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How prevalent are co-crystals? American microscopist Walter McCrone once famously said ‘every compound has different polymorphic forms, and that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound’ [22]. McCrone’s words may yet also prove true for co-crystals. At January 2012, only 3.1% of crystals reported in the Cambridge Structural Database (CSD) were known to be polymorphic (https://www.ccdc.cam. ac.uk/products/csd/statistics/). This figure of course does not take into account unpublished proprietary data on pharmaceuticals, which are often the most studied compounds in terms of their solid-state behaviour. By contrast, approximately 15% contain water either in a co-crystal or as part of a metal aquo complex. A perhaps truer representation (albeit on a much smaller sample size) is given by data from contract solid-form screening company SSCI, reported in 2007 [20]. Of 245 polymorph screens on organic compounds, 91% existed in more than one crystal form and approximately half proved to have more than one pure polymorph. A total of 64 sets of screening experiments designed to reveal co-crystals indicated that approximately 61% of compounds studied formed co-crystals, whereas approximately one-third of the compounds studied formed hydrated or solvated forms. These numbers are consistent with the proportions in the 1999 European Pharmacopeia [23]. The behaviour of salt and non-salt forms proved to be somewhat different. Although both types of compounds existed as multiple solid forms approximately 91% of thetime, non-salts showed a greater polymorphic tendency (55% vs 39%), whereas salts, with their greater propensity to hydrate ionic sites, exhibited a significantly greater tendency to form hydrates (48% vs 30%) (Figure 2). A recently launched web resource, Hydrateweb.org, collated published data on pharmaceuticals monographed in Pharmacopeia Europa 6.2, 2012 and revealed that of 955 substances, 287 form hydrates (30.1%), with an overall 415 different hydrated crystal forms. Interestingly, there seems to be a statistically significant relationship between co-crystal formation and crystallisation with multiple symmetryindependent molecules (a kind of ‘self-co-crystallisation’). For example, based on the unusual three independent molecule structure of the antibacterial ornidazole, the existence of co-crystal forms was predicted and a co-crystal with dinitrobenzoic acid prepared experimentally shortly after the discovery of a hydrate form [24,25]. |
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共晶体有多普遍？美国显微镜专家Walter McCrone曾经有一句名言说：“每一个化合物具有不同的多态形式，在一般情况下，对于一个给定化合物的形态多样性与花在该研究化合物的时间和经费成正比“ [22] 。McCrone的话也适用于共晶体研究。2012年1月，在剑桥结构数据库（ CSD ）里只有3.1％已经报道的晶体是多态的（https://www.ccdc.cam. ac.uk/products/csd/statistics/） 。当然，这个数字并没有计入未公布的药物公司的私有的晶体数据，药物公司最经常研究化合物在固态时的表型。与此相反，约15％（的化合物）包含共晶体中的水或是金属水合配合物的一部分。也许更真实的代表（尽管是一个更小的样本）是于2007年报道的来自固态形式筛选公司SSCI的数据 [20]。对有机化合物的245个多形态筛选中，91％在一个以上的晶体形式存在，约一半被证明有一个以上的纯多晶形态。总共64组旨在揭示共晶体的筛选实验表明约61％的化合物形成共晶，而约三分之一的化合物形成水合物或溶剂化物形式。这些数字与1999年版的欧洲药典[23] 是一致的。 盐和非盐形式的表型被证明是有所不同的。虽然这两种类型化合物月91%作为多个固体形式存在，非盐表现出较大的多晶型的倾向（55％比39％ ） ，而更偏爱水合离子位点的盐显示更大的、更显著的形成水合物的趋势（48％比30％ ） （图2） 。最近推出的网络资源 Hydrateweb.org 整理了2012年欧洲药典（6.2版本）已经发表的数据，发现在955种物质中， 287种形成水合物（30.1％） ，整体上有415种不同的水合结晶形态。有趣的是，共晶体形成和多个独立于对称性的分子结晶（一种“自共结晶' ）之间的关系似乎有一个统计学上的显著性。例如，在不寻常的三个独立的抗菌硝唑分子结构的基础上，共晶体形式被预测存在，在发现水合物形式[24,25]后不久，在实验室制备了与二硝基苯甲酸的共晶体。 |
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