Here is the link to the paper http://www.sciencemag.org/content/185/4154/862.long
AMINO ACID DIFFERENCE FORMULA TO HELP EXPLAIN PROTEIN EVOLUTION
A formula for difference between amino acids combines properties that correlate best with protein residue substitution frequencies: composition, polarity, and molecular volume. Substitution frequencies agree much better with overall chemical difference between exchanging residues than with minimum base changes between their codons. Correlation coefficients show that fixation of mutations between dissimilar amino acids is generally rare.
In case you are looking for the code, you can modify this python code and use it: