Temperature-induced transition between polyproline I and II helices: quantitative fitting of hysteresis effects.

The conformational properties of the polyproline I (PPI) helix of oligoprolines toward heating were examined. Oligoproline H-Pro(12)-NH(2) served as a model which adopts in n-PrOH a pronounced PPI conformation with all cis amide bonds, whereas a polyproline II (PPII) conformation with all trans amide bonds is predominant in pure aqueous buffer. CD spectroscopic studies revealed that a conformational change from the PPI to the PPII helix takes place upon heating and back to the PPI helix upon cooling. This conformational transition cycle is characterized by a strong hysteresis. With a quantitative fitting of the experimentally observed hysteresis loops by a newly developed iterative integration with different starting conditions, kinetic and thermodynamic parameters for the transition from the PPI to the PPII helical conformation were determined. The transition is as expected for cis-trans isomerizations of amide bonds comparatively slow (k = 0.003 s(-1) at 80 °C) and characterized by an activation energy E(a) of 81.1 ± 3.6 kJ mol(-1). Thermodynamically, the transition from the PPI to the PPII helix is characterized by a positive standard enthalpy (ΔH(0) = 33.5 ± 2.1 kJ min(-1)) and a positive standard entropy (ΔS(0) = 102 ± 6.6 J mol(-1) K(-1)).