Рентгеноструктурный анализ

Естествознание пользуется двумя главными способами для изучения строения атомов и молекул. Эти способы — химия и оптика в широком смысле слова, т. е. изучение взаимодействия вещества со светом во всем диапазоне длин волн — от рентгеновских лучей до радиоволн. Химия расшифровывает первичную структуру биополимеров, структуру функциональных центров белковых глобул и т. д. Однако химия как таковая не может установить пространственное строение белка или нуклеиновой кислоты.

Рентгенография дает прямую информацию о расположении атомов в молекулах и кристаллах. Рентгеновские лучи, т. е. электромагнитные волны с длиной порядка 0,1 нм, рассеиваются на электронных оболочках атомов. Интерференция волн, рассеянных веществом, приводит к возникновению дифракционной картины. При рассеянии на кристалле можно рассматривать дифракцию как отражение рентгеновских лучей плоскостями кристаллической решетки. Дифракция наблюдается, если рассеянные волны находятся в фазе, т. е. разность хода равна целому числу волн.

UNIT 32 Nuclear Magnetic Resonance

Although X-ray diffraction is a highly useful technique for studying the structure of biological molecules, it does suffer from the disadvantage that the molecules have to be investigated in an environment that contrasts with the one in which they normally operate; the diffraction method requires crystals whereas the individual molecules normally function in aqueous or membranous surroundings. Moreover, molecules such as proteins are dynamic structures whose function depends to a considerable extent on the flexibility of their outer regions. It is true that the individual atoms in a protein, for example, are not prevented from vibrating just because that protein is part of a crystalline array, but the vibrational excursions will not be the same as those observed when the molecule is floating in solution. The technique of nuclear magnetic resonance permits study of biological molecules in their natural environments, and in this respect it can be said to complement the X-ray method.

If one gently gives a sideways tap to a freely suspended compass needle, it will oscillate about its original quiescent position of alignment in the earth's magnetic field. And the oscillations will gradually decrease in amplitude because of frictional forces. Ultimately, the needle will once again be stationary. It is well known that there are magnetic effects associated with some processes at the atomic level. The motions of electrons in incomplete electronic shells of an atom give rise to such effects, for example, and it is also found that atomic nuclei can have magnetic moments. These too will be aligned when subjected to an external magnetic field, and they can be perturbed by an applied force, just as in the case of the tapped compass needle. Indeed, although atomic nuclei are not subject to the sort of mechanical friction we are familiar with in the macroscopic domain, their oscillations will nevertheless gradually decay if there are retarding forces. Such forces are in fact present because the nuclear moments display coupling to those associated with the electrons, and indeed to other nuclei. The usefulness of the nuclear magnetic resonance technique stems from the dependence of this coupling to the surrounding environment, the energy absorption characteristics being exquisitely sensitive to the local magnetic neighbourhood.