Khristenko Nina, Amato Jussara, Livet Sandrine, Pagano Bruno, Randazzo Antonio, Gabelica Valérie
Laboratoire Acides Nucléiques: Régulations Naturelle et Artificielle, Université de Bordeaux, Inserm & CNRS (ARNA, U1212, UMR5320), IECB, 2 rue Robert Escarpit, 33607, Pessac, France.
Department of Pharmacy, University of Naples Federico II, via Domenico Montesano 49, 80131, Naples, Italy.
J Am Soc Mass Spectrom. 2019 Jun;30(6):1069-1081. doi: 10.1007/s13361-019-02152-3. Epub 2019 Mar 28.
Ion mobility spectrometry (IMS) has become popular to characterize biomolecule folding. Numerous studies have shown that proteins that are folded in solution remain folded in the gas phase, whereas proteins that are unfolded in solution adopt more extended conformations in the gas phase. Here, we discuss how general this tenet is. We studied single-stranded DNAs (human telomeric cytosine-rich sequences with CCCTAA repeats), which fold into an intercalated motif (i-motif) structure in a pH-dependent manner, thanks to the formation of C-H-C base pairs. As i-motif formation is favored at low ionic strength, we could investigate the ESI-IMS-MS behavior of i-motif structures at pH ~ 5.5 over a wide range of ammonium acetate concentrations (15 to 100 mM). The control experiments consisted of either the same sequence at pH ~ 7.5, wherein the sequence is unfolded, or sequence variants that cannot form i-motifs (CTCTAA repeats). The surprising results came from the control experiments. We found that the ionic strength of the solution had a greater effect on the compactness of the gas-phase structures than the solution folding state. This means that electrosprayed ions keep a memory of the charging process, which is influenced by the electrolyte concentration. We discuss these results in light of the analyte partitioning between the droplet interior and the droplet surface, which in turn influences the probability of being ionized via a charged residue-type pathway or a chain extrusion-type pathway.
离子淌度光谱法（IMS）已广泛用于表征生物分子折叠。众多研究表明，在溶液中折叠的蛋白质在气相中仍保持折叠状态，而在溶液中未折叠的蛋白质在气相中会呈现更伸展的构象。在此，我们探讨这一原则的普遍程度。我们研究了单链DNA（具有CCCTAA重复序列的人端粒富含胞嘧啶序列），由于形成了C-H-C碱基对，其会以pH依赖的方式折叠成插入基序（i-基序）结构。由于在低离子强度下有利于i-基序的形成，我们能够在较宽的醋酸铵浓度范围（15至100 mM）内研究pH约为5.5时i-基序结构的电喷雾离子淌度质谱行为。对照实验包括pH约为7.5时相同序列（此时序列未折叠）或不能形成i-基序的序列变体（CTCTAA重复序列）。令人惊讶的结果来自对照实验。我们发现，溶液的离子强度对气相结构紧凑性的影响大于溶液的折叠状态。这意味着电喷雾离子保留了对充电过程的记忆，而充电过程受电解质浓度影响。我们根据分析物在液滴内部和液滴表面之间的分配来讨论这些结果，这反过来又影响了通过带电残基型途径或链挤出型途径被电离的概率。
