Circular dichroism temperature dependence

The crossover from the nonhelical to the helical polymeric state occurs for 5 = 1, and becomes sharper the smaller the value of the parameter a. It is for this reason that a is often viewed as a parameter that describes the cooperativity of the helical transition [72], This quantity is accessible experimentally by means of circular dichroism ectroscopy because to leading order in the temperature we have In 5 = A/ihCT - Th)/k 1 and (d9/dT)T=Ti, = A/th/4Va B T. Here, Th denotes the helical transition temperature where 9 = j, and Ahh the excess enthalpy associated with the formation of a helical bond [3]. The steepness of the measured helicity versus temperature curve (obtained, for example, by means of circular dichroism spectroscopy) depends on the ratio Ahbl /o, where Ahb is obtainable independently from microcalorimetry [71], Values of = 0.01 - 0.001 and Ahb 50 kJmoP have been obtained for the discotic of Figure 13, suggesting that the helical transition in supramolecular polymers can indeed be highly cooperative [3,73]. 

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... 

Figure 9.19 Temperature dependent circular dichroism spectra of 1.2 x 10 " M melittin in a 43% (w/w) l-palmitoyl-2-linoleoyl-L-3-phosphatidylcholine(PLPC), cubic phase (10 irtM tris-HCl buffer, pH 7.4). Spectra taken during a heating cycle (1, 5 °C 2, 15 °C 3, 25 °C 4, 35 °C 5,45 °C) [0] is the mean residue ellipticity. (Adapted from Landau and Luisi, 1993.)...

Fig. 51a. Circular dichroism spectra for hydrated lipid 27 (0.24 mM). b Temperature dependencies of [01 values for hydrated lipid 27(0.24 mM). The circles and triangles show the molecular ellipticity at 246 and 231 nm, respectively [367]...

Farrell, H.M. Jr., Wickham, E.D., Unmh, J.J., Qi, P.X., and Hoagland, P.D. 2001. Secondary structural studies of bovine caseins Temperature dependence of J-casein structure as analyzed by circular dichroism and FTIR spectroscopy and correlation with micellization. Food Hydrocolloids 15 341-354. 

Variable temperature magnetic circular dichroism (MCD) studies have revealed similar electronic and magnetic properties for the nickel centres in Jack bean and Klebsiella aerogenes urease. Native and 2-mercaptoethanol and acetohydroxamic inhibited forms of both enzymes have been investigated and in each case the energy of the temperature dependent MCD for the nickel(II) d-d transitions indicates octahedral coordination with mainly oxygen donors. 

Circular dichroism (CD) and optical rotatory dispersion (ORD) spectra (71PMH(3)397) are very sensitive to the spatial disposition of the atoms in a molecule, and conformational changes may yield rather dramatic changes in the appearance of a CD or ORD spectrum of a chiral molecule. The analysis of the temperature dependence of the CD spectrum may give information on populations and free energy differences. Except for nucleosides, the use of the chiroptical method in conformational analysis is rather limited, which may be accounted for by the complexity of the theory for optical activity. 

Studies involving temperature dependence of the oscillator strengths of the hypersensitive transitions and circular dichroism (CD) and magnetic circular dichroism (MCD) of optically active complexes may give some useful information from which the relative contributions of static and dynamic coupling mechanisms to the observed oscillator strengths may be ascertained. 

In order to understand the charge transfer features of the Blue Copper site, the variable-temperature optical absorption, room-temperature circular dichroism, and magnetic circular dichroism spectra of plastocyanin, stellacyanin, and azurin were studied355. As can be seen for plastocyanin in Fig. 12, the relative intensities (and signs, in the case of CD and MCD) of these transitions vary among the different types of spectra. This is a result of the difference in selection rules for absorption, CD, and MCD spectra, as mentioned in the Introduction. A careful comparison of the three types of spectra and the absorption bandshape temperature dependence (see moment analysis in Ref. 35, pp. 176-177)... 

These data have led to the development of a catalytic mechanism, shown in Scheme 6, that has been further refined by kinetic isotope effect (KIE) experiments. Substrate binds to Cu(II), replacing bound solvent. The metal coordination facilitates the deprotonation of the substrate hydroxyl group. The proton is transferred to Tyr495, which dissociates from copper. The temperature and pH dependence of the visible absorption and circular dichroism spectra indicate that galactose oxidase exists as an equilibrium of the Tyr495-Cu(II) form (TyroN) and the protonated Tyr495 state. 

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