Optical rotatory dispersion and circular dichroism of cytochrome oxidase.
Optical rotatory dispersion of the oxidized and the reduced cytochrome oxidase has been measured from 196 to 550 nm. The experimental data have been analyzed in order to separate the extrinsic cotton effects from the rotations of the protein backbone. From the resolution, two cotton effects of similar sign are obtained in the Soret region. For the reduced enzyme, the cotton effect centered at 446.5 nm shows a molar amplitude of 192 k-degrees with a band width of 5.3 nm and rotational strength of 0.25 Debye magneton; the second cotton effect centered at 444 nm shows a molar amplitude of 45 k-degrees with a band width of 12.9 nm and rotational strength of 0.14 Debye magneton. Upon oxidation, the centers of these cotton effects shift to 432 and 423.7 nm. The amplitudes become 114 and 122 k-degrees with band widths of 6.1 and 11.2 nm and rotational strengths of 0.18 and 0.35 Debye magneton, respectively. In addition, a less defined third cotton effect centered at 376 nm appears with an amplitude of 28 k-degrees, band width of 18 nm, and rotational strength of – 0.15 Debye magneton. The circular dichroic curves constructed from the optical rotatory dispersion data by the Kronig-Kramers transform are in general agreement with the experimentally determined spectra. 
 
 
 
The backbone rotations are analyzed and expressed in the terms of Shechter-Blout's parameters. The effective helicity thus computed is 50% for the reduced and 44.5% for the oxidized form. 
 
 
 
The sources of the Soret cotton effects are discussed along with the possibilities of the components present (cytochromes a and a3 and copper), the intrinsic property of the Soret transition, possible splitting of the Soret band, interactions with the protein helix, and heme-heme and heme-copper couplings. The fact that the optical rotatory dispersion and circular dichroism behavior is dependent upon the oxidation state of cytochrome oxidase could arise either from an altered conformation of the protein backbone and/or from an altered interaction with the heme group. Regardless of the possibilities, the geometrical conformation of cytochrome oxidase and the electronic relation between the prosthetic group and the apoenzyme is dependent upon the oxidation state of the enzyme.
Optically active quantum-dot molecules.
Chiral molecules made of coupled achiral semiconductor nanocrystals, also known as quantum dots, show great promise for photonic applications owing to their prospective uses as configurable building blocks for optically active structures, materials, and devices. Here we present a simple model of optically active quantum-dot molecules, in which each of the quantum dots is assigned a dipole moment associated with the fundamental interband transition between the size-quantized states of its confined charge carriers. This model is used to analytically calculate the rotatory strengths of optical transitions occurring upon the excitation of chiral dimers, trimers, and tetramers of general configurations. The rotatory strengths of such quantum-dot molecules are found to exceed the typical rotatory strengths of chiral molecules by five to six orders of magnitude. We also study how the optical activity of quantum-dot molecules shows up in their circular dichroism spectra when the energy gap between the molecular states is much smaller than the states' lifetime, and maximize the strengths of the circular dichroism peaks by optimizing orientations of the quantum dots in the molecules. Our analytical results provide clear design guidelines for quantum-dot molecules and can prove useful in engineering optically active quantum-dot supercrystals and photonic devices.
High and low energy states of cytochromes. I. In mitochondria.
Abstract Methods for the observation of reversed electron transfer reactions by a quick freezing, trapped steady state method are described. Adenosine triphosphate-activated cytochrome oxidation in the trapped steady state is comparable to that observed at room temperature. Cytochrome oxidation by ATP activation of reversed electron transfer in terminally inhibited mitochondria containing endogenous substrate shows difference spectra of recognized components of the respiratory chain; no unusual compounds are observed. Supplements of oxalacetate or malonate to terminally inhibited mitochondria show on ATP addition not only oxidation of cytochromes in reversed electron transfer, but the formation of an absorption band between those of cytochromes b and c, here interpreted as the formation of the compound b555. Addition of uncoupling agents to terminally inhibited mitochondria containing endogenous substrate causes the decomposition of a new compound which has been tentatively designated as cytochrome b555, on the basis of (a) the positions of the α and γ bands at 77° K, 555 and 425 mµ, respectively; (b) the high ratio of the intensities of the γ to the α bands characteristic of cytochromes of type b; (c) the positions of the α and γ bands (560 and 428 mµ, respectively) at room temperature; and (d) the shift of the α band to a wave length shorter by 5 mµ on cooling from room temperature to a low temperature. With the assumption that the molecular extinction coefficient and low temperature enhancement factor of cytochrome c are applicable to cytochrome b555, the amount of this b555 compound present in pigeon heart mitochondria can be calculated and approaches 50% of that of cytochrome c. The b555 compound is relatively stable, reaching a maximum concentration less than 30 sec after sulfide is added and being detectable for approximately 15 min thereafter. The compound appears at a very low concentration in the presence of highly active reductants such as succinate. The concentration of Dicumarol required for half-maximal decomposition of the compound is 5 µm at room temperature. Other uncoupling agents are effective, for example, 2,4-dinitrophenol and m-chlorodicyanocarbonylphenylhydrazone.