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Accessible surface area
The accessible surface area (ASA) is the surface area of a biomolecule (protein, DNA, etc.) that is accessible to a solvent. Is usually quoted in square ångstrom (a standard unit of measurement in molecular biology). ASA was first described by Lee & Richards in 1971 is sometimes called the Lee-Richards molecular surface. ASA is typically calculated using the 'rolling ball' algorithm developed by Shrake & Rupley in 1973. This algorithm uses a sphere (of solvent) of a particular radius to 'probe' the surface of the molecule. The choice of the 'probe radius' does have an effect on the observed surface area, as using a smaller probe radius detects more surface details and therefore reports a larger surface. A typical value is 1.4Å, which approximates the radius of a water molecule. Another factor that affects the results is the definition of the VDW radii of the atoms in the molecule under study. For example, the molecule may often lack hydrogen atoms which are implicit in the structure. The hydrogen atoms may be implicitly included in the atomic radii of the 'heavy' atoms, with a measure called the 'group radii'.
Additional recommended knowledge
The ASA is closely related to the concept of the solvent-excluded surface (also known as the molecular surface or Connolly surface), which is imagined as a cavity in bulk solvent (effectively the inverse of the solvent-accessible surface). It is also calculated in practice via a rolling-ball algorithm developed by Fred Richards and independently implemented three-dimensionally by Michael Connolly in 1983 and Tim Richmond in 1984. Connolly spent several more years perfecting the method and it is thus sometimes called the Connolly surface.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Accessible_surface_area". A list of authors is available in Wikipedia.|