66 



The Structure of Protoplasm 



much more exacting than others in the type of residue or residue 

 combination to which they can become attached. For example, 

 small, uncharged organic molecules, such as acetaldehyde, ethyl 

 alcohol, etc., may require only a single polar residue; whereas 

 small ionized monocarboxylic acids, such as formic and acetic, may 

 require either a hydrogen-donating residue or a basically ionized 

 group; while others, such as hexose-diphosphate and the dicar- 



•^♦Vi 



Fig. 13. Model photograph showing dimensional relationship between ad- 

 jacent amino acid residues on the protein and the possible points of attach- 

 ment on the phospholipid. 



boxylic acid, succinic, may require two adjