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On heating, bovine 8-lactoglobulin A (8-lg A) dimers dissociate and then aggregate. The extent of aggregation of 8-19 A after a heat-quench treatment was studied quantitatively using photon correlation spectroscopy (PCS) ; circular dichroism (CD) spectroscopy was also carried out on heated solutions of 8-1s A to examine changes in the secondary and tertiary structure of t h e protein. In pH 7.0 solution the protein underwent a change in tertiary structure at 67 "C; by contrast, the secondary structure, containing ca. 50% 8-sheet, showed very little change up to 90°C. PCS showed that t h e extent of aggregation of 8-lg A depended on pH and buffer concentration; under some conditions it was observed that there was a temperature above which t h e rate of aggregation decreased. The pH-dependence of t h e temperature of onset of aggregation correlated with the conformational stability of 8-19 A. Detailed examination of the light scattering data suggested initial aggregation of the protein molecules to form linear, rod-like particles. It is proposed that, at later stages in the aggregation process, the rods themselves aggregate and start to form a network.

The catalytic roles of the two reductively acetylatable lipoic acid residues on each lipoate acetyltransferase chain of the pyruvate dehydrogenase complex of Escherichia coli were investigated. Both lipoyl groups are reductively acetylated from pyruvate at the same apparent rate and both can transfer their acetyl groups to CoASH, part-reactions of the overall complex reaction. The complex was treated with N-ethylmaleimide in the presence of pyruvate and the absence of CoASH, conditions that lead to the modification and inactivation of the S-acetyldihydrolipoic acid residues. Modification was found to proceed appreciably faster than the accompanying loss of enzymic activity. The kinetics of the modification were fitted best by supposing that the two lipoyl groups react with the maleimide at different rates, one being modified at approximately 3.5 times the rate of the other. The loss of complex activity took place at a rate approximately equal to that calculated for the modification of the more slowly reacting lipoic acid residue. The simplest interpretation of this result is that only this residue is essential in the overall catalytic mechanism, but an alternative explanation in which one lipoic acid residue can take over the function of another was not ruled out. The kinetics of inactivation could not be reconciled with an obligatory serial interaction between the two lipoic acid residues. Similar experiments with the fluorescent N-[p-(benzimidazol-2-yl)phenyl]maleimide supported these conclusions, although the modification was found to be less specific than with N-ethylmaleimide. The more rapidly modified lipoic acid residue may be involved in the system of intramolecular transacetylation reactions that couple active sites in the lipoate acetyltransferase component.

The ultrasonic attenuation of skimmed milk, of the serum containing the soluble whey proteins and small casein aggregates and of ultrafiltered serum was measured in the frequency range 15–96 MHz. From these measurements the attenuation due to the colloidal casein micelles in serum was obtained. For the serum, α∝f1.86±0.02; for the soluble proteins, Δα∝f1.16±0.04; and for the casein micelles, Δα∝f1.06±0.05. The ultrasonic attenuation of the soluble proteins is similar to that of other protein solutions; however, ultrasonic attenuation of the casein micelles differed from that of the soluble proteins. Using the theory of Allegra and Hawley [J. Acoust. Soc. Am. 36, 1545–1564 (1972)] for the ultrasonic attenuation of suspensions of elastic spheres, the expected attenuation for a model of the casein micelle suspension was computed. The frequency-dependent attenuation, α′(f ), of the spheres is taken as an unknown for this model and, by varying α′(f ), a fit between the experimental data and the predictions of the theory was obtained. This fit requires the attenuation of the micelles to vary as f1.40±0.10 and the micellar attenuation to be higher than for a typical protein solution of the same concentration. This suggests that relaxation processes of the casein polypeptide chains in the casein micelles differ from those of most soluble proteins studied so far by ultrasonic attenuation measurements. This conclusion is consistent with results from nuclear magnetic resonance spectroscopy.

VOLUME 43, NUMBER 15 PHYSICAL REVIEW LETTERS 8 OCTOBER 1979 ently badly missing. It is in this energy range that detailed information can be obtained on the balance between the contributions to the opticalmodel potential of the attractive and repulsive components of the nucleon-nucleon interaction.The Dirac-Hartree model could be tested in some detail, since it makes definite predictions on the spin-orbit potential. 4 The latter depends only very weakly on energy and can thus essentially be taken over from RefSc 4 and 6. It will be advisable to perform the analysis of these data with a relativistic optical-model equation, which is best adapted to the Dirac-Hartree theoretical model. Such relativistic wave equations have already been used by Arnold, Clark, and Mercer, 19 in conjunction with phenomenological potentials wells U 0 {r) and U e {r). These were fitted to the available data which correspond to energies larger than 500 MeV. After the submission of the first version of the present Letter, a paper by Arnold and Clark 20 appeared in which a relativistic optical-model potential is constructed at low energy. These authors also use a a-and c^-exchange interaction. However, they adopt a folding prescription rather than a self-consistent theory; hence, they use densities as input. Moreover, they do not discuss the value and the shape of the potential at intermediate energy.We describe both the theory and first experimental realization of a new light-scattering method for studying nonspherical particles in dilute solution. It is based on measuring the cross correlation of signals from two spatially separated detectors each of which receives light from the same small number of scatterers. Possible reasons for observed differences between experiment and theory are discussed.We report an experiment which essentially ob-A dilute solution of tobacco mosaic virus (TMV), serves the rotational Brownian motion of a sue-a rod-shaped particle of length about 300 nm and cession of single, nonspherical macromolecules.diameter of about 15 nm, was illuminated by las-1100