1. Evidence is presented which indicates that inactivation of the mitochondrial ATPase from bovine heart by the reagent 4-chloro-7-nitrobenzofurazan results from modification of one tyrosine residue per enzyme molecule. Activity can be restored by a variety of sulphydryl reagents.2. In sodium dodecyl sulphate, the nitrobenzofurazan group on tyrosine is transferred to newly exposed sulphydryl groups on the enzyme.3. The rate of transfer of the nitrobenzofurazan moiety from the enzyme to sulphydryl compounds is compared with that for transfer from the model compound N-acetyl-tyrosine-O(7-nitrobenzofurazan) ethyl ester, the synthesis and properties of which are also described.4. The ligands ATP and ADP exert a protective effect on the rate of reaction between the mitochondrial ATPase and 4-chloro-7-nitrobenzofurazan. The variation in rate of this reaction with change in pH has also been examined and a pK, of 9.5 estimated for the tyrosine residue.5. The modification does not prevent substrate binding as judged by changes in the fluorescence of aurovertin, an antibiotic with specific affinity for mitochondrial ATPases.6. When the ATPase activity of submitochondrial particles is inhibited by 4-chloro-7-nitrobenzofurazan, there is a parallel decrease in the extent of the energy-linked fluorescence enhancement of 1-anilino-naphthalene-8-sulphonate induced by ATP hydrolysis. Both ATPase activity and the fluorescence enhancement are restored by sulphydryl reagents.The various theories of oxidative phosphorylation ascribe specific and distinct roles to the mitochondrial ATPase [1,2]. It is therefore important to elucidate features of this enzyme which might lead to a firmer mechanistic understanding of the final step in oxidative phosphorylation, the synthesis of ATP. When isolated as a soluble protein from mitochondrial membrane fragments, the mitochondrial ATPase from both bovine heart and rat liver apparently has a complex subunit structure, a,&yGc [3,4] and a molecular weight of approximately 360 000 [5]. A recent report suggests, however, that the enzyme from bovine heart can be obtained in a fully active form without the y and 6 subunits [6]. Mitochondrial ATPase has five tight dhhrruiarions. Nbf-Cl, 4-chloro-7-nitrobeiizofurazan (this compound has bcen named 7-ch~oro-4-nitrobenzo-?-oxd-l ,3-diazole and abbreviated NBD-chloride elsewhere; the name and abbreviation used here follow CBN recornmcndations). AcTyr(NbQ-OEt, Nacetyltyrosine-O (7-nitrobenzofurazaii) ethyl ester; AcCys(Nbf), N-acetyl-cystcine3 (7-nitrobenzofura~an).Enzyme. ATPasc or adenosine triphosphatase (EC 3.6.1.3).binding sites for adenine nucleotides [7] but little is known of the function or interrelationships of these sites.The most significant observations about the mechanism of the mitochondrial ATPase have come from the study of isotope exchange reactions [8]. Such studies have indicated that ATP hydrolysis occurs by attack of water on the terminal phosphate group with concomitant breakage of the bond between the terminal bridging oxygen and phos...

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The critical thickness for Si1−xGex strained layers for the alloy range 0<x<0.15 has been determined from annealed epilayers using mapping techniques which allow single dislocation detection and composition thickness measurements over large areas (∼50 cm2 ). A series of Si1−xGex layers was deposited by molecular beam epitaxy in which the composition (x) and thickness (h) were continuously varied across the substrate to produce a slowly changing strain energy density through the stable/metastable transition. On annealing at either 750 or 900 °C for 30 min, an abrupt transition in relaxation behavior was found at critical values of thickness and composition (hc,xc ). Increasing the anneal temperature or time did not shift the transition giving identical (hc,xc ) values. At strain thicknesses above these critical values a large increase in defect density was observed (>∼104 , cm−2) whereas in thinner strained epilayers, below the thermodynamic stability curve, no misfit dislocations were found. Nomarski microscopy of defect etched surfaces and x-ray topography were used to reveal misfit dislocations formed during the initial stages of relaxation. The appearance of single misfit dislocations at a density ≊1 cm−2 was taken as the criterion for a ‘‘relaxed’’ layer. The critical strain and thickness in the vicinity of these transition points were determined on the as-grown wafer by x-ray diffraction and Rutherford backscattering spectrometry with confirmation of layer thicknesses by cross-sectional transmission electron microscopy. The Matthews–Blakeslee [J. Cryst. Growth 27, 118 (1974)] equilibrium critical thickness he (nm), vs Ge atom fraction curve given by xe =0.55/he  ln(4he /b) for 1/2 a0〈110〉, 60° glide dislocations with a Burgers vector b ∼0.4 nm, is an excellent fit to these experimental data, i.e., xc =xe and hc =he .

A series of Si/Si,-,Ge, strained-layer superlattice structures has been studied by x-ray double-crystal diffractometry, Raman spectroscopy and transmission electron microscopy. The periodicity of the superlattices, the alloy composition and the degree of relaxation have been measured. The precisions of the three techniques are discussed and the results critically compared.

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