手性分析

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手性what is chirality?
我们知道，生命是由碳元素组成的，碳原子在形成有机分子的时候，4个原子或基团可以通过4根共价键形成三维的空间结构。由于相连的原子或基团不同，它会形成两种分子结构。这两种分子拥有完全一样的物理、化学性质。比如它们的沸点一样，溶解度和光谱也一样。但是从分子的组成形状来看，它们依然是两种分子。这种情形像是镜子里和镜子外的物体那样，看上去互为对应。由于是三维结构，它们不管怎样旋转都不会重合，如果你注意观察过你的手，你会发现你的左手和右手看起来似乎一模一样，但无论你怎样放，它们在空间上却无法完全重合。如果你把你的左手放在镜子前面，你会发现你的右手才真正与你的左手在镜中的像是完全一样的，你的右手与左手在镜中的像可以完全重叠在一起。实际上，你的右手正是你的左手在镜中的像，反之矣然。所以又叫手性分子。
在化学中，这种现象被称之为“手性”(chirality)，有时也称为“手性对称”或“镜面对称”。几乎所有的生物大分子都是手性的。两种在分子结构上呈手性的物质，它们的化学性质完全相同，唯一的区别就是：在微观上它们的分子结构呈手性，在宏观上它们的结晶体也呈手性。
手性分子——“反应停”
反应停是一种谷氨酸生物，英文名Thalidomide，又名沙立度胶、酞胺哌啶酮、酞咪哌啶酮、酞谷酰亚胺、K-17等。为白色结晶性粉末，无臭无味，分子式C13H10N2O4，分子量258。
什么叫叫手性分子？
生命最基本的东西氨基酸的手性之分：
作为生命的基本结构单元，氨基酸也有手性之分。也就是说，生命最基本的东西也有左右之分。
惊人的发现---组成地球生命体的几乎都是左旋氨基酸，而没有右旋氨基酸。
我们已经发现的氨基酸有20多个种类，除了最简单的甘氨酸以外，其它氨基酸都有另一种手性对映体！那么，是不是所有的氨基酸都是手性的呢？答案是肯定的，检验手性的最好方法就是，让一束偏振光通过它，使偏振光发生左旋的是左旋氨基酸，反之则是右旋氨基酸。通过这种方法的检验，人们发现了一个令人震惊的事实，那就是除了少数动物或昆虫的特定器官内含有少量的右旋氨基酸之外，组成地球生命体的几乎都是左旋氨基酸，而没有右旋氨基酸！
右旋分子是人体生命的克星!
因为人是由左旋氨基酸组成的生命体，它不能很好地代谢右旋分子，所以食用含有右旋分子的药物就会成为负担，甚至造成对生命体的损害。
在手性药物未被人们认识以前，欧洲一些医生曾给孕妇服用没有经过拆分的消旋体药物作为镇痛药或止咳药，很多孕妇服用后，生出了无头或缺腿的先天畸形儿，有的胎儿没有胳膊，手长在肩膀上，模样非常恐怖。仅仅4年时间，世界范围内诞生了1.2万多名畸形的“海豹婴儿”。这就是被称为“反应停”的惨剧。后来经过研究发现，反应停的R体有镇静作用，但是S-对映体对胚胎有很强的致畸作用。
正是有了60年代的这个教训，所以现在的药物在研制成功后，都要经过严格的生物活性和毒性试验，以避免其中所含的另一种手性分子对人体的危害。
在化学合成中，这两种分子出现的比例是相等的，所以对于医药公司来说，他们每生产一公斤药物，还要费尽周折，把另一半分离出来。如果无法为它们找到使用价值的话，它们就只能是废物。在环境保护法规日益严厉的时代，这些废品也不能被随意处置，考虑到可能对公众健康产生的危害，这些工业垃圾的处理也是一笔不小的开支。
因此，医药公司急切地寻找一种方法来解决这个问题，比如，他想要左旋分子，那么他就得想办法把另一半右旋分子转化成左旋分子。现在，这个令人头痛的问题已经得到了解决。科学家用一种叫做“不对称催化合成”的方法解决了这一问题。这个方法可以广泛地应用于制药、香精和甜味剂等化学行业，给工业生产一下子带来了巨大的好处，这项研究也获得了2001年度的诺贝尔化学奖。毫无疑问，这个成果具有重要意义。
看来，手性真是一种奇妙的东西！手性的氨基酸甚至决定着我们这个世界存在的方式！对手性的研究，在造就工业奇迹的同时，也启发了我们对地球生命，甚至宇宙起源的重新认识。
我们知道，在自然界的各个方面，尤其是物理和化学中，都广泛地存在着许多对称的概念：带负电的电子与带正电的反电子，磁场的南极和北极，以及化学中的分解和合成反应。就连遥远的河外星系也存在着正旋和逆旋的旋涡结构。科学家们不禁感到疑惑：这是否在提示我们在宇宙中存在着一种奇特的普适性的对称规律？
地球上没有右旋氨基酸生命，但是，按照手性的原则，它们确实是可能存在的，甚至，有智慧的右旋氨基酸生命也是存在的。
如果真是这样的话，他们又有些什么跟我们不一样的特性呢？左旋氨基酸组成的我们习惯于使用右手，右旋氨基酸组成的他们是不是习惯于使用左手呢？他们的思维是不是会采取另一种与我们相对映的方式呢？这种对映的规则又是什么呢？
假如有一天你遇见了外星人，为了表示作为地球人的友好态度，你打算请他回家喝一杯，在干杯之前，你一定要记得问问他们的氨基酸是右手性还是左手性的。如果是左手性的那就可以快乐的干杯。如果是右手性的，你可就得费点事——麻烦你去拿瓶右手性的啤酒！

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What is chirality?
Imagine that you had a left hand transplanted in the place of your right hand. Now try and shake hands with a friend. You're in trouble! Whichever way the left hand was transplanted, your fingers would seem to be inverted. The moral is your left hand and your right hand are different. On the other hand, your left hand and your right foot are also different. However, you may feel there exists a relationship between your left hand and your right hand that doesn't exist between your left hand and your right foot. Here's a hint. Put your left hand up to a mirror. The image that appears in the mirror is a right hand. The answer: left and right hands are mirror images.
Not all objects behave like a pair of hands. The common water glass, for example, is identical to its image in the mirror. The underlying issue is the identity of an object and its reflection. Any object that is different from its reflection is said to be chiral. Otherwise, it is called achiral. Your left hand and your right hand are chiral, whereas the water glass is achiral. This geometric phenomenon is called chirality in the physical sciences. Basically, chirality is "handedness," that is the existence of left/right opposition. Even if you had never heard this word in your life, you do know the phenomenon. Otherwise, you would not be able to distinguish left from right.
Chirality in life
Thalidomide is a sedative drug that was prescribed to pregnant women, from 1957 into the early 60's. It was present in at least 46 countries under different brand names. "When taken during the first trimester of pregnancy, Thalidomide prevented the proper growth of the foetus, resulting in horrific birth defects in thousands of children around the world" [1]. Why? The Thalidomide molecule is chiral. There are left and right-handed Thalidomides, just as there are left and right hands. The drug that was marketed was a 50/50 mixture. One of the molecules, say the left one, was a sedative, whereas the right one was found later to cause foetal abnormalities. "The tragedy is claimed to have been entirely avoidable had the physiological properties of the individual thalidomide [molecules] been tested prior to commercialization [2]."
Aspartame is a sweetening agent that is more than a hundred times sweeter than sucrose. And yet, the mirror image molecule is bitter. "(S)-carvone possesses the odor perception of caraway while [the mirror image molecule] (R)-carvone has a spearmint odor [2]."
These examples are just the tip of the iceberg. DNA, proteins, amino acids, sugars are all chiral. Mirror image amino acids are called L- and D-aminoacids. Human proteins are exclusively built from L-aminoacids. The origin of this fundamental dissymmetry is still mysterious. When interacting, molecules recognize each other just as your right hand distinguishes another right hand from a left when you shake hands. This is why mirror image molecules, like mirror image Thalidomides, so often have radically different fates in our bodies.
Drug synthesis is an enormous worldwide market. As a consequence, issues related to chirality have gradually pervaded chemical research. This background is to be kept in mind when appreciating the importance of chirality, whether in science or in everyday life.
To whom is this site proposed?
You could simply be curious about symmetry issues. No background in science is needed to browse through the "beginners" section (still to be written!). This site was tentatively conceived to provide you information, hopefully without boring you with scientific jargon. If you feel that this site is a failure in that respect, you are encouraged to complain here.
Besides, research results will also be presented. Chirality is an old theme in many branches of chemistry and biology, with overwhelming practical importance, and physicists have long paid interest in symmetry matters. As a result, there is a wealth of published articles dealing with chirality in the physical sciences. However, this site is primarily focused on recent theoretical progress in a quite unexpected direction: the description of chirality. What we mean by describing chirality is presented in the research results section. Some scientific background is required in this part of the site.
Have a look at the site map for more details.

By the way...... please pronounce "ky-rality" with a "k", like chiropractor. The greek name kheir means "hand." So chirality indeed means "handedness." Following Louis Pasteur's discoveries, the name was coined by the Irish physicist William Thomson, alias Lord Kelvin.

[ 本帖最后由 aa_tang 于 2008-10-4 15:28 编辑 ]