Distribution multiply labeled

The pathway in Scheme 4 was tested33 by feeding to the cactus the amide (42) multiply labelled with 14C at both C(8) and the carbonyl group. The alkaloid (43) was isolated in radioactive form but the distribution of the label ruled out an intact incorporation. Thus, the amide (42) is not an intermediate. 

Each of these considerations has been discussed In detail In a variety of reviews (10-16) and those discussions will not be repeated here. Instead, we should like to focus attention on two important but not generally appreciated, limitations on the methodology of evaluating the population of at a particular site using the relative Intensities of singlet to multiplet resonances of multiply labeled molecules which are usually encountered In metabolic studies. These limitations arise from the non-statlstical distribution of labels that can result from the operation of certain metabolic pathways In biological systems and from "special" relaxation effects that can complicate intensity/population relationships In multiply labeled molecules. We stress this subject because as the following discussion will illustrate, the proper treatment of these relative Intensities can provide information about metabolic pathways unobtainable with any other method. 

The availability of mass spectrometrie techniques (such as electrospray) for observing molecular ions from intact enzymes is an important advance in the study of the covalent chemistry of proteins. Although the use of radioactively labelled molecules has previously allowed the average level of protein modification to be deduced, mass spectrometry is much quicker, more convenient and safer it also shows the distribution between the various multiply modified species and, most importantly, gives the molecular weight of the modified species, allowing deductions to be drawn as to the nature of the reaction occurring. 

The hrst multiplier in the right-hand side of Equation 5.10 expresses the number of ways indistinguishable molecules can be distributed among M labeled sites the partition function for a single adsorbed molecule is g = where q qy, and q are one-dimensional harmonic-oscillator... 

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