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A protein ( tertiary, quaternary, primary, secondary ) structure is the most thermodynamically stable structure achievable in aqueous solution. It is primarily driven by the ( hydrophobic, hydrophilic ) effect and by a series of many ( non-covalent, covalent ) forces. There are however some addition stabilising factors which aid in the formation of the most appropriate ( tertiary structure, primary structure, quaternary structure, secondary structure ).
Many proteins are additionally stabilised by ( disulphide, phosphodiester, hydrogen ) bonds between segments of secondary structure in the native state. These proteins go through many intermediates on the folding pathway and tend to be more stable ( than tertiary structures, than secondary structure, than primary structure, than quaternary structure ) without covalent interactions. Proteins that have ( disulphide bonds, phosphodiester bonds, hydrogen bonds ) (formed between appropriately positioned cyteine residues), include immunoglobulins, ribonuclease, insulin and pancreatic trypsin inhibitor (shown here).