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Relative accessible surface area or relative solvent accessibility of a protein residue is a measure of residue solvent exposure. It can be calculated by formula:
RSA = ASA / MaxASA {\displaystyle {\text{RSA}}={\text{ASA}}/{\text{MaxASA}}}
where ASA is the solvent accessible surface area and MaxASA is the maximum possible solvent accessible surface area for the residue. Both ASA and MaxASA are commonly measured in Å 2 {\displaystyle {\mathrm {\AA} }^{2}}.
To measure the relative solvent accessibility of the residue side-chain only, one usually takes MaxASA values that have been obtained from Gly-X-Gly tripeptides, where X is the residue of interest. Several MaxASA scales have been publishedand are commonly used.