School of Fundamental Sciences, Massey University, Palmerston North, New Zealand; MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington, New Zealand.
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 30076, Taiwan, ROC.
Carbohydr Polym. 2022 Nov 15;296:119958. doi: 10.1016/j.carbpol.2022.119958. Epub 2022 Aug 11.
The solution state structure of κ-carrageenan is typically described as a 'random coil', to indicate a lack of defined secondary structure elements. From this starting point the assignment of an optical-rotation-detected change that follows the introduction of particular ions to such solutions as a 'coil-to-helix transition' seems unambiguous, and thus the canonical description of this important biopolymer's gelling behaviour was born. However, the notion that κ-carrageenan exists in solution as a random coil, devoid of secondary structure, has been questioned a number of times previously in the literature, particularly by the molecular modelling and NMR communities. Regrettably, there has been little desire to-date to address these largely overlooked studies or consider their implications for the nature of the so-called 'coil-to-helix transition'. Despite evidence to the contrary, the random-coil-paradigm has prevailed. Here, new data from synchrotron-enabled solution-state x-ray scattering experiments, combined with state-of-the-art atomistic molecular dynamics simulations, are used to show that the solution-state structure of κ-carrageenan in fact retains many of the helical motifs present in the solid-state, as inferred from fibre diffraction data. Furthermore, no evidence is found to suggest that single chains undergo any uni-molecular conformational transition upon the addition of ions. These findings once again challenge the paradigm that κ-carrageenan exists as a 'random coil' in the solution state, and thereby question the long held assumption that a uni-molecular 'coil-to-helix transition' precedes the dimerization of helices.
κ-卡拉胶的溶液状态结构通常被描述为“无规线团”，以表示缺乏明确的二级结构元件。从这个起点出发，当特定离子被引入到这些溶液中时，光学旋转检测到的变化被分配为“无规线团到螺旋的转变”，这似乎是明确的，因此这种重要的生物聚合物胶凝行为的规范描述就诞生了。然而，κ-卡拉胶在溶液中以无规线团形式存在，没有二级结构的观点在之前的文献中已经被多次质疑，特别是在分子建模和 NMR 社区。遗憾的是，迄今为止，人们几乎没有意愿去解决这些在很大程度上被忽视的研究，也没有考虑它们对所谓的“无规线团到螺旋的转变”性质的影响。尽管有相反的证据，无规线团范式仍然占主导地位。在这里，新的同步加速器溶液态 X 射线散射实验数据，结合最先进的原子分子动力学模拟，用于表明 κ-卡拉胶的溶液状态结构实际上保留了许多在固态中存在的螺旋模体，这是从纤维衍射数据推断出来的。此外，没有发现任何证据表明单链在加入离子后会发生任何单一分子构象转变。这些发现再次挑战了κ-卡拉胶在溶液状态下以“无规线团”形式存在的范式，从而质疑了长期以来的假设，即单一分子的“无规线团到螺旋的转变”先于螺旋的二聚化。
