Polarized Absorption Bands Research Articles

Cytochrome c interaction with membranes. Absorption and emission spectra and binding characteristics of iron-free cytochrome c.

A cytochrome c derivative from which iron is removed has been prepared and characterized. Several lines of evidence indicate that native and porphyrin cytochrome c have similar conformations: they have similar elution characteristics on Sephadex gel chromatography; in both proteins the tryptophan fluorescence is quenched and the pK values of protonation of the porphyrin are identical. Porphyrin cytochrome c does not substitute for native cytochrome c in either the oxidase reaction or in restoring electron transport in cytochrome-c-depleted mitochondria. It does however competitively inhibit native cytochrome c in these reactions, the Ki for inhibition being larger than the Km for reaction. The absorption and emission spectra, and the polarized excitation spectrum of the porphyrin cytochrome c are characteristic of free base porphyrin. The absence of fluorescence quenching of porphyrin cytochrome c when the protein is bound to cytochrome oxidase suggests that heme to heme distance between these proteins is larger than 0.5 to 0.9 nm depending upon orientation. Binding of the porphyrin cytochrome c to phospholipids or to mitochondria increases the fluorescence polarization of a positively polarized absorption band, which indicates that the bound form of the protein does not rotate freely within the time scale of relaxation from the excited state.

Optical absorption associated with superlattice in silicon carbide crystals

AbstractAbsorption of polarized light in the near IR and visible region has been measured in n‐type silicon carbide crystals of seven modifications: 4H, 6H, 8H, 15R, 27R, 33R, and 21R. All the samples showed clearly resolved polarized absorption bands at wavelengths characteristic for each crystal structure. It is shown that the energy position, temperature behaviour, and selection rules for these bands are in good agreement with those predicted theoretically for interminizone transitions in the Superlattice. The period of the super‐lattice is identified for several groups of SiC structures defined according to the principle of their unit cell formation.

Absorption Band Profile of the Origin Region of the b Polarized 4000 Å Anthracene Crystal Transition

New low temperature measurements of the reflectivity of crystalline anthracene are reported. Attention is confined to the important b axis polarized absorption band in the vicinity of the 0–0 transition. In this way line broadening by ``two particle'' excitations are eliminated. It is shown how low temperature reflectivity data, even when limited to only a region of the spectrum, can provide interesting information concerning the exciton dynamics. For this crystal transition optical constants are obtained from the observed reflectance data by use of a Kramers—Kronig analysis and a General Dispersion analysis. Discussed in some detail are those methods of obtaining optical constants, the values of the various band parameters and the implications of such values, the asymmetry of the absorption band profile and what that asymmetry may indicate. The interpretation of the data is not without its difficulties especially in view of the effects which may be caused by surface irregularities and by impurities and surrounding defects. Nevertheless, strong inferences are drawn from the measurements and their subsequent analysis. These include (i) the absorption coefficient in the b axis polarized 0–0 transition of crystalline anthracene is extremely large ∼1.6× 106 cm−1 at 7°K; (ii) the absorption line shape is asymmetric; (iii) the coupling between the exciton and optical phonons probably exceeds that between the exciton and acoustic phonons. The quantitative aspects of these observations are consistent with the strong coupling line shape theory of Fischer and Rice and of Grover and Silbey. The consequences of the analysis are employed to suggest areas for further experimental and theoretical work.

Infra-red dichroism of trans-dihalogenobis(ethylenediamine)-cobalt (III) complexes

The polarized infra-red absorption spectra of single crystals of trans-[CoX 2(en) 2]Y (X = Cl −, Br −, Y = ClO 4 −, NO 3 −, I −) have been observed from 5000 to 400 cm −1. The spectra show no distinct dichroism in the NaCl region where most of absorption bands are due to the vibrations of CH 2 and NH 2 groups of ligand ethylenediamine. However, the spectra in the KBr region exhibit three polarized absorption bands at about 590, 510 and 470 cm −1, which are characteristic to the chelate ring. The dichroism of these bands has been discussed in relation to the crystal structure of these compounds. The band at 510 cm −1 is mainly due to the CoN stretching vibration, whereas the other two bands, 590 and 470 cm −1, are assigned to the skeletal ring deformation coupled with the CoN stretching vibration.

Spectrum of the Iodine Crystal. II

The absorption spectrum of single and oriented crystals of iodine has been investigated in the wavelength region 7000–3000 Å. Four strongly polarized bands and one weakly polarized absorption band have been observed, all of which are polarized in the ac plane of the crystal. All of the absorption bands are approximately 1000 cm−1 wide. The crystal-band theory for iodine given by Rosenberg has been extended, and the effects of exchange and of spin—orbit coupling have been qualitatively considered. Three models are discussed with varying relative importance of spin—orbit and crystal energies. These models are correlated, and the strong crystal force model is used to interpret the experimental observations.

Polarization of the π*→π and π*→n Phosphorescence Spectra of N-Heterocyclics

Phosphorescence polarization measurements have been performed by the method of photoselection. Since this technique requires a knowledge of the polarization of at least two perpendicularly polarized absorption bands, the method can be fully exploited in the case of the N-heterocyclics where the (n, π*) and (π, π*) absorptions are perpendicular to each other. The results on pyrazine (1,4-diazabenzene) show that the π*→n emission is polarized along the N···N axis, in agreement with previous work. The polarization is uniform throughout the 600-cm—1[ν6(ag)] progression and does not alternate as does the intensity and the vibrational frequency. This behavior might arise from a resonance-type interaction which produces a progression with mixed Franck—Condon characteristics. Quinoxaline (1,4-diazanaphthalene) is found to have a π*→π phosphorescence spectrum which is highly polarized out of the plane. Approximate calculations show that the (n, π*) singlet state in quinoxaline is at least as important as the (σ, π*) states in mixing with the emitting (π, π*) triplet state. The general conclusion is reached that in nitrogen heterocyclics when the phosphorescent state is of the (n, π*) type, the mixing singlet is of the (π, π*) type, or in-plane Rydberg state, whereas if the emitting triplet state is of the (π, π*) type, the mixing singlet might be of the (n, π*), (σ, π*) and/or perpendicularly polarized Rydberg state.

Absorption and rotation of light by helical oligomers: The nearest neighbor approximation

AbstractExplicit expressions for the DK and RK associated with the excitation of regular oligomers from their ground states to the Kth exciton state as functions of N, P, μ, μ∥, μt, and μr have been derived on the assumption that the only perturbation Vij to the N singly excited states results from exciton transfer between adjacent residues, V12. The DK relation predicts the occurrence of a perpendicularly polarized absorption band arising from μ⟂ and a parallel polarized band from μ∥ with maxima at K = [2(N + 1)/P] and 1, which are shifted from the monomer band by 2V12 cos(2π/P) and 2V12cos[π/(N + 1)], respectively. Overlapping of the bands, which may occur in oligomers, decreases rapidly with N, leaving two perfectly polarized bands in the polymer. Within each band which includes only a few levels near the maxima, the most probable transition may be associated with all of the intensity for very short chains but only 40.2–50% (⟂) and 80.9% (∥) as N → ∞ (polymer case). A relation was derived by which V12 may be simply calculated from μ, μr, μt, μ∥, a, and z. The nearest neighbor theory was compared with experiment by calculating a reasonable set of the relevant parameters which exactly account for known spectral shifts.