Drittes Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany.
J Phys Chem B. 2012 Mar 1;116(8):2446-54. doi: 10.1021/jp2106007. Epub 2012 Feb 22.
Broadband ultrasonic attenuation spectra (100 kHz to 2 GHz) of aqueous solutions of vesicles from 1,2-dimyristoyl-l-3-phosphatidylcholine, with different amounts of cholesterol admixed, have been measured at temperatures between 20 and 28 °C. The spectra have been evaluated in terms of suitable relaxation functions. They are discussed in view of the effect of cholesterol on the membrane behavior around the gel-fluid phase transition temperature T(m). In addition to a frequency-independent asymptotic high-frequency term, all spectra reveal a critical term and a Debye-type relaxation term with relaxation time around 0.5 ns. The former is evaluated in the light of the Bhattacharjee-Ferrell dynamic scaling theory. It is assigned to the critical domain structure fluctuations of the membranes. Critical slowing of fluctuations is demonstrated. Also shown are relations of the critical amplitudes to thermodynamic parameters. The Debye term reflects the rotational isomerization of the phospholipid alkyl chains. The relaxation time of isomerization reveals a significant steplike change at T(m). At moderate cholesterol content an additional Debye relaxation term exists. It is assigned to the axial diffusion of the membrane molecules. Because it likewise shows effects of slowing near T(m), the diffusion appears to be coupled to the domain structure fluctuations. A further relaxation term at small cholesterol concentration is assumed to be due to small-range shape fluctuations of vesicles near the phase transition temperature.
在 20 至 28°C 的温度范围内，测量了含有不同胆固醇含量的 1,2-二肉豆蔻酰基-sn-甘油-3-磷酸胆碱囊泡水溶液的宽带超声衰减谱（100 kHz 至 2 GHz）。根据合适的弛豫函数对光谱进行了评估。考虑到胆固醇对膜行为在凝胶-流体温相变温度 T(m) 附近的影响，对其进行了讨论。除了频率无关的渐近高频项外，所有光谱都显示出临界项和德拜型弛豫项，弛豫时间约为 0.5 ns。前一项是根据 Bhattacharjee-Ferrell 动态标度理论进行评估的。它被分配给膜的临界域结构波动。波动的临界减速得到了证明。还展示了临界幅度与热力学参数的关系。德拜项反映了磷脂烷基链的旋转异构化。异构化的弛豫时间在 T(m) 处显示出明显的阶跃变化。在中等胆固醇含量下，存在额外的德拜弛豫项。它被分配给膜分子的轴向扩散。由于它在接近 T(m) 时也表现出减速效应，因此扩散似乎与域结构波动耦合。在低胆固醇浓度下，进一步的弛豫项归因于相变温度附近小范围的囊泡形状波动。
