like ribonuclease a, lysozyme from t4 phage is a model enzyme for understanding the energetics and pathways of protein folding. unlike ribonuclease a, t4 lysozyme does not contain any disulfide bonds. a number of studies have quantified the thermodynamic contributions individual amino acid residues and their interactions make to lysozyme folding. an ion pair between an asp residue and a his residue in lysozyme contributes 13–21 kj/mol of favorable folding energy at ph 6.0. however, this ion pair contributes much less to lysozyme folding at either ph 2.0 or ph 10.0.