Laboratoire Léon Brillouin, CEA/CNRS UMR 12, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France.
J Chem Phys. 2012 Jun 7;136(21):215101. doi: 10.1063/1.4723871.
We report circular dichroism measurements on the helix-coil transition of poly(L-glutamic acid) in solution with polyethylene glycol (PEG) as a crowding agent. The PEG solutions have been characterized by small angle neutron scattering and are well described by the picture of a network of mesh size ξ, usual for semi-dilute chains in good solvent. We show that the increase of PEG concentration stabilizes the helices and increases the transition temperature. But more unexpectedly, we also notice that the increase of concentration of crowding agent reduces the mean helix extent at the transition, or in other words reduces its cooperativity. This result cannot be taken into account for by an entropic stabilization mechanism. Comparing the mean length of helices at the transition and the mesh size of the PEG network, our results strongly suggest two regimes: helices shorter or longer than the mesh size.
我们报告了在聚（L-谷氨酸）的螺旋-卷曲转变中，使用聚乙二醇（PEG）作为拥挤剂的圆二色性测量。PEG 溶液已通过小角中子散射进行了表征，并通过网眼尺寸 ξ 的网络图像得到了很好的描述，这对于良好溶剂中的半稀释链来说是常见的。我们表明，PEG 浓度的增加稳定了螺旋并提高了转变温度。但更出乎意料的是，我们还注意到，拥挤剂浓度的增加降低了转变时的平均螺旋程度，或者换句话说，降低了其协同性。这种结果不能用熵稳定机制来解释。将转变时的平均螺旋长度与 PEG 网络的网眼尺寸进行比较，我们的结果强烈表明存在两种状态：网眼尺寸小于或大于螺旋长度。
