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The properties of proteins are determined not only by the sequence of amino-acid residues in the polypeptide chains, but also by the con figuration of the chains-the way in which the chains are coiled or folded. It is probable that denaturation, the loss of some of the specific properties of a native protein, may in many cases be the result simply of a change in configuration of the polypeptide chains, without any change whatever in the sequence of amino-acid residues. During the past few years great progress has been made in the attack on the determination of the sequence of amino-acid residues in the poly peptide chains of proteins, through the work of SANGER and his collabora tors (Io9, IIO) and of other investigators. There has also been significant progress in the attack on the problem of the configuration of polypeptide chains, largely through the application of the X-ray diffraction technique.
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During the past three decades the organic chemist has become in creasingly used to take advantage of more and more complex instrumenta tion and physical measurements in lieu of laborious, time-consuming and often ambiguous chemical transformations. Mass spectrometry is perhaps the most recent, most complex and most expensive addition to this field. In view of the astonishingly quick acceptance of nuclear magnetic reso nance by the organic chemist it is, in retrospect, surprising that he has neglected mass spectrometry for such a long time. This can be explained, in part, by the complexity of the instrumentation and some technical shortcomings of the earlier commercially available instruments but, to an even greater extent, it reflects also the prejudices against a technique that was originally mainly used for quantitative gas analysis. The usefulness of mass spectrometry as a qualitative technique in organic chemistry rather than a tool for quantitative analysis was more and more recognized towards the end of the last decade. A rather spectacular development followed during the intervening few years to the point that now any reasonably well equipped modern organic laboratory is supplied with, or at least has access to, one or more mass spectrometers suitable for work on organic compounds. Within the realm of organic chemistry the technique has become much more important, if not indispensable, for the natural products chemist while its application to synthetic problems is much less pro nounced.
In agreement with SCHLENK (I78) "nucleotide" is here taken to mean a phosphoric ester of an N-glycoside of a heterocyclic base. Although they fall outside the scope of this definition, phosphorylated derivatives of riboflavin are added in recognition of their close relation to the main body of nucleotides. A complete review of even the organic chemistry of nucleotides and their derivatives would now form a substantial monograph. The smaller and less characteristic fragments of the nucleotides, namely the hetero cyclic bases, the component sugars and the derived sugar phosphates, are therefore excluded from this article. * Their chemistry is of longer standing and is covered in standard works. On the other hand knowledge of the "nucleosides", the N-glycosides of heterocyclic bases, has expanded very considerably in recent years and an account of the relevant work is therefore included. With regard to the nucleotides themselves attention is focussed on work published in the last few years leading to improved characterisation, more exact structural knowledge and increased availa biIity. Only such enzymatic experiments as bear direct1y on structural problems are discussed and therefore much fascinating work by KALCKAR, FRIEDKIN et al. and by KORN BERG has been omitted. I t would be unthinkable to present a review on nucleotides without mentioning that the foundations of the subject were laid by P. A. LEVENE. His monograph with BASS (I36) was published in 1931 and two reviews by LYTHGOE (I50, I5I) cover the intervening period until 1944.