CD-spectroscopy

Although circular dichroism (CD) is not observed for achiral molecules, when they form complexes with DNA they are placed within a chiral environment and give 

Interestingly, the 14-membered ring compounds most of the time do not show characteristic UV minima or maxima despite the presence of the styrylamine unit which, due to conjugation, should absorb intensively. The reason for this anomalous behavior is the strain of the ring system which hinders p-orbital overlap. This phenomenon sometimes is described as end absorption . Some of the compounds belonging to this category show a weak absorption around 280 nm. When tryptophan is present, characteristic absorptions are observed at ca. 220, 270 and 290 nm. In 13-membered cyclopeptide alkaloids the strain is relieved and their UV spectra reveal absorptions around 270 (log s 4.0) and 320 nm (log s 3.8). 15-Membered cyclopeptide alkaloids exhibit a maximum around 275 nm (log s 4.2) and sometimes a shoulder at ca. 220 nm. 

The CD spectra of 13-membered cyclopeptide alkaloids reveal strong negative. Cotton effect bands at 324, 276, 254 and 219 nm and a small positive one at 232 nm. The spectra of 14-membered cyclopeptide alkaloids exhibit a weak positive band at 287 and an intense negative one at 239 nm. This behavior is due to the existence of the L-amino acid in position B or C. For the D-enantiomer the negative peak becomes intensely positive. This is illustrated by scutianine-D (77) which has L-P-OH-Leu and L-P-OH-Phe as the B and C units respectively and its isomer scutianine-E (78) having the corresponding D-forms. 

In every group of compounds e.g. 5(14)-scutianine-A type) every letter that is used to denote a fragment is uniquely attributed to one fragment in this particular group. 

1 Fragmentation of 4(14)-Frangulanine- and -Integerrine-lype Cyclopeptide Alkaloids 

R= Basic terminal amino acid p-OH-amino acid R = ring amino acid 

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An especially interesting recent example is Benedetti et al 5 use of circular dicliroism (CD) spectroscopy to detect a pressure-induced change of the configuration at the metal centre of the octahedral chiral A- and A-tris... 

Method Abs, chemical reduction, monitored by absorption spectroscopy CD, chemical reduction, monitored by CD spectroscopy CD/OTTLE, electrochemical reduction using an optically transparent thin layer (OTTLE) cell, monitored by CD spectroscopy CV, cyclic voltammetry EPR, chemical reduction, monitored by EPR. 

As a prelude to our binding studies, the secondary structure of aPNA itself was examined using CD spectroscopy [52]. The first aPNA to be studied was the tail-to-tail bl dimer, [Ac-Cys-Gly-Ser -Asp-Ala-Glu-Ser -Ala-Ala-Lys-Ser -Ala-Ala-Glu-Ser -Ala-Aib-Ala-Ser -Lys-Gly-NH2]2- The far-UV CD spectra of this aPNA in water at 30 °C showed the double minimum at 220 nm (n-n transition) and 206 nm (n-n transition) as well as the maximum at 193 nm (n-n transition), characteristic of a peptide a-hehx. Upon increasing the temperature, the intensity of the minimum at 200 nm decreased indicating a transition from a-helix to random stracture. An isodichroic point at 202 nm was suggestive of a temperature-depen-dent a-helix to random coil transition. The helical content of this T5(bl)-dimer at 20°C in water was estimated to be 26% [40]. 

CD spectroscopy has been used extensively to study peptide-nucleic acid interactions. Conformation changes induced by either the peptide or nucleic acid can be detected readily using this spectroscopic technique. Although the CD spectra of... 

Chiral-at-metal cations can themselves serve as chirality inducers. For example, optically pure Ru[(bipy)3] proved to be an excellent chiral auxihary for the stereoselective preparation of optically active 3D anionic networks [M(II)Cr(III)(oxalate)3]- n (with M = Mn, Ni), which display interesting magnetic properties. In these networks all of the metalhc centers have the same configuration, z or yl, as the template cation, as shown by CD spectroscopy and X-ray crystallography [43]. 

Preparation of mixed peptide fibrils was similar for both of these labelled peptides 1% (by weight) was incubated at 37 °C with the other, unlabelled peptide in 10% CH3CN/H20atpH2.ForTTRio i9with l%dansyl-TTRio5 n5, a blue shift and dansyl anisotropy increase were observed, indicating the inclusion of dansyl-TTRios-i 15 into fibrils. CD spectroscopy proved that the stmcture was primarily p-sheet. 

Because membrane filtration is the only currently acceptable method of sterilizing protein pharmaceuticals, the adsorption and inactivation of proteins on membranes is of particular concern during formulation development. Pitt [56] examined nonspecific protein binding of polymeric microporous membranes typically used in sterilization by membrane filtration. Nitrocellulose and nylon membranes had extremely high protein adsorption, followed by polysulfone, cellulose diacetate, and hydrophilic polyvinylidene fluoride membranes. In a subsequent study by Truskey et al. [46], protein conformational changes after filtration were observed by CD spectroscopy, particularly with nylon and polysulfone membrane filters. The conformational changes were related to the tendency of the membrane to adsorb the protein, although the precise mechanism was unclear. 

