question about mutagenesis

[warlord]

Hi guys, I am wondering what could be the best a.a. substitution for glycine? Under what circumstances could you make this substitution without disrupting the structure of the protein? (Assume you could look at a crystal structure). If you did not have a crystal structure, how could you predict whether substitution for glycine might be tolerated? If tolerated, a substitution for glycine by other amino acids can increase thermal stability of proteins. Why might this be so? I can think of two reasons, one an effect on the folded state and one an effect on the unfolded state.