MOLEC'TTLAR STRUCTURE IN I'HOTOPLASM 



171 



these substances, protein and water, and at 

 the present time certain characteristics and 

 properties may be said to be well estab- 

 lished. A few of these properties are of 

 direct importance in our protoplasmic pic- 

 ture, and it is to a discnssion of these to 

 wiiicli we now turn in order to interpret 

 the ultramicroscopic particles and other 

 structures which, on account of their 

 nature, do not diffract light. 



The proteins with which we are prima- 

 rily concerned are classed as simple pro- 

 teins; that is, those which, upon liydrolysis, 

 yield amino acids (about 15 to 20 different 

 kinds) almost exclusively. That these are 

 attached to one another through peptide 

 linkages is generally well known and 

 accepted. This type of attachment should 

 produce a chain-like molecule of which the 

 amino acid residues are the links, and all 

 bonds are primary valence. It is often 

 spoken of as a "primary valence chain." 

 The simple proteins are the ones most likely 

 to occur in protoplasm where they may be 

 found with or without attached groups of 

 substances other than amino acids. These 

 attached groups, called prosthetic groups, 

 may include fatty acids, lecithin, nucleic 

 acid, carbohydrates and various other mole- 

 cules, such as those of the resj^iratory pig- 

 ments. 



Although all of the proteins are pro- 

 duced wdthin the living cell, investigations 

 are almost always carried on after extrac- 

 tion from the cells; and our knowledge of 

 them depends to a great extent upon 

 studies of the material which has been re- 

 moved from the living matrix. Much of 

 the knowledge which is useful in our molec- 

 ular studies of protoplasm involves an 

 assumption that the proteins and particles 

 of protein w'hich are studied in this form 

 are directly comparable to those w^hicli 

 exist in the living cell. We have accepted 

 this assumption, tentatively, as a working 

 basis, and as we proceed shall attempt to 

 reduce the uncertainties. 



The conception of a molecular chain as 

 the primary structure of proteins has been 

 confirmed in several ways and may be 

 taken as fairly well established. The X- 

 ray diffraction patterns which are formed 



by silk fibroin show that the component 

 molecules are long extended polypeptide 

 chains. There seems to be very little doubt 

 on the part of the chemist that the amino 

 acid residues are almost exclusively linked 

 to one another through peptide groups in- 

 volving a particular carbon atom of the 

 amino acid, the a carbon atom. These 

 polypeptide chains occur in various pro- 

 teins as slender structures, about 4.5 A 

 thick by about 10.5 A wide, but of unknow^n 

 length. The amount of folding and attach- 

 ment through sidechains into packets is 

 uncertain for the most part. A molecular 

 w^eight of about 85,000 has been ascribed to 

 many proteins, and if the chains were fully 

 extended, they w'ould be about 1000 A long. 

 These molecular dimensions in Angstroms, 

 magnified to a scale equivalent to inches 

 and feet, would enlarge the chain to about 

 one inch in thickness by tw^o inches in 

 width by twenty feet in length. 



AA^ith the conception of a protein chain 

 as a thin, fiber-like molecule in mind, the 

 particles shown by the ultramicroscope and 

 by the ultracentrifuge take on a new sig- 

 nificance, especially when a folding, such 

 as postulated by Pauling and Mirsky 

 (1936) and Astbury (1935), into globular 

 or cubical structures is considered. A 

 thousand Angstrom length of the chain, 

 whether in one piece, folded back and 

 forth, or in several pieces of, say, 50 A in 

 length, whether linked through their side 

 chains or not, could form a packet about 

 30 X 30 X 50 A. A particle similar to this 

 or perhaps slightly larger might be one of 

 those seen as a point of light in the ultra- 

 microscope ; it would be recognized by the 

 ultracentrifuge; and it might readily be 

 one of the minute reflecting packets wdiich 

 help produce the diffraction pattern of X- 

 rays. This picture of the particle must be 

 considered as a first approach and may be 

 somewhat misleading. To present a pic- 

 ture which better represents various ex- 

 perimental facts we must turn to several 

 different lines of investigation. 



Our knowledge of proteins is obtained 

 from widely varying fields of experimental 

 effort; and when information from these 

 sources is brought to bear upon the struc- 