Of the visible spectroscopic techniques, CD spectroscopy has seen the most rapid and dramatic growth. The far-UV circular dichroism spectrum of a protein is a direct reflection of its secondary structure [71]. An asymmetrical molecule, such as a protein macromolecule, exhibits circular dichroism because it absorbs circularly polarized light of one rotation differently from circularly polarized light of the other rotation. Therefore, the technique is useful in determining changes in secondary structure as a function of stability, thermal treatment, or freeze-thaw. 

To obtain statistically significant comparisons of ordered and disordered sequences, much larger datasets were needed. To this end, disordered regions of proteins or wholly disordered proteins were identified by literature searches to find examples with structural characterizations that employed one or more of the following methods (1) X-ray crystallography, where absence of coordinates indicates a region of disorder (2) nuclear magnetic resonance (NMR), where several different features of the NMR spectra have been used to identify disorder and (3) circular dichroism (CD) spectroscopy, where whole-protein disorder is identified by a random coil-type CD spectrum. 

Before discussing details of their model and others, it is useful to review the two main techniques used to infer the characteristics of chain conformation in unordered polypeptides. One line of evidence came from hydrodynamic experiments—viscosity and sedimentation—from which a statistical end-to-end distance could be estimated and compared with values derived from calculations on polymer chain models (Flory, 1969). The second is based on spectroscopic experiments, in particular CD spectroscopy, from which information is obtained about the local chain conformation rather than global properties such as those derived from hydrodynamics. It is entirely possible for a polypeptide chain to adopt some particular local structure while retaining characteristics of random coils derived from hydrodynamic measurements this was pointed out by Krimm and Tiffany (1974). In support of their proposal, Tiffany and Krimm noted the following points ... 

Relative Amplitude of Burst Phase Measured by Time-Resolved Far-UV CD Spectroscopy for Various Globular Proteins a... 

Structural characterization of the biomolecule-AMP lamellar nanocomposites using FTIR and circular dichroism (CD) spectroscopies showed no change in the... 

Analyses by native polyacrylamide gel electrophoresis and circular dichroism (CD) spectroscopy revealed that spontaneous coiled-coil associations between EGF-E5-His and EGF-K5-His promoted heterodimer (dEGF-His) formation. The CD spectroscopic analysis suggested that the E5 peptide in monomeric EGF-E5-His had a disordered structure. However, the ot-helical structure was induced in the E5 peptide when it associated with EGF-K5-His. These findings are shown schematically in Fig. 6. 

These observations suggest an interesting question can CD spectroscopy be so sensitive to minute conformational details as to challenge apparently sophisticated computational methods ... 

Saaby S, Knudsen KR, Ladlow M, Ley SV (2005) The Use of a Continuous Flow-Reactor Employing a Mixed Flydrogen-Liquid Flow Stream for the Efficient Reduction of Imines to Amines. Chem Commun 23 2909-2911 Seebach D, Overhand M, Kilhnle FNM, Martinoni D, Oberer L, Hommel U, Widmer H (1996) Beta-Peptides Synthesis by Arndt-Eistert Homologation with Concomitant Peptide Coupling. Structure Determination by NMR and CD Spectroscopy and by X-ray Crystallography. Helical Secondary Structure of a Beta-Hexapeptide in Solution and its Stability Towards Pepsin. Helv Chim Acta 79 913-941... 

The double-helical rope formation in T-10-T was unexpected, since neither T-10-T nor its precursor thymine is chiral, as verified by CD spectroscopy. The authors speculate that photodimerization of two thymine derivatives leads... 

Nuckolls and Katz have synthesized discotic liquid crystalline molecules in which the core is a helix in its own right.37 Nonracemic helicene 33 was found to assemble into a columnar mesophase in which the helicenes stack on top of each other. CD spectroscopy showed a strong increase of the Cotton effect upon going from the molecularly dissolved state to the aggregated state, exhibiting an amplification of chirality. These helical columns give rise to a strong expression of chirality because the intrinsic shape of the helicenes... 

Metal phthalocyanines functionalized with four helicenes (62) have also been reported to form chiral columnar aggregates.76 In chloroform solutions of these metal phthalocyanines aggregation into columns occurred upon addition of ethanol, as was observed by UV-Vis spectroscopy. CD spectroscopy revealed that the chromophores within the columnar aggregates are in a chiral environment, implying that the chirality of the peripheral helicenes has been transferred to the supramolecular aggregates. These phthalocyanines stack with a typical intermolecular distance of 3.4 A, and calculations have indicated that to allow this distance the two phthalocyanine moieties have to be rotated because of the bulkiness of the helicenes. It can easily be imagined that a phthalocyanine provided with both R and S helicenes cannot stack in such a defined manner because of the steric interactions between the nonconform helicenes. 

Optical activity also manifests itself in small differences in the molar extinction coefficients el and er of an enantiomer toward the right and left circularly polarized light. The small differences in e are expressed by the term molecular ellipticity [9 J = 3300(el — r). As a result of the differences in molar extinction coefficients, a circularly polarized beam in one direction is absorbed more than the other. Molecular ellipticity is dependent on temperature, solvent, and wavelength. The wavelength dependence of ellipticity is called circular dichroism (CD). CD spectroscopy is a powerful method for studying the three-dimensional structures of optically active chiral compounds, for example, for studying their absolute configurations or preferred conformations.57... 

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