Thermodynamic parameters of helix–coil transition in polypeptide chains. II. Poly-L-lysine

✍ Scribed by T. V. Barskaya; O. B. Ptitsyn

• Publisher: Wiley (John Wiley & Sons)
• Year: 1971
• Tongue: English
• Weight: 768 KB
• Volume: 10
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360101112

✦ Synopsis

The helix-coil transitions for poly-L-lysine (PL) were investigated by the methods of spectropolarimetry, viscometry and potentiometric titration in 0.2M NaCl a t different temperatures as well as in 0.2M NaBr, 1M KC1, and in mixtures of 0.2M NaCl or NaBr with methanol a t room temperature. The enthalpy and entropy differences between the helical and coillike states of uncharged PL molecules in 0.2M NaCl were determined from the potentiometric titration curves. The cooperativity parameters u for PL in different solvents were determined by two methods (from the sharpness of the transition and from the dependence of t.he intrinsic viscosity on the helical content in the transition region). I n 0.2M NaCl u has a value of (2.3 =! = 0.5) X 10-4 and does not depend on temperature, i.e., the cooperativity of the helix-coil transition, as for PGA, is mainly of an entropy origin (the initiating of the helical region is accompanied by the entropy decrease ASi = -12 eu/mole of helical regions). A comparison of the obtained results for PGA and PL with the molecular theories of the helix-coil transitions shows that the role of dipole-dipole interactions of nonneighboring peptide groups is greatly overestimated in these theories, leading to a considerable enthalpy contribution to the free energy of initiating helical regions which is not observed in the experiment.