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.

Substances belonging to this group of organic compounds are wi,lely distributed in Nature, being found in plants, both higher and lower fungi and, upto the present time, in one animal source, sheep wool-fat. For a long time no real differentiation was possible between the tetra- and penta cyclic triterpenes and the sterols. Then the two latter became disting uishable by their selenium. ·dehydrogenation products, i. e. , picene and naphthalene derivatives from the pentacyclic triterpenes and DIELS' hydrocarbon from the sterols. It is now apparent that those compounds yielding predominantly I: 2: 8-trimethylphenanthrene on dehydro genation represent yet another class, and this property is regarded as typical of the tetracyclic triterpenes. The group contains both C and C 30 3l compounds and, although the latter fall outside RUZICKA'S strict definition of triterpenes (I42), it seems desirable to permit this deviation. Members with thirty-two carbon atoms may well be discovered in due course. In a most· valuable account of the triterpenes written in I949, JEGER. (II3) was able to summarize all that was known about the tetra cyclic group in a very small compass. Most of the work discussed in the present article has been published during the last five years and in that time the structures of some twenty compounds have been elucidated. Of outstanding importance is the revelation of close structural relation ships tQ the steroids, and the presence of C and C side-chains, skeletally 8 9 identical with those of cholesterol and ergosterol.