The three replacements Thr109/Asp, Asn116/Asp, and Asn144/Asp in T4 lysozyme introduce acid groups at the N2 position of an alpha helix and in so doing increase protein stability.This is very much in line with the enhanced frequencies of acidic groups at the N2 and N3 positions. Bell substitued Thr59 , an amino cap residue, with other residues and determined the stabilities and crystal structures of the mutants. All mutations caused destabilization ranging from 0.2kcal/mol for serine to 1.6kcal/mol for glycine, alanine, valine. The loss of protein stability for the Val, Ala and Gly replacements of Thr59 appears to be due to the loss of hydrogen bonding.For Thr59Asn mutant, different backbone conformation is the reason of decreasing protein stability, althugh Asn is observed much more frequently than Thr at the Ncap position