Physical organic methods in carotenoid research.

The ever increasing importance of physical methods in the elucidation of the structure and configuration of carotenoids is reviewed. The use of solvent partition, chromatography, X-ray crystallography, infra-red spectroscopy and optical rotary dispersion are briefly discussed. Special emphasis is given to the application of absorption spectroscopy and to 220 MHz proton magnetic resonance spectroscopy of carotenoids. Mass spectroscopic studies of the characteristic fragmentation of the carotenes, and a possible mechanism for the fragmentation of the polyene chain, are discussed in detail. * * * Physical organic methods have exerted an extremely stimulating influence on carotenoid research. Prior to the routine application of these methods polyene chemistry has been very troublesome in comparison with today's efforts in this field. About twenty years ago, the main tool for routine analysis of, for instance, chromatographic fractions was the Carr-Price reaction: in chloroform solution an unsaturated compound when mixed with antimony trichioride, trichloroacetic or sulphuric acid gives a very pronounced bathochromic shift. Vitamin A is the most prominent example because here the Carr-Price reaction is still in use as an analytical method. In the colourless chloroform solution a transient deep blue colour arises. The draw-back of the Carr-Price reaction was that, lacking spectrophotometric apparatus, one had to guess the colour of an intermediate polyene and one usually guessed wrong. It might even be said, that if the intermediates did not crystallize and thus could not be sent for C.H.-analysis, the proof of their structure was more or less the identity of the final product with the known natural material. This caused time consuming arduous syntheses but, in retrospect, it also increases our admiration for these early efforts. A possible mechanism for the reaction has been discussed: the mesomeric allylic carbonium ions shown in Figure 1 for vitamin A and anhydro-vitamin A are considered responsible for the bathochromic shift'. Since the protonation of lycopene has been studied2, one might argue that, in analogy, the colour reaction of the carotenes can be explained as shown in Figure 2. The approximate position of the absorption maximum of the Carr-Price reaction can be calculated: Amax 3O05+655Xn where n represents the number of effective double bonds (i.e. 5 for vitamin A, 10 for /3-carotene)3. This equation has been found to give good values 365 PAC—B U. SCHWIETER, G. ENGLERT, N. RIGASSI and W. VETTER