Molar ellipticity, mean

Fig. 2.48 CD analysis of oligoureas 177 and 178. CD spectra were recorded in MeOH at room temperature at a concentration of 0.2 mM. Molar ellipticity [0] in deg cm drnol . The mean-residue ellipticities measured at 204 nm for 177 and 178 are 26600 and 41300 deg cm dmol, respectively...

Fig. 5 Poly(L-glutamic acid) containing 56 mol% azobenzene units in the side chains (III). CD spectra in trimethylphosphate before (continuous line) and after (dashed line) irradiation at 360 nm. Below 250 nm, molar ellipticity is based on the mean residue weight above 250 nm, the ellipticity is referred to one azo-Glu residue.

Most CD spectra of proteins are reported in terms of mean residue ellipticity, which is the molar ellipticity of the protein divided by the number of residues. This allows the CD spectra of proteins to be reported on a normalized scale. Infrequently, the reader may find the spectra reported in terms of differences in molar absorptivity (Aa). Recall that molar ellipticity can be obtained by multiplying Aa by 3298. Conversions between other units of optical activity can be found in a review article by Woody [14]. 

As determined from HPLC, the purity of the peptide was greater than 90%. Steady-state fluorescence spectra of this peptide were collected from 310-480 nm with an excitation wavelength of 264 nm. A 1 cm pathlength cuvette was used with concentrations of 8.6 pM. The emission quantum yields were determined relative to N-acetyl-L-tryptophanamide at pH = 6.9 (f = 0.13) (17). Steady-state circular dichroism spectra were obtained using a 0.5 mm pathlength cylindrical cell and concentrations of 0.26 mM. The mean molar ellipticity [0] (deg cm dmol" ) was calibrated with (+)-10-camphorsulfonic acid. Concentrations of the solutions were determined by measuring the absorbance of 4-methylaminobenzoic acid. 

Commonly, for the sake of comparison, the molar ellipticity [6>] = 100 Ofcd in units of deg cm dmol" is recorded, where c is the concentration in mol and d is the optical path length. If, in the case of polymers such as proteins, the molar concentration is related to the molar mass of the residue, i.e. to the repeating (base) unit, the mean residue weight ellipticity [0]mrw is obtained. 

Optical activity of a substance leads at least to three spectroscopic phenomena which are, however, related Optical Rotatory Dispersion (ORD), i.e. the rotation of the plane of linearly polarized light by traversing the substance. Circular Dichroism, expressed as Ae = e - e (if the relative molar mass of the molecules is known, or - in case of biopolymers - the mean residual relative molar mass), and Molar Ellipticity, [e] = 3300 x Ae. A necessary condition for optical activity of a substance is the Chirality of its molecules, i.e. the property of being not superposable onto its mirror image. This allows still the presence of (proper) axes of rotation Cp, but not improper ones (Sp, including... 

Molar ellipticity (6 or [6]) is expressed in degrees cm dmol" and is defined by equation 3, in which c and L have the meanings given earlier. 

Early CD spectrometers actually measured ellipticity, but most modem instmments measure Ae more directly and sensitively by switching a light beam rapidly between right- and left-circular polarization (Chap. 1). In studies of proteins or polynucleotides, the molar ellipticity usually is divided by the number of amino acid residues or nucleotide bases in the macromolecule to obtain the mean residue ellipticity. 

Figure 14.10 Effect of irradiation and dark adaptation on CD spectra of poly(L glutainic acid) bearing 85 mol% spiropyran units in HFP at 25 °C 1, kept in the dark 2, exposed to sunlight dotted lines are CD spectra recorded during decay in the dark over 8h. Below 250nm, CD data are expressed in terms of molar ellipticity based on the mean residue molecular weight above 250 nm, the molar ellipticity is referred to one sfriropyran-glutamy) residue...

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