Small organic molecules are used as solution additives in electrospray ionisation mass spectrometry (ESI-MS) to increase the charge states of protein ions and improve the performance of intact protein analysis by tandem mass spectrometry. The properties of the additives that are responsible for their charge-enhancing effects (e.g. dipole moment, gas-phase basicity, Brønsted basicity, and surface tension) have been debated in the literature. We report a series of solution additives for ESI-MS based on cyclic alkyl carbonates and sulphites that have alkyl chains that are from two to ten methylene units long. The extent of charging of [Val [5]]-angiotensin II, cytochrome c, carbonic anhydrase II, and bovine serum albumin in ESI-MS using the additives was measured. For both the alkyl carbonate and sulphite additives with up to four methylene units, ion charging increased as the side chain lengths of the additives increased. At a critical alkyl chain length of four methylene units, protein ion charge states decreased as the chain length increased. The dipole moments, gas-phase basicity values, and Brønsted basicities (i.e. the pKa of the conjugate acids) of the additives were obtained using electronic structure calculations, and the surface tensions were measured by pendant drop tensiometry. Because the dipole moments, gas-phase basicities, and pKa values of the additives did not depend significantly on the alkyl chain lengths of the additives and the extent of charging depended strongly on the chain lengths, these data indicate that these three additive properties do not correlate with protein charging under these conditions. For the additives with alkyl chains at or above the critical length, the surface tension of the additives decreased as the length of the side chain decreased, which correlated well with the decrease in protein charging. These data are consistent with protein charging being limited by droplet surface tension below a threshold surface tension for these additives. For additives with relatively high surface tensions, protein ion charging increased as the amphiphilicity of the additives increased (and surface tension decreased) which is consistent with protein charging being limited by the emission of charge carriers from highly charged ESI generated droplets.

中文翻译：

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关于电喷雾电离质谱中蛋白质增压的机制：对具有碳酸酯和亚硫酸盐末端基团的短链和长链烷基成分的添加剂充电的影响

有机小分子在电喷雾电离质谱 (ESI-MS) 中用作溶液添加剂，以增加蛋白质离子的电荷状态并提高串联质谱分析完整蛋白质的性能。导致电荷增强效应（例如偶极矩、气相碱度、布朗斯台德碱度和表面张力）的添加剂的特性在文献中已存在争议。我们报告了一系列用于 ESI-MS 的溶液添加剂，这些添加剂基于具有 2 到 10 个亚甲基单元长的烷基链的环状碳酸烷基酯和亚硫酸盐。[Val [5]]-血管紧张素II、细胞色素c、碳酸酐酶II和牛血清白蛋白在使用添加剂的ESI-MS中的充电程度被测量。对于最多含有四个亚甲基单元的碳酸烷基酯和亚硫酸盐添加剂，离子电荷随着添加剂侧链长度的增加而增加。在四个亚甲基单元的临界烷基链长处，蛋白质离子电荷状态随着链长的增加而降低。使用电子结构计算获得添加剂的偶极矩、气相碱度值和布朗斯台德碱度（即共轭酸的 pKa），并通过悬滴张力测定法测量表面张力。因为添加剂的偶极矩、气相碱度和 pKa 值不显着依赖于添加剂的烷基链长度，并且充电程度强烈依赖于链长，这些数据表明这三种添加剂特性不在这些条件下与蛋白质充电相关。对于烷基链处于或高于临界长度的添加剂，添加剂的表面张力随着侧链长度的减少而降低，这与蛋白质电荷的减少密切相关。这些数据与蛋白质充电受到液滴表面张力低于这些添加剂的阈值表面张力的限制一致。对于具有相对高表面张力的添加剂，蛋白质离子电荷随着添加剂的两亲性增加（和表面张力降低）而增加，这与蛋白质电荷受到来自高电荷 ESI 生成的液滴的电荷载流子发射的限制一致。这些数据与蛋白质充电受到液滴表面张力低于这些添加剂的阈值表面张力的限制一致。对于具有相对高表面张力的添加剂，蛋白质离子电荷随着添加剂的两亲性增加（并且表面张力降低）而增加，这与蛋白质电荷受到来自高电荷 ESI 生成的液滴的电荷载流子发射的限制一致。这些数据与蛋白质充电受到液滴表面张力低于这些添加剂的阈值表面张力的限制一致。对于具有相对高表面张力的添加剂，蛋白质离子电荷随着添加剂的两亲性增加（和表面张力降低）而增加，这与蛋白质电荷受到来自高电荷 ESI 生成的液滴的电荷载流子发射的限制一致。