Biomolecular simulation: historical picture and future perspectives

Over the last 30 years, computation based on molecular models is playing an increasingly important role in biology, biological chemistry and biophysics. Since only a very limited number of properties of biomolecular systems are actually accessible to measurement by experimental means, computer simulation complements experiments by providing not only averages, but also distributions and time series of any definable, observable or non-observable, quantity. Biomolecular simulation may be used (i) to interpret experimental data, (ii) to provoke new experiments, (iii) to replace experiments and (iv) to protect intellectual property. Progress over the last 30 years is sketched and perspectives are outlined for the future.

Definitions, Summary, and Some Issues

This Joint Discussion has been titled Massive Star Birth. Perhaps it is appropriate here to define what we mean by a massive star. The very word massive suggests we consider aminimummassMbelow which one would speak of low (or intermediate) mass evolution, and above which is the realm of massive stars. It is natural to take this mass limit as that in which a (single) star will end its life as a supernova: 8M⊙. This corresponds to a (minimum) luminosityLof a few × 103L⊙, a (minimum)Teff of 20000 K, and a ZAMS spectral type of about B1.5V. Note that this mass division refers to the final evolution of a star, and might well have nothing to do with difference in physical processes between massive and low mass starbirth. For example, the minimumTeff for a star to produce an UCHII region, a readily observable quantity, corresponds to aTeffcloser to 30000 K and a mass of 15M⊙.

First Ranked Galaxy Morphology and Morphological Content in Groups of Galaxies

V. Ambartsumian (1956) has shown that the observable quantity of double and multiple galaxies much exceeds that expected, based on the assumption about dissociative balance. He has concluded that the members of double and multiple systems, as well as the clusters, could not arise independently of each other, and only then to be united in systems, by mutual capture. They should arise in common. Moreover, when in a group are large luminous central galaxies, the origin of the weak members of the group should be caused by activity of a nucleus of the central galaxy (Ambartsumian 1962). On the other hand, recently, there was widespread the opinion that the properties of central galaxies of groups and clusters are caused by interactions with environmental galaxies.

Cathode ray oscillography of gas adsorption phenomena I-A method for measuring high-velocity approach to certain physical and chemical equilibria

The essential feature of many experiments on adsorption, evaporation, surface diffusion, and surface chemistry consists of observing a progress towards equilibrium after altering A the pressure of gas which has access to a solid, or B the temperature of a solid exposed to gas or gas mixture. But such observations only become adequate for investigating probabilities of transition between the relevant physical and chemical states if the true rate of approach to equilibrium is not obscured or distorted by time lags in the experimental methods employed. If the term "reaction velocity" be generalized to include rate of simple phase change, owing to the similarity of the latter to chemical change in its thermodynamic treatment, then the limits to accessible range of such velocity are set by the following: ( a ) the maximum rate at which A or B can be made to stimulate reaction, ( b ) the rate at which resulting energy exchanges affect some observable quantity chosen as indicator, and ( c ) the rate at which the indicator can record itself. Such limitations become serious in the study of reactions occurring in interfacial layers of monomolecular thickness. These present uniquely simple kinetics owing to the elimination of transmission delays in the actual reaction, since all the atoms may be exposed simultaneously to any agency of change; but reaction velocity becomes for that reason so large, in the interesting cases where intrinsic probabilities of transition are not extremely small, that detailed investigation by ordinary means becomes impossible. The difficulty is most acute when the solid surface is reduced to the dimensions of a filament capable of high-temperature flashing, to obey modern needs of reproducible cleanliness; the gas content is then so minute that all optical methods are inapplicable and no micromanometric or thermal observation can keep pace with the reaction. Time constants for the faster of these would seem essentially inaccessible except electron emission from a surface offers an observable quantity indicative of the state of surface layers, and responding instantaneously to any reactions which modify that state. Such thermionic acid photoelectric emission has not hitherto been combined with fast enough recording to take full advantage of this rapidity of response; Langmuir and others have used auxiliary vapours to avoid temperature ranges in which gas reaction would be rapid, while Oliphant and Moon, and Evans, have used mechanical oscillography for alkali ions only, whose behaviour is important in a low-velocity range. The extension which we here put forward, to phenomena rapid enough to justify introducing a cathode ray oscillographic technique, requires firstly certain improvements in the timing, etc., controls of such instruments not needed for their normal use; secondly, it requires theoretical and experimental determinations of the limits imposed on observable velocity by the time taken in attaining pressure equilibria and thermal and electrical steady state, and by the speed of photography. These requirements in establishing the method for rapid surface processes are completed in Part I, in relation to the two standard way A and B of initiating reaction by pressure and temperature. The conclusions are exhibited in photographic examples from phenomena well known but previously inaccessible to exact analysis, before proceeding, Part II, to apply them in the solution of particular problems.