The folding of most proteins occurs during the course of their translation while their tRNA-bound C termini are embedded in the ribosome. How the close proximity of nascent proteins to the ribosome influences their folding thermodynamics remains poorly understood. Here, we have developed a mass spectrometry–based approach for determining the stabilities of nascent polypeptide chains using methionine oxidation as a folding probe. This approach enables quantitative measurement subglobal folding stabilities of ribosome nascent chains within complex protein mixtures and extracts. To validate the methodology, we analyzed the folding thermodynamics of three model proteins (dihydrofolate reductase, chemotaxis protein Y, and DNA polymerase IV) in soluble and ribosome-bound states. The data indicate that the ribosome can significantly alter the stability of nascent polypeptides. Ribosome-induced stability modulations were highly variable among different folding domains and were dependent on localized charge distributions within nascent polypeptides. The results implicated electrostatic interactions between the ribosome surface and nascent polypeptides as the cause of ribosome-induced stability modulations. The study establishes a robust proteomic methodology for analyzing localized stabilities within ribosome-bound nascent polypeptides and sheds light on how the ribosome influences the thermodynamics of protein folding.

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The mode of formation of the shadow cast by the inner cone of a Bunsen flame has been studied. On the basis of a simple model the main features of the shadow picture have been accounted for both qualitatively and quantitatively. The method of Sherratt and Linnett for determining burning velocities gives results that are in error by an amount depending on the magnification employed and the level in the cone which is considered. With their particular experimental arrangement the measured burning velocity over the whole cone is about 4 yo too high. Necessary modifications are suggested.Direct shadow and Schlieren photographs are being used increasingly to locate and follow the movement of flame fr0nts.l suggested, for example, that the direct shadow of the Bunsen cone could be used in employing the burner method for determining burning velocities. Garner, Long and Ashforth 3 have used both the shadow and visible cones in combination in a new method they have recently suggested. The burner method has many' advantages as a means of determining the burning velocity V,, and reasons were advanced by Sherratt and Linnett €or using the shadow cone rather than the visible cone. Moreover, since the shadow cone does not depend on the light emitted from the flame for its formation, flames of low luminosity may be studied easily.However, if the shadow photograph is to be used, it is essential that we should understand clearly how the shadow of the flame is produced. In the present work we have made a more detailed study than that carried out by Sherratt and Linnett of the way in which the shadow of a Bunsen flame cone is formed. As a result we have found that some modifications must be made in the method they suggest for calculating burning velocities. The values obtained for V , using the new method are slightly lower than those resulting from the procedure suggested by Sherratt and Linnett.The results obtained by Garner, Long and A ~h f o r t h , ~ using the simple shadow picture of a Bunsen flame, are different in form from those obtained by Sherratt and Linnett. This is exemplified by Fig. 5 of their paper.s The combustible mixtures used were not the same and may have had different burning properties. Differences in refractive indices in the two sets of experiments would lead to differences in the production of the shadow picture and this might account for the lack of agreement. Also the dimensions of the experimental arrangements may have been different. However, whatever the true explanation, we must come t o the general conclusion that the differences between the results of Garner, Long and Ashforth and those of Sherratt and Linnett make it important that we should understand more fully how the shadow picture is produced.

Using two wave functions for the hydrogen molecule, we have calculated the changes in the average electronic kinetic energy both parallel and perpendicular to the internuclear axis as the two hydrogen atoms approach one another. We have considered, also, the variations in average electronic and nuclear potential energy. The changes in electronic kinetic energy are related to changes in spreading and curvature of the wave function and an attempt is made to understand the changes in average potential and kinetic energy. At the equilibrium separation R, the average potential energy is very much less than when the atoms are far apart, the average electronic kinetic energy parallel t o the axis is slightly less, and that perpendicular t o the axis considerably greater a t R, than when the atoms are far apart.

Recently Tanford and Pease have shown that in a number of carbon monoxide flames the burning velocity increases as the calculated equilibrium concentration of hydrogen atoms in the flame front increases. They point out that this would be so if the propagation of the flame were dependent on the diffusion of hydrogen atoms ahead of the flame. It is shown in the present paper that, by combining calculated ignition and flame temperatures, the variation in burning velocity with composition can also be accounted for on an essentially thermal theory of flame propagation.