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The dichroism for photochemical attachment of a psoralen derivative to Mg2+-stabilized chromatin fibres is used to deduce the orientation of nucleosomal disks and linker DNA in the 30-nm fibre. The new technique of photochemical electric dichroism should have general applicability to problems of nucleic acid organization in cellular subunits and viruses.

T1 values of the His-35 C-2 proton resonance of reduced Pseudomonas aeruginosa azurin were determined at 25C and pH values 4.5, 7.3, and 9.0 in the presence of different fractional amounts of oxidized azurin. The C-2 proton of His-35 undergoes very rapid spin relaxation in oxidized azurin because of its close proximity to the paramagnetic copper. In the presence of oxidized protein, the T1 values of this proton in reduced azurin depend on the lifetime of the reduced protein. From the T1 data, the electron self-exchange rate constant for azurin was calculated to be 1.4 x 104 M-s-, 4.3 x 10 M-l s'l, and 6.0 x 103 M-l s'1 at pH values 4.5, 7.3, and 9, respectively. At pH 7.3, the C-2 proton of His-35 is in slow exchange between the imidazole and imidazolium forms and gives rise to two separate resonances at 9.39 and 8.00 ppm. By using these two resonances, the electron self-exchange rate constants were determined separately for the two species of azurin for which the His-35 residue is in the imidazole or the imidazolium forms; results showed that both species participate in self-exchange of electrons with equal efficiency.Azurins are low molecular weight bacterial proteins that contain one "blue" copper redox center per molecule of protein. Because of their convenient size and the unusual characteristics of their redox center, the structure and the electron exchange properties of this protein have been examined in detail (e.g., refs. 1-12). In several of these studies, the electron self-exchange rate for azurin has been calculated based on Marcus theory and electron transfer rates between azurin and a variety of redox partners (7-10). Given the important role played by this parameter in theoretical considerations of the overall electron exchange process, it would be highly desirable to determine its value experimentally.In this paper we report direct measurements of the selfexchange rate constant for Pseudomonas aeruginosa azurin using 1H nuclear magnetic resonance spectroscopy. The measurements rely on determinations of the spin-lattice relaxation time (T,) gen by the Grain Processing Corp., Muscatine, IA. Azurin was purified as described (12). The final product had a purity ratio of (Ag25/A280) > 0.52 and also was checked by the appearance of a single band corresponding to azurin in a polyacrylamide gel.Azurin samples were prepared for NMR by taking lyophilized protein and incubating in excess 2H20 at pH 3.5 and 50'C for 1 hr. The pH was then adjusted to 7.5, and the samples were lyophilized and subsequently redissolved in 0.1 M NaCl in 2H20. Reduced azurin was prepared by addition of dithionite and subsequent dialysis against 0.1 M NaCl in 2H20 under an argon atmosphere. Oxidized azurin concentrations were measured at 625 nm with an extinction coefficient of 5.7 x 103 M-cm-(2). The pH values of the solutions were adjusted with NaO2H or 2HCl in 2H2Q. The pH readings reported represent direct electrode readings and were not corrected for the deuterium isotope effect.The 1H NMR spectra were recorded at 25...

The disposition of the heme groups in cytochrome cd1 oxidase from Pseudomonas aeruginosa is studied by emission spectroscopy. This protein of molecular weight 120 000 is composed of two monomers each with a heme c and a heme d1. It has been shown by electron microscopy to be oblong in shape and by preliminary X-ray crystallography to have a twofold axis of rotation. Three electronic energy donors, a singlet tryptophan, a triplet tryptophan, and an attached 8-dimethylamino-1-naphthalenesulfonyl group, all exhibit normal decay lifetimes. It follows that there are parts of the protein at least 80 A from the nearest heme. The conclusion is that the hemes are all at one end of the molecule.