Accessible surface area
（重定向自Accessible surface）
![Illustration of the solvent accessible surface in comparison to the van der Waals surface. The van der Waals surface as given by the atomic radii is shown in red. The accessible surface is drawn with dashed lines and is created by tracing the center of the probe sphere (in blue) as it rolls along the van der Waals surface. Note that the probe radius depicted here is of smaller scale than the typical 1.4Å.](/Images/godic/202412/19/Accessible_surface.svg3539.png")
The accessible surface area (ASA) or solvent-accessible surface area (SASA) is the surface area of a biomolecule that is accessible to a solvent. Measurement of ASA is usually described in units of square ångstroms (a standard unit of measurement in molecular biology). ASA was first described by Lee & Richards in 1971 and 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.