In order to determine whether the new amino acid being replaced has distinct properties, we must first record which features of the amino acid each location possesses. At physiological pH, the side chains of some amino acids will be negatively charged, others positively charged, and yet others will be neutral.
Methionine and arginine have chemically distinct properties. While arginine is a positively charged amino acid and would presumably be facing the protein outside, methionine is a nonpolar side chain that would likely be hidden inside the protein core. The structure of a protein would likely be altered or disrupted if arginine were substituted for methionine.
