COMMUNICATION: Culture conditions producing structure entities composed of Cellulose I and II in bacterial cellulose

Culture conditions for the production of Cellulose I and/or II structures have been investigated by transmission electron microscopy using smooth colonies of Acetobacter xylinum ATCC23769. Cells prepared from smooth colonies produce the band material composed of Cellulose II in phosphate buffer (pH 7) at 4 °C. In contrast, the same cells produce the normal twisting ribbons of Cellulose I when the incubation temperature is raised to 28 °C. The band material is also produced at 4 °C in 2% buffered glucose solution and in the standard Hestrin-Schramm medium. This revised version was published online in August 2006 with corrections to the Cover Date.     

两位置交换问题魔角旋转核磁共振时域信号的准确解

已知两位置交换分子运动速率对于静态¹³CNMR谱的影响是由一简单代数关系决定,它已成为分析固体中分子运动的常规方法.慢速的魔角旋转可以保留边带,所产生的边带型原则上也可用来分析分子运动,只是缺乏象静态谱那样的简单的关系,以至于在实际应用上是不可行的.本文给出作者所发现的存在分子运动时魔角旋转核磁共振自由诱导衰减信号的解析表达式的理论推导,提供分析固体分子运动的理论谱计算公式.我们选择二甲矾(dimethylsulfone)作为模型化合物,记录其不同温度下¹³CNMR谱,又计算了两个甲基做两位置交换运动的理论谱.理论谱与实验谱定性一致.定量上,边带强度存在一些差别,但边带宽度符合很好.通过理论谱与实验谱的比较,得到交换速率与温度的对应关系.虽然本文限于化学位移各向异性相互作用两位置交换问题,但容易推广到偶极作用多位置交换问题.