Linear Dichroism Measurements

As noted in the figure, these designs can accommodate samples with coaxial birefringence and dichroism. It can be shown that the presence of coaxial birefringence will not appear in the measured signals. All of the designs produce signals of the form, 

Of these designs, only the (P/PEM)psg fails to produce a signal that is proportional to the term Sg S2e Since it only returns the term S5,c2q , there is insufficient information to simultaneously determine the extinction, 8 , and the orientation angle, 6 . For that reason, it can be used only in cases where 0 is known. In this case, the optimal signal is obtained by setting the orientation of the sample to 0 = 45° relative to the initial polarizer of thePSG. It is evident that the coefficients is simplified by setting the modulation amplitude so that JQ (A) =0 when a PEM is used. 

TABLE 8.4 Constants for the Intensity Signal for the Dichroism Polarimeter 

Once the intensity has been measured, it is left to analyze the Fourier content of the signal to extract the coefficients multiplying the desired terms,, Sg c20 and Sg,s2e  

It is also necessary to isolate the underlined term in equation (8.27) for the purposes of normalization so that the coefficient (/QCg,/)/p can be eliminated. 

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The non-covalently bound BPDEs to DNA formed initially appear to be intercalation complexes (1 6,52-55) Meehan et al. (1 6) report that the BPDE intercalates into DNA on a millisecond time scale while the BPDE alkylates DNA on a time scale of minutes. Most of the BPDE is hydrolyzed to tetrols (53-56). Geacintov et al. (5l ) have shown with linear dichroism spectral measurements that the disappearance of intercalated BPDE l(+) is directly proportional to the rate of appearance of covalent adducts. These results suggest that either there may be a competition between the physically non-covalently bound BPDE l(+) and an externally bound adduct or as suggested by the mechanism in the present paper, an intercalative covalent step followed by a relaxation of the DNA to yield an externally bound adduct. Their results for the BPDE i(-) exhibit both intercalative and externally bound adducts. The linear dichroism measurements do not distinguish between physically bound and covalent bound forms which are intercalative in nature. Hence the assumption that a superposition of internal and external sites occurs for this isomer. 

The linear dichroism measurement demonstrated that MOPPV chains formed the planar orientation chains in the LB films dichroic ratio Ap/As... 

In the following we will compare the order parameter of the dve u Table 4) which is an azoderivative, dissolved in the 1-l.c. (M2 in Table 4) andtT Id in the side chain polymer (M3), which has a very similar chemical Issove to the 1-l.c.57). The linear dichroism measurements for the polymer/d°nSt,tUt,°n M3, is shown in Fig. 13, measured at X = 430 nm, which is the absorptiony ShStdm,... 

Using this method in Fig. 15, the order parameters vs. temperature of the polymers are compared with the corresponding monomer mixture Ml. The monomer Ml exhibits a higher value of S than the polymer by about 10 % and confirms the previous linear dichroism measurements. If we compare the magnitude and temperature dependence of S for the polymers Cl to C3, which differ in the length of the flexible spacer, no difference in S can be found within the experimental error. 

Well organized Langmuir-Blodgett (LB) films have been obtained from mixtures of a push-pull carotenoid and co-tricosenoic acid as shown in 7. These mixed films exhibit a very good cohesion, with an area of about 25 A2 per carotenoid molecule. They can easily be transferred onto solid substrates. Examination by UV-visible linear dichroism measurements confirms that the carotenoid chains are oriented perpendicularly to the surface of the substrate in card-packed aggregates, in which the polyenic chains interact via excitonic coupling, as indicated by the large hypsochromic shift of the tc-tc transition (20). 

Figure 8.7 Polarimeter for linear dichroism measurements. Any of the PSGs listed can be...

On the basis of novel electron diffraction methods, Unwin and Henderson have shown that each protein molecule consists of seven ot-hexical rods, about 40 A long and 10 A apart, all perpendicular to the plane of the membrane (39,40). The lattice structure results in clustering of the BR molecules as trimers around one of the threefold axes. Analysis of CD spectra in terms of exciton interactions between the chromophores (41,42), as well as linear dichroism measurements (43-45), have yielded values of 20-24° for the tilt angle (out of the membrane plane) of the retinal transition moment, which closely coincides with the long axis of the molecule. The absence of any rotational mobility of bacteriorhodopsin in the purple membrane (on a time scale of 60 min) was also confirmed by linear dichroism measurements (45). 

Infrared linear dichroism measurements may be performed on unidirectionally oriented samples. In the case of DNA, the favored axis of orientation is the axis of the double helix. Films are prepared by unidirectional stroking while the sample is gently dried. Two spectra are recorded with the electric field of the incident light oriented parallelly... 

Matrix photolysis of several cyano-substituted phenyl azides with plane polarized light, combined with qualitative linear dichroism measurements in the IR spectra of the resulting matrices, has been used to deduce information about... 

Alegria and Dutton used the Langmuir-Blodgett (LB) technique to deposit multiple (mono) layers of either the Rp. viridis reaction-center complex or of the corresponding chromatophores, in either case dispersed in a phospholipid matrix. The absorption spectmm of the LB-film was almost identical to that of the reaction center complex in solution, as shown in Fig. 10 (A), indicating that the in vitro properties of the photosynthetic apparatus are stable and essentially retained in the LB-films. The authors used a combination of redox potentiometry and absorption spectroscopy to identify the four Rp. viridis hemes and their redox potentials, and also determined their orientations by linear-dichroism measurements. 

The B-DNA structure given by Amott and Hukins [171] is taken as initial conformation for the DNA fragment as this structure is close to the native DNA at physiological conditions. According to electric linear dichroism measurements [154], there are several possibilities for the mutual geometrical arrangement of the components of the physical complex. Hence, different initial orientations and conformations of PAH metabolites were considered with the aim to find the most stable structure of the intercalated complex. 

It was shown [832] that linear dichroism measurements following H/D exchange allow the tertiary protein structure to be characterized. In addition, the disappearance upon H/D exchange of the residual amide II band at 1550 cm in the spectrum of adsorbed protein, which occurs when the protons in the protein core become accessible, can be used as an indication of the protein unfolding [833]. 

RCs are asymmetrically partioned within the primary donor (8). However, linear dichroism measurements show that the pigment organization in His OO ( eu ap - j5M200 RQ5 js... 

In conclusion, the method of fluorescence depolarization measurements on particles in compressed gels can be performed on chlorosomes and they lead to results, which are in good agreement with the theoretical predictions from [1], and results fromt linear dichroism measurements [2]. The results support the further use of the expressions in... 

DYNAMIC INFRARED LINEAR DICHROISM MEASURED WITH A MONOCHROMATOR... 

Figure 2 Schematic diagram of a spectropolarimeter adapted for linear dichroism measurements. LS, light source M, monochromator P, polarizer PEM, photoelastic modulator producing time-dependent retardation jisin cot, S, sample PM, photomultiplier A, d.c. amplifier LA, lock-in amplifier operating at frequency 2co REC, recorder. Adapted by permission from Michl J and Thulstrup EW (1995) Spectroscopy with Polarized Light Solute Alignment by Photoselection, in Liquid, Polymers, and Membranes. New York Wiley-VCH.

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