Concentration spectra, isotropic

Figure 8-12 (A) 31P NMR spectra of different phospholipid phases. Hydrated soya phosphatidylethanolamine adopts the hexagonal Hn phase at 30°C. In the presence of 50 mol% of egg phosphatidylcholine only the bilayer phase is observed. At intermediate (30%) phosphatidylcholine concentrations an isotropic component appears in the spectrum. (B) Inverted micelles proposed to explain "lipidic particles" seen in freeze fracture micrographs of bilayer mixture of phospholipids, e.g., of phosphatidylethanolanine + phosphatidylcholine + cholesterol. From de Kruijft et al.m Courtesy of B. de Kruijft.

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

As we will see in Chapter 4, g-matrices are often difficult to interpret reliably. The interpretation of isotropic g-values is even less useful and subject to misinterpretation. Thus isotropic ESR spectra should be used to characterize a radical by means of the hyperfine coupling pattern, to study its dynamical properties through line width effects, or to measure its concentration by integration of the spectrum and comparison with an appropriate standard but considerable caution should be exercised in interpreting the g-value or nuclear hyperfine coupling constants. 

Since the binding constants of Mn2 + to the tight and weak metal ion sites of unadenylylated enzyme are 5.0 x 10 7 and 4.5 x 10 5 M, respectively, the tight site can be selectively populated under conditions where [enzyme] > [Mn2 +]. Figure 24 shows EPR spectra obtained with a solution of 0.79 mil/ enzyme subunit concentration and 0.7 mAf Mn2+ concentration (113). This spectrum represents Mn2+ bound only at the tight sites with no free Mn2 + present. It shows that bound Mn2 + is in a relatively isotropic environment (i.e., the zero-field splitting is small). 

The probability profile on the right is indicative of the absorption characteristic of each individual chromophore. Note the use of the symbols c and a to indicate the half amplitude points of this profile. Note also that the difference in two exponential profiles remains an exponential profile, albeit with modified parameters. These parameters, and the nominal center energy, represent the conjugated-dipole-molecular absorption band of the Rhodonines. This spectrum is frequently reported in the literature for the chromophores of vision. It is usually attributed to the putative rhodopsin. However, the presence of opsin is not required. Only a conventional concentration of Rhodonine is required to record this isotropic spectrum since it is the only feature in the visual range of the spectrum for these materials. 

The contribution of dissolved surfactant, whose concentration was only 0.001M, compared to 7M of decane, to the observed Class I peaks must have been negligible. Class II peaks were not observed in Spectrum 13 of the birefringent phase, and Class I peaks were broadened (linewidth about 30 Hz) compared to the peaks in Spectrum 12 (linewidth less than 5 Hz). Therefore it seems quite possible that the dispersed birefringent phase did give Class I peaks in Spectrum 12, but that these peaks, due to either the surfactant or to absorbed decane or to both, merged with those of the decane in the isotropic phase. 

NMR spectra of ubiquitin in the presence of bicelles at a concentration of 25% w/v have been recorded under sample spinning for different angles of rotation . For an axis of rotation equal to the magic angle, the coupled H- N HSQC corresponds to the classical spectrum obtained in an isotropic solution and allows the measurement of scalar couplings. For an angle of rotation smaller... 

---