Fundamental Studies on Preparation of Cellulose Oligomers with Narrow Molecular Weight Distribution

长期以来，人们对纤维素的功能化进行了大量的研究，但对纤维素低聚物包括纤维素低聚糖的功能化研究却很少。这是因为这些低聚糖非常昂贵，而且很难获得。本研究项目涉及(i)通过分步沉淀和酶降解方法制备DP值在7到20之间的窄分布纤维素低聚物，以及（ii）利用其特征反应性对还原端基进行功能转换。结果概述如下1。以商品三醋酸纤维素（CTA）为原料，经均相酸水解及去乙酰化，制备DP值为10 ~ 20，多分散比（Mw/Mn）为1.50的纤维素低聚物，并作为起始原料。热DMSO(100。C)，通过DMSO/H_2O热体系进一步分馏得到分布较窄的纤维素低聚物（Mw/Mn=1.18）， DP为9.9（产率14%）.2。由天野制药株式会社（Amano Pharmaceutical Co., Ltd）用cellulose T酶解得到分布较窄的纤维素低聚物（Mw/Mn=1.22-1.26）， DP值约为20。值得注意的是，降解产物的结晶度比起始纤维素的结晶度要高得多，这反映了样品低聚物中末端的非结晶区比结晶区更优先被纤维素降解。首先用HBr/CH_3COOH对三丙酸纤维素（CTP）进行酸水解解聚，得到端溴化CTP低聚物。随后以n -保护氨基醇为糖基受体的糖基化反应和Pd-C/HCOONH_4促进的去保护反应导致具有末端初级氨基功能的CTP低聚物。^1H-NMR和GPC分析表明，氨基的官能团接近统一。

A number of studies on the functionalization of cellulose have been made for a long time, however few on the functionalization of cellulose oligomers including cello-oligosaccharides. This is because these oligosaccharides are very expensive and hardly available. This research project concerns (i) the preparation of narrow-distributed cellulose oligomers with DP values ranging from 7 to 20 by using fractional precipitation and enzymatic degradation methods, and (ii) the functionality conversion of the reducing-end groups by taking advantage of their characteristic reactivity. The results are outlined below.1. Cellulose oligomers having a DP value of 10-20 and a polydispersity ratio (Mw/Mn) of 1.50 were prepared by homogeneous acid-hydrolysis of commercial cellulose triacetate (CTA) and the subsequent deacetylation, and used as a starting material. The extraction with hot DMSO (100。C) and the further fractional precipitation by hot DMSO/H_2O system afforded the relatively narrow-distributed cellulose oligomer (Mw/Mn=1.18) with a DP of 9.9 (yield 14%).2. The narrow-distributed cellulose oligomers (Mw/Mn=1.22-1.26) having a DP value of ca. 20 were obtained through enzymatic degradation with Cellulose T (Amano Pharmaceutical Co., Ltd.). It is worth noting that the degree of crystallinity of the degraded product became much higher than that of the starting cellulose, reflecting that the terminal non-crystalline region in the sample oligomers is preferentially degraded with celluose compared with the crystalline region.3. Cellulose tripropionate (CTP) was first depolymerized via acid-hydrolysis using HBr/CH_3COOH to obtain the end-brominated CTP oligomer. The subsequent glycosylation employing N-protected aminoalcohol as a glycosyl acceptor and the deprotection promoted with Pd-C/HCOONH_4 lead to the CTP oligomer with terminal primary amino function. ^1H-NMR and GPC analyzes revealed that the functionality of amino groups was close to unity.