화학공학소재연구정보센터
Journal of Chemical Engineering of Japan, Vol.51, No.1, 111-115, 2018
Behavior of t-Butoxycarbonyl-L-asparagine (Boc-Asn) during the Initial Stage of Crystallization
The crystallization mechanism of amino acid derivative t-butoxycarbonyl-L-asparagine (Boc-Asn) was investigated. NMR spectroscopy was used to observe the behavior of Boc-Asn molecules in under-and supersaturated solution. Nuclear Overhauser Effect (NOE) NMR measurements were performed to provide information concerning the intra-or intermolecular access of a particular hydrogen atom to other hydrogen atoms in solution. We compared the interactions identified by NOE measurements for under-and supersaturated solutions of Boc-Asn with those in the crystal, as determined by X-ray analysis. According to the X-ray analysis, intramolecular NOE should not be observed, but intermolecular NOE might be observed between the methyl protons of the t-butoxycarbonyl group and the protons belonging to other functional groups. The time course of the NOE intensity was followed for three solutions with different saturation ratios S (= concentration of solute/solubility). For undersaturated and nearly saturated conditions (S = 0.8 and 1.12 at 0 degrees C), the NOE intensity did not change for at least for 250 h. However, for the supersaturated solution (S = 1.62 at 10 degrees C), the NOE intensity increased suddenly and rapidly after about 120 h, and no crystals were observed during the NOE measurements. As the observed NOEs agreed with those expected from the arrangement of molecules in the crystal, it was suggested that Boc-Asn molecules aggregated after about 120 h with the same structure as that in the crystal. In addition to the behavior of the Boc-Asn solution before nucleation, we also investigated the effect of aggregation on the crystallization process. Solutions that were preincubated at 10 degrees C for various periods were further cooled and crystallized at -30 degrees C. It was found that when the solution was preincubated under the supersaturated condition for a long time before crystallization, nucleation was enhanced, and consequently, the crystal size distribution became homogeneous.