Among these end-products, or amino-acids, are leucin, tyrosin, aspartic acid, glutaminic acid, glycocoll, arginin, lysin, histidin, and likewise the peculiar aromatic body tryptophan. The chemical make-up of these substances may be indicated by the following structural formulæ, which, if even only partially understood, will suggest to the non-chemical mind some idea of close chemical relationship: CH(NH_{2})COOH / CH(NH_{2})COOH CH_{2} | \ CH_{2}-COOH CH_{2}-COOH Glutaminic acid Aspartic acid CH_{3} CH_{2}-NH_{2} \ | CH-CH_{2}-CH(NH_{2})-COOH COOH / CH_{3} Glycocoll Leucin OH / C_{6}H_{4} \ CH_{2}-CH(NH_{2})COOH Tyrosin C·CH_{2}·CH·(NH_{2})·COOH / \\ C_{6}H_{4} CH \ / NH Tryptophan CH_{2}-NH CH_{2}-NH_{2} CH--N | \ | || \\ CH_{2} C--NH_{2} CH_{2} || CHµ | // | || / CH_{2} NH CH_{2} C--NH | | | CH·NH_{2} CH_{2} CH_{2} | | | COOH CH-NH_{2} CH-NH_{2} | | COOH COOH Arginin Lysin Histidin In these various decomposition products there is apparent certain definite lines of resemblance, on which is based one or more suggestions regarding possible ways in which these chemical groups are linked, or bound together, in the proteid molecule. Thus, there is apparently present a complex or nucleus which may be indicated as | | HC-NH-CO- also HC-NH-C(NH)- | |