ISOMERISM OF BIOLOGICAL COMPOUNDS 



An examination of the alanine, CHg.CHNHg.COOH, of 



bacterial protoplasm will show that it consists almost entirely 



of the laevo- form, whereas the alanine synthesised in the 



laboratory will consist of equal parts of laevo- and dextro- 



forms. If the organism is given synthetic alanine as a source 



of nutrient, it will utilise the laevo- form, while leaving the 



dextro-alsiiome almost untouched, although it has been shown 



recently that very small amounts of D-alanine are taken up 



by the growing cells. If we hydrolyse the proteins of bacteria 



we find that they are composed almost entirely of the laevo- 



forms of the various amino-acids, although in some cases, as 



in the antibacterial peptides excreted by certain Bacilli, a 



proportion of the constituent amino-acids are found to be of 



the dextro- form. Some twenty odd amino-acids have been 



isolated and proteins consist of these amino-acids condensed in 



various permutations and combinations into peptide chains. 



Proteins differ in the order and sequence of the amino-acids in 



the chain, but so far the synthesis of any single protein has 



eluded the chemist in his laboratory. 



Further complications of positional isomerism are met in 



structures of the nature of coenzyme I, adenine-nicotinamide- 



dinucleotide, which plays a part as a carrier of hydrogen in 



cellular respiratory processes. Analysis of hydrolytic and 



enzymatic degradation products of this substance leads to a 



structural formula: 



H 

 N = C-N 



! II 



H ^/°\ /^ C = C-C-NH2 



C^u n^C OH OH i ii 



c— c CH2OP-0-P-OCH2 ,0^ / r. 

 II II -^c-^ ^/ " 



H \^cf H 

 I I 

 OH OH 



I I 



H H 



CONH; 



but synthesis in vitro is made difficult by the facts that the 

 whole molecule is essential for biological activity and that 

 any alteration in the nature or position of the linkages around 



