I'm reading the papers linked below and all three of them mention a 20% cut-off for buried/exposed residues, by calculating a relative solvent accessibility (RSA) value.
I understand how the RSA is calculated, by dividing the calculated solvent accessibility against its total solvent accessibility values from paper 4 table 2.
RSA = calculated/total
e.g. if arginine is calculated to have a solvent accessibility of 55.43 and its total solvent accessibility is 241 then the RSA = 55.43/241 = 23%, so this arginine is considered exposed (see statement 1 below).
What leaves me confused is the definition or lack of the 20% method for defining an exposed or buried residue.
I am assuming it means one of the following:
If an amino acids RSA is below 20% it is buried and above 20% it is exposed. So for an amino acid with an RSA of 21% is considered exposed, this value seems a little low for me. I think statement 2 would make for sense.
If an amino acids RSA is below 20% it is buried and above 80% it is exposed.
Which statement if any is correct?
Paper 1 - see methods section first paragraph
Paper 2 - see figure 5 and table 3
Paper 3 - see abstract and dataset
Paper 4 - see table 2 for total values