H H_{ 2} | ¦ C C // \ / \ HOC C CH_{ 2} | ¦ | C C N-CH_{ 3} / \\/ \ / / C CH O | | \ HC CH_{ 2} \ / \ / HC C | ¦ H_{ 2}C CH \ / C ¦ H_{ 2}
=Codeine=, C_{ 17}H_{ 18}(OCH_{ 3})NO_{ 2}, which is also found in opium, is a methyl derivative of morphine. =Papaverine=, =laudanosine=, =narcotine=, and =narceine= are four other alkaloids found in opium. They each contain an isoquinoline nucleus, combined by one bond to a benzene ring, with one or more methyl groups and three or more methoxy (OCH_{ 3}) groups attached at various points around the three characteristic rings. The following formula for laudanosine will illustrate their structure:
/ \ / \ CH_{ 3}O | | | | | OCH_{ 3} CH_{ 3}O | N-CH_{ 3} / \ \ / \ / | |OCH_{ 3} | | | | \ / H_{ 2}C----------------CH
The above discussions of the composition of typical alkaloids clearly indicate the extreme complexity of their molecular structure. It is generally supposed that they are formed by the decomposition of proteins. But they are developed in only a few particular species of plants and are always present in these plants in fairly constant quantities. Hence, it appears that, in these species, the production of alkaloids is in some way definitely connected with protein metabolism; but it is certain that this is not a common relationship, as it is manifested by such a limited number of species of plants, and there is absolutely no knowledge as to its character and functions. Some authorities prefer to regard the alkaloids as waste-products of protein metabolism; but here, again, it is difficult to understand why such products should result in certain species of plants and not in others.
This is a group of compounds, widely distributed in both plant and animal tissues, all of which are derivatives of the compound known as _purine_, C_{ 5}H_{ 4}N_{ 4}. All of the naturally occurring compounds of this group may be regarded as derived from purine, either by the addition of oxygen atoms, or by the replacing of one or more of its hydrogen atoms with a methyl (CH_{ 3}) group or an amino (NH_{ 2}) group. The following structural formula represents the arrangement of the purine nucleus, the numbers being used to designate the nitrogen or carbon atoms to which the additional atoms, or groups, are attached in the more complex compounds of the group. In purine itself, the four hydrogen atoms are attached in the 2, 6, 7, and 8 positions.