Kim Kihyun, Lee Jong Wha, Chang Taihyun, Kim Hugh I
Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, 790-784, Korea.
J Am Soc Mass Spectrom. 2014 Oct;25(10):1771-9. doi: 10.1007/s13361-014-0949-1. Epub 2014 Jul 8.
We investigated the effect of stereoregularity on the gas-phase conformations of linear and cyclic polylactides (PLA) using electrospray ionization ion mobility mass spectrometry (ESI-IM-MS) combined with molecular dynamics simulations. IM-MS analysis of PLA ions shows intriguing difference between the collision cross section (ΩD) value of poly-L-lactide (PLLA) and poly-LD-lactide (PLDLA) ions with respect to their chain architecture and stereoregularity. In the singly sodiated linear PLA (l-PLA∙Na(+)) case, both l-PLLA and l-PLDLA up to 11mer have very similar ΩD values, but the ΩD values of l-PLLA are greater than that of l-PLDLA ions for larger ions. In the case of cyclic PLA (c-PLA), c-PLLA∙Na(+) is more compact than c-PLDLA∙Na(+) for short PLA ions. However, c-PLLA exhibits larger ΩD value than c-PLDLA for PLA ions longer than 13mer. The origin of difference in the ΩD values was investigated using theoretical investigation of PLAs in the gas phase. The gas-phase conformation of PLA ions is influenced by Na(+)-oxygen coordination and the weak intramolecular hydrogen bond interaction, which are more effectively formed in more flexible chains. Therefore, the less flexible PLLA has a larger ΩD value than PLDLA. However, for short c-PLA, concomitant maximization of both Na(+)-oxygen coordination and hydrogen bond interaction is difficult due to the constricted chain freedom, which makes the ΩD value of PLAs in this range show a different trend compared with other PLA ions. Our study facilitates the understanding of correlation between stereoregularity of PLAs and their structure, providing potential utility of IM-MS to characterize stereoisomers of polymers.
我们使用电喷雾电离离子淌度质谱（ESI-IM-MS）结合分子动力学模拟，研究了立构规整性对线性和环状聚丙交酯（PLA）气相构象的影响。PLA离子的IM-MS分析表明，聚-L-丙交酯（PLLA）和聚-LD-丙交酯（PLDLA）离子的碰撞截面（ΩD）值在链结构和立构规整性方面存在有趣的差异。在单钠化线性PLA（l-PLA∙Na(+)）的情况下，l-PLLA和l-PLDLA直至11聚体都具有非常相似的ΩD值，但对于更大的离子，l-PLLA的ΩD值大于l-PLDLA离子的ΩD值。在环状PLA（c-PLA）的情况下，对于短PLA离子，c-PLLA∙Na(+)比c-PLDLA∙Na(+)更紧凑。然而，对于长度超过13聚体的PLA离子，c-PLLA的ΩD值比c-PLDLA大。使用气相中PLA的理论研究来探究ΩD值差异的起源。PLA离子的气相构象受Na(+) - 氧配位和弱分子内氢键相互作用的影响，这些相互作用在更灵活的链中更有效地形成。因此，柔性较差的PLLA比PLDLA具有更大的ΩD值。然而，对于短的c-PLA，由于链自由度受限，同时使Na(+) - 氧配位和氢键相互作用最大化是困难的，这使得该范围内PLA的ΩD值与其他PLA离子相比呈现出不同的趋势。我们的研究有助于理解PLA的立构规整性与其结构之间的相关性，为IM-MS表征聚合物立体异构体提供了潜在的应用价值。
