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葡萄糖的环状结构和变旋现象产生的原理
来源:http://www.sspp8.com 日期:2019-07-05 发布人:admin
结晶葡萄糖有两种,一种是从乙醇中结晶出来的,熔点146℃。它的新配溶液的[α]D为+112°,此溶液在放置过程中,比旋光度逐步降落,到达+52。17°以后维持不变;另一种是从吡啶中结晶出来的,熔点150℃,新配溶液的[α]D为+18。7°,此溶液在放置过程中,比旋光度逐步上升,也到达+52。7°以后维持不变。糖在溶液中,比旋光度自行转变为定值的现象称为变旋现象。显然葡萄糖的开链构造不能解释此现象。
There are two kinds of crystalline glucose, one is crystallized from ethanol, the melting point is 146 C. The [alpha] D of its new solution is + 112. The specific rotation of this solution decreases gradually and remains unchanged after reaching + 52.17. The other one is crystallized from pyridine at 150 and the new solution has a [alpha] D of + 18.7. During the process of placement, the specific rotation of this solution increases gradually and reaches + 52.7. It remains unchanged. The phenomenon that the specific rotation of sugar changes itself into a fixed value in solution is called the rotation phenomenon. Obviously, the open-chain structure of glucose can not explain this phenomenon.
从葡萄糖的开链构造可见,它既具有醛基,也有醇羟基,因而在分子内部能够构成环状的半缩醛。
From the open-chain structure of glucose, we can see that it has both aldehyde group and alcohol hydroxyl group, so it can form a cyclic semiacetal inside the molecule.
成环时,葡萄糖的羰基与C-5上的羟基经加成反响构成稳定的六元环。葡萄糖分子固然具有醛基,但在反响性能上与普通的醛有许多差别,例如对NaHSO3的加成十分迟缓,其缘由是在溶液中,葡萄糖简直以环状的半缩醛构造存在的缘故。
The carbonyl group of glucose and the hydroxyl group of C-5 react by addition to form a stable six-membered ring. Glucose molecule has aldehyde group, but there are many differences in reactivity with ordinary aldehydes. For example, the addition of NaHSO3 is very slow. The reason is that in solution, glucose exists almost in the form of cyclic semiacetals.
成环后,使原来的羰基碳原子(C-1)变成了手性碳原子,C-1上新构成的半缩醛羟基在空间的排布方式有两种可能。半缩醛羟基与决议单糖构型的羟基(C-5上的羟基)在碳链同侧的叫做α型,在异侧的称为β型。α型和β型是非对映异构体。它们的不同点是C-1上的构型,因而又称为异头物(端基异构体)。它们的熔点和比旋光度都不同。
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After cyclization, the original carbonyl carbon atom (C-1) is transformed into chiral carbon atom. There are two possibilities for the spatial arrangement of the newly formed semi-acetal hydroxyl groups on C-1. The hydroxyl groups of hemiacetal and monosaccharide (the hydroxyl groups on C-5) are called alpha on the same side of the carbon chain and beta on the opposite side. Types alpha and beta are non-enantiomers. The difference between them is the configuration of C-1, so they are also called heteropoly (terminal isomer). Their melting point and specific rotation are different.
葡萄糖的变旋现象,就是由于开链构造与环状构造构成均衡体系过程中的比旋光度变化所惹起的。在溶液中α-D-葡萄糖可转变为开链式构造,再由开链构造转变为β-D-葡萄糖;同样β-D-葡萄糖也变转变为开链式构造,再转变为α-D-葡萄糖。经过一段时间后,三种异构体到达均衡,构成一个互变异构均衡体系,其比旋光度亦不再改动。
The variation of specific rotation of glucose is caused by the change of specific rotation in the equilibrium system formed by open-chain structure and ring structure. In solution, alpha-D-glucose can be converted to open-chain structure, and then from open-chain structure to beta-D-glucose. Similarly, beta-D-glucose can also be converted to open-chain structure and then to alpha-D-glucose. After a period of time, the three isomers reach equilibrium, forming a tautomeric equilibrium system, and the specific rotation does not change.
不只葡萄有变旋现象,凡能构成环状构造的单糖,都会产生变旋现象。
It is not only the grape that has the phenomenon of rotation, but also the monosaccharide that can form the ring structure.
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