Lipoids may be subdivided according to different criteria.

I. According to the polar group.

A. Lipoids classified according to the nature of their polar group.

1. Lipo carboxylic acids (-COOH)

2. Lipo thiols (—SH)

3. Lipo sulfonic acids (—SO2H)

4. Lipo amines (—NHS)

5. Lipo amides (—CONH,)

6. Lipo alcohols (-OH)

7. Lipo aldehydes (-CHO)

8. Lipo ketones (=CO)

9. Lipo halogens (— CI, etc.)

10. Lipo metals (—Na, etc.) etc.

B. Lipoids classified according to the predominant element of the polar group.

1. Lipo sulfur compounds a. Lipo thiols (-SH) b. Lipo sulfonic acids (—SO.H) c. Lipo sulfidcs (=S) d. Lipo sul fox ides (—SO) e. Lipo sulfones (=SO2) f. Lipo sulfites ( = SO2) etc.

2. Lipo nitrogen derivatives a. Lipo amines (—NH,) b. Lipo amides (—CONH,) c. Lipo nitriles (— CN) d. Lipo isocyanides (—NC) e. Lipo nitro derivatives (— NO2) etc.

C. Lipoids classified according to the energetic character of their polar group.

A. Lipoids with negative polar groups

1. Lipoacids a. Lipo carboxylic acids b. Lipo thiols c. Lipo sulfonic acids, etc.

2. Lipo aldehydes

B. Lipoids with positive polar groups

3. Lipobases a. Lipo amines b. Lipo guanidines c. Lipo imines, etc.

4. Lipo alcohols

II. According to the nonpolar group.

A. Lipoids classified according to the structure of their hydrocarbon chain.

1. Aliphatic

2. Alicyclic

3. Aromatic

4. Heterocyclic, etc.

B. Lipoids classified according to their carbon bonds.

1. Saturated

2. Unsaturated a. Ethenic (mono-, di-, poly-) b. Ethynic

Some aspects of this classification require discussion.

The generic term lipoacid has been employed to describe simple lipoids having polar groups with acid functions. While the principal lipoacids are the fatty acids, other members have other acid polar groups, such as SO2, SH, NO2, etc. The significance of this grouping together of lipoids with negative polar groups has become evident especially in studying the similarities in the biological effects of these substances. In certain aspects of our research, this correlation has permitted us to substitute one category of lipoids (lipo thiols or lipo aldehydes) for another (lipo carboxylic acids), thereby avoiding certain undesirable effects of the latter group of substances.

Lipoids having polar groups energetically opposite to those of the acids have been grouped together. Of these, the members with a polar group with alkaline functions have been classified as lipobases. The term base is generally applied to ionizable compounds which influence the pH of solutions and combine readily with acids by losing an OH- and gaining a proton. Another group is formed by the lipoidic alcohols. Recent evidence indicates that, in many circumstances, the differences in the reactions of alcohols and common bases are quantitative rather than qualitative. Quite often the reaction of an alcohol with an acid is analogous to the reaction between an acid and sodium hydroxide, the H+ of the acid combining with the OH- of the alcohol. The differences between reactions are considered to be matters of time rate. Whereas the reaction of the base is almost instantaneous, that of alcohol is a slow reaction and is less complete. This behavior of alcoholic substances is particularly clear when the hydroxyl group of an alcohol is replaced by a halogen to prepare the alkyl halides. For instance, according to Karrer (24): "This can be done by the action of the concentrated halogen acids on the alcohol:

CH3OH + HCl → CH3CI + H2O

The reaction corresponds superficially to the formation of a salt from an acid and a base:

NaOH + HCl→ NaCl + H2O

There is, however, a difference between the two processes. Bases and acids are largely dissociated; when they come together, hydrogen ions and hydroxyl ions combine almost at once to give the very little electrostatically dissociated water, so that the reaction which really occurs is:

Na+ + OH- + H+ + Cl- → Na+ + CI- + H2O

Ionic reactions always occur instantaneously. Reaction between alcohol and hydrogen halides is governed by other laws. Alcohol is only very slighdy ionized. For the removal of the hydroxyl group a certain time is required. The reaction between alcohol and acid with elimination of water, known as esterification. is therefore a time reaction." No essential difference exists between the reaction of lipo alcohols or lipo amines, for instance, with organic acids.

These considerations would have been sufficient to allow lipobases and lipo alcohols to be grouped together. There are other considerations as well. Their common biological activity and mutual interchangeability also appear to justify grouping them together. Furthermore, the recognition of the existence of a general mutual antagonism between lipoacids on the one hand and lipobases and lipo alcohols on the other hand, chemically, physically and biologically, has proven of considerable value in explaining a variety of experimentally observed facts in many aspects of our research.

Following this through, we have found it advantageous to define the two groups of lipoids by a more general character, the electrical aspect of the polar part, negative for the lipoacids and lipo aldehydes and positive for the group of lipo alcohols and lipobases. The terms, "positive and negative lipoids," serve also to emphasize the nature of their antagonism.

The structure of the nonpolar group as it confers physical, chemical and biological properties on the lipoids permits further subdivisions. Lipoids may be classified on the basis of the aliphatic, alicyclic, aromatic or heterocyclic character of the nonpolar group. While the negative lipids are principally formed by fatty acids, the positive are made up principally of sterols. The presence or absence of double bonds defining saturated and unsaturated carbon chains has been one subject of our study and considerable biological importance has been found to be related to this character as well as to the positional relationship of the double bond to the polar group and the polarity induced by the double bond.

The study of lipoids has shown that, besides properties contributed by the elements and groups which compose them, they have additional physico chemical and even biological properties which are characteristic. We have termed these "lipoidic properties" to indicate that they are considered to result directly from the particular constitution of the lipoids.