X-ray analysis on the nanogram to microgram scale using porous complexes

X-ray single-crystal diffraction (SCD) analysis has the intrinsic limitation that the target molecules must be obtained as single crystals. Here we report a protocol for SCD analysis that does not require the crystallization of the sample. In our method, tiny crystals of porous complexes are soaked in a solution of the target, such that the complexes can absorb the target molecules. Crystallographic analysis clearly determines the absorbed guest structures along with the host frameworks. Because the SCD analysis is carried out on only one tiny crystal of the complex, the required sample mass is of the nanogram–microgram order. We demonstrate that as little as about 80 nanograms of a sample is enough for the SCD analysis. In combination with high-performance liquid chromatography, our protocol allows the direct characterization of multiple fractions, establishing a prototypical means of liquid chromatography SCD analysis. Furthermore, we unambiguously determined the structure of a scarce marine natural product using only 5 micrograms of the compound. Chemists need reliable methods to analyse and determine molecular structures. Nuclear magnetic resonance (NMR) and mass spectrometry are indispensable tools in daily chemical research for rapidly analysing molecular structures, but, strictly speaking, they provide only speculative molecular structures that are sometimes assigned incorrectly. However, X-ray SCD provides direct structural information at the atomic level and is recognized as the most reliable structure determination method 1–3 . Unfortunately, X-ray SCD has some critical limitations. First, the crystallization of samples before measurement can not be automated and usually requires a time-consuming trial-and-error procedure. Second, the method is in principle not applicable to non-crystalline molecules. In this Article, we describe an advance in crystallographic analysis based on a new X-ray analysis protocol that does not require the crystallization of the sample molecules themselves. Our idea is to use networked porous metal complexes 4–7 as ‘crystalline sponges’ 8 . Owing to the high molecular-recognition ability of the pores, the crystalline sponges can absorb target sample molecules from their solution into the pores, rendering the incoming molecules regularly ordered in the crystal. Accordingly, the molecular structure of the absorbed guest will be displayed, along with the host framework, by the crystallographic analysis of the networked porous complexes. We emphasize that even trace amounts of samples (,0.1mg) can be analysed by this method because the experiment can be performed with only one tiny crystal (,0.1 mm to a side). In the following discussion, we thus describe the crystallographic analysis of non-crystalline compounds and nanogram–microgram-scale X-ray crystallography based on our method. The great advantage of trace-amount X-ray analysis is particularly emphasized by its application to liquid chromatography SCD analysis (see below), where high-performance liquid chromatography (HPLC) fractions are directly collected by the crystalline sponge and analysed by X-ray crystallography. Furthermore, we successfully determine the structure of a scarce marine natural product, miyakosyne A, including the absolute configuration of its chiral centre, which could not be determined by conventional chemical and spectroscopic methods. X-ray crystallography of liquid samples