Abstract
The analysis of thermal melting curves of (PPG)~n~ (n = 10, 12, 14, and 15) and (PPG)~n~(APG)~m~ (PPG)~n~ (2__n__ + m = 15; m = 1, 3, and 5) revealed that the enthalpy and entropy changes accompanying the transition from the random coil to the triple helix are −2500 cal and −6.3 e.u. per one mole of the tripeptide of the form of Pro‐Pro‐Gly, and −3100 cal and −11.2 e.u. per one mole of the tripeptide of the form of Ala‐Pro‐Gly.
The thermal instability of the triple helix composed of Ala‐Pro‐Gly sequences, compared to the helix of Pro‐Pro‐Gly sequences, is due to the larger entropy change of Ala‐Pro‐Gly (−11.2 e.u.) compared to that of Pro‐Pro‐Gly (−6.3 e.u.), not from the difference in the enthalpy change.
The difference in the enthalpy change between Pro‐Pro‐Gly and Ala‐Pro‐Gly arises from the hydrophobic bond between two pyrrolidine rings of proline residues formed in the triple helix. Since the enthalpy change for the formation of hydrophobic bonds is positive, it is also concluded that only one hydrogen bond is formed in a tripeptide unit, regardless of the amino acid sequence. The enthalpy change for the formation of this hydrogen bond is −3100 cal/mol, and that of the hydrophobic bond between two pyrrolidine rings is +600 cal/mol.