23,462 research outputs found
Force-clamp experiments reveal the free energy profile and diffusion coefficient of the collapse of proteins
We present force-clamp data on the collapse of ubiquitin polyproteins in
response to a quench in the force. These nonequilibrium trajectories are
analyzed using a general method based on a diffusive assumption of the
end-to-end length to reconstruct a downhill free energy profile at 5pN and an
energy plateau at 10pN with a slow diffusion coefficient on the order
of~100nm^2/s. The shape of the free energy and its linear scaling with the
protein length give validity to a physical model for the collapse. However, the
length independent diffusion coefficient suggests that internal rather than
viscous friction dominates and thermal noise is needed to capture the
variability in the measured times to collapse.Comment: 12 pages, 4 figure
On the Structure of the Orion A Cloud and the Formation of the Orion Nebula Cluster
We suggest that the Orion A cloud is gravitationally collapsing on large
scales, and is producing the Orion Nebula Cluster due to the focusing effects
of gravity acting within a finite cloud geometry. In support of this
suggestion, we show how an elliptical rotating sheet of gas with a modest
density gradient along the major axis can collapse to produce a structure
qualitatively resembling Orion A, with a fan-shaped structure at one end,
ridges or filaments along the fan, and a narrow curved filament at the other
end reminiscent of the famous integral-shaped filament. The model produces a
local concentration of mass within the narrow filament which in principle could
form a dense cluster of stars like that of the Orion Nebula. We suggest that
global gravitational contraction might be a more common feature of molecular
clouds than previously recognized, and that the formation of star clusters is a
dynamic process resulting from the focusing effects of gravity acting upon the
geometry of finite clouds.Comment: 23 pages, 6 figures, to appear in the Astrophysical Journa
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