Photon correlation spectroscopic measurements on monomeric actin have yielded a translational diffusion constant of 8.13×10−7 cm2 s−1 after correction for the contribution of the back-reflections of the main beam. This value corresponds to a sphere with a radius of 2.6 nm and 50% hydration, or to a prolate ellipsoid with axes 2.0 and 4.0 nm and 30% hydration.

The dependence of phospholipid vesicle size on lipid composition is investigated by photon correlation spectroscopy. For each lipid composition prolonged ultracentrifugation was used to isolate a nearly uniform population of minimum-sized vesicles. The residual size variations in the samples were sufficient to cause polydispersity that made comparisons between samples difficult. Analyses of the data by the method of cumulants and by a method for approximating the particle size distributions directly are presented. The latter method made possible unambiguous comparisons that revealed small but systematic dependences of vesicle size on composition in vesicles containing mixtures of egg phosphatidylcholine and phosphatidylethanolamine, egg phosphatidylcholine and beef brain sphingomyelin, and in single lipid vesicles of egg phosphatidylcholine, dioleylphosphatidylcholine, and beef brain sphingomyelin. These size dependences are quantified within the resolution limits of the technique and their implications are discussed.

A new method is described for the preparation of homogeneously sized, single-lamellar phospholipid vesicles. This method, wich is based on differential high-speed ultracentrifugation, has the advantages of a higher vesicle yield without dilution and rapidity of preparation when compared to the molecular-sieve technique. The homogeneity of vesicle dispersions, prepared by this new method, is examined by several physical techniques and found to be comparable to the best samples prepared by molecular-sieve chromatography.

The gelation processes of gelatin and acto heavy meromyosin solutions were investigated by a new method of light scattering. The sample was given a constant slow rotation around a vertical axis in a typical dynamic light-scattering setup. The measurements were made on solutions in which polystyrene latex spheres were added as the scattering probes. When a sample reached the gel state, the intensity of the light scattered from the sample fluctuated highly. The relative standard deviation of the intensity fluctuation has been shown to be a good measure of gelation. In addition, computer simulations of this scattering system were found to simulate well the experimental results.

Viscosity, turbidity, and laser-light fluctuation autocorrelations of acto-heavy merymyosin (HMM) and acto-subfragment 1 (S-1) solutions were measured under conditions where the actin-activated ATPase is close to its maximal value. The results were compared to similar data obtained in the absence of ATP where the actin and myosin fragments were completely domplexed, and in the presence of ATP but at 0.1 M KLC where the actin and HMM or S-1 were almost completely dissociated. It was found that at maximal actin activation, the viscosity, turbidity, and autocorrelation data were all much closer to the values for the completely dissociated systems than to the values for the completely complexed systems. Assuming that viscosity, turbidity, and autocorrelation measurements approximate a linear measure of binding between actin and HMM or S-1, the results suggest that at maximal actin activation less than 10% of the HMM or S-1 are bound to the actin. Therefore as was suggested previously by ultracentrifuge and kinetics studies, it appears that under conditions of maximal actin activation, most of the HMM and S-1 occur in a refractory state unable to bind to actin.

Intensity fluctuation autocorrelation measurements of laser light scattered from solutions of F-actin and F-actin complexes with myosin subfragments were made in order to estimate the flexibility and other dynamic characteristics of these molecules. F-actin behaves as an unbound diffusing particle. The measurements gave an infinite relaxation time (zero bandwidth spectrum) in the limit of zero scattering angle and, therefore, offer no firm evidence of flexibility according to the theoretical criterion of Fujime & Ishiwata (1971). F-actin complexes with heavy-meromyosin and the myosin subfragment-1 do not exhibit free diffusion. Their scattered light fluctuations are characteristic of bound particles in the gel state, for which a model and theory are proposed. These results leave open the question of the flexibility of F-factin and its complexes and raise the possibility that the presently available theory for inferring flexibility characteristics of macromolecules is inadequate.

1. Heat production, tension development and phosphorylcreatine (PC) splitting have been measured simultaneously during isometric contractions of iodoacetate‐poisoned frogs' sartorii at 0° C. The muscles were stimulated to produce a series of 30 twitches or a 10 sec tetanus or a 30 sec tetanus.2. Of the several possible methods of calculating PC breakdown the best appears to be the one based on the assumption that PC/(total creatine) was originally the same in the two muscles of a pair.3. The difficulty of expressing PC break‐down, heat production etc. in terms of the muscle's size is demonstrated.4. Several artifacts are discussed, including the heat produced by the stimulus and the possible role of reactions other than PC splitting.5. Even when these have been substantially eliminated there remains the rather intractable statistical problem of establishing a functional relationship between heat produced and PC split when both of these variables contain errors, so that ordinary regression analysis is misleading. It is shown that this problem can be dealt with satisfactorily by introducing other instrumental variables that can be determined experimentally.6. The final conclusion is that the in vivo enthalpy of hydrolysis of PC is about 10·6 kcal/mole, and is the same in twitches and in tetani.