sheet structure is favored. Thus, cylindrical brushes with PLL side chains seemed to be ideal candidates for investigating the influence of a coil to a-helix to p-sheet transitimi of the side chains on the main chain conformation. Indeed, circular dichroism (CD) measurements confirmed that all three side chain conformations also occur in the side chains of cylindrical brush polymers (Fig. 18). 

The bulky pendants of neighboring repeat units may not be coplanar due to steric constraints. When the pendants are chiral, their cooperative twisting may generate an asymmetric force field to induce the segments of the polymer backbone to spiral in a helical fashion. To ch k wheftier the chain helicity is really induced in our polymers, we measured their circular dichroism (CD) spectra. 

Synthesis and characterization of a new type of crown ethers (81)-(83) have been deseribed by Huszthy. The electronic circular dichroism (BCD) spectra of the ehiral crown ethers (i ,i )-(81) and (i ,i )-(82) eontaining an alkyl diarylphosphinate moiety, showed a strong exciton splitting in the Bb spectral region of the aromatic chromophores. In the case of the proton-ionizable ehiral derivative (i ,i )-(83) containing the phosphinic acid unit, the BCD speetrum measured in MeCN, suggested molecular dimerization or aggregation. 

---