Organic chemistry

Abstract

Studies of the segregation behavior of particulate systems indicate the simultaneous occurrence of both segregation and mixing. The specific rate constants for these processes are calculated from statistical data and are shown to be a function of the particulate velocities. The results suggest the analogy of mechanical agitation in these idealized systems to thermal motion in molecular systems. Arrhenius-type plots of first-order rate constants versus the square of the reciprocal particulate velocities make possible the prediction of segregation behavior at various agitation intensities.N PREVIOUS reports (1, 2), the segregation I occurring in particulate systems of steel and glass spheres has been characterized partially.The over-all process results in a separation of particles of different physical properties and is a function of density, particle size, and the size of the container in which the segregation occurs. The present work indicates the manner in which energy input affects both the segregation and mixing processes which are shown to occur simultaneously. Donald and Roseman (3, 4), in experiments with a drum mixer, have reported that increased mixer speed reduced the final degree of mixing. The results reported here suggest that this may not always be the case, but that the segregation characteristics of granulations can be expected to vary with the physical properties of their constituents. The mechanical agitation of the systems under study shows a marked similarity to the thermal motion of molecular systems. The specific forward and reverse rate constants, representing segregation and mixing, respectively, have been evaluated from statistical data for the over-all process of segregation. These rate constants, when plotted as a function of the particulate energies, substantiate the analogy of agitation to thermal motion. THEORYSegregation has been shown to proceed by apparent over-all first-order kinetics to an equilibrium state in which the rates of mixing and unmixing balance (1). This suggests that both mixing and segregation, which are shown to occur simultaneously here at the particulate level, are first-order processes and that the observed rate constant, ko, is the sum of the constant for segregation, k1, and that for mixing, kz. The process may be considered analogous to a reversible chemicaf reaction in which both forward and reverse steps occur simultaneously. The over-all process, if followed, will reilect the predominance of one reaction step (forward or reverse) over the other. A system of particles following this behavior ( 5 ) can be represented by The concept of concentration used here relates to the relative number of spheres which may be classified as mixed or unmixed on the basis of their immediate neighbors. Thus, particles within a sample are considered unmixed in so far as they exceed their mean proportion of the total system. The remainder of particles within the sample are

Accuracy and precision in protein structure analysis: restrained least-squares refinement of the structure of poplar plastocyanin at 1.33 Å resolution

“…A revised assignment of the intra-and intermolecular hydrogen bonds and salt bridges is given in Tables 4 and 5. There is no clear-cut definition of a hydrogen bond, and the decision whether a particular contact should be labelled as a hydrogen bond usually involves the application of subjectively chosen criteria. Both geometrical (Baker & Hubbard, 1984) and energetic (Kabsch & Sander, 1983) criteria have been proposed. In the present work we have adopted an even more conservative geometrical approach than in our earlier papers.…”

“…(e) Contact was not identified as a hydrogen bond at 1.6 A resolution (Guss & Freeman, 1983). (f) Contact was identified as a hydrogen bond in a ~/-bend (1-3 bend) by Baker & Hubbard (1984).…”

“…ofGuss & Freeman (1983), with the addition of two contacts identified as '1-3' or 'y' hydrogen bonds byBaker & Hubbard (1984). However, six of the peptide-peptide interactions violate at least one of the more rigorous geometrical criteria mentioned above.…”