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Potentiometric titration of poly‐ L ‐lysine: the coil‐to‐β transition
Author(s) -
Pederson Dennis,
Gabriel Don,
Hermans Jan
Publication year - 1971
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360101109
Subject(s) - enthalpy , chemistry , helix (gastropod) , potentiometric titration , random coil , entropy (arrow of time) , gibbs free energy , crystallography , electromagnetic coil , thermodynamics , mole , circular dichroism , organic chemistry , physics , ecology , electrode , quantum mechanics , snail , biology
We have performed potentiometric titrations of poly‐ L ‐lysine. From these data we have calculated the free energy and enthalpy changes for the folding of the random coil to the α‐helix in 10% ethanol (−120 and −120 cal/mole) and from the random coil to the β‐structure in water (−140 and 870 cal/mole) and in 10% ethanol (−180 and 980 cal mole). Comparison of these values with each other and with values for the coil → α‐ helix transition in water (−78 and −880 cal/mole) led to the following conclusions. The stabilization by ethanol of ethanol of the α‐helix with respect to the coil is that predicted from the known free energy of transfer of the peptide group from water to 10% ethanol. Similar data to explain the enthalpy difference are not available. The thermodynamic functions for the transition from α‐helix to β‐structure, obtained by subtracting those for the coil → α‐helix and coil → β‐structure transitions, are explained from a consideration of the structural differences: non bonded interactions of the polypeptide backbone are less favorable in the β‐structure than in the α‐helix, causing an increase in the energy, while hydrophobic contacts between side chains raise the entropy of the β‐structure as compared with the α‐helix, so that the free energy difference between the two structures is small, but enthalpy and entropy differences are large. The observation of only small differences in the free energy and enthalpy changes for the transition from coil β‐structure upon going from water to 10% ethanol is expected by considering both the free energy of transfer of the peptide group (as for the α‐helix) and the free energy and enthalpy of transfer of the apolar part of the side chain involved in hydrophobic bond formation.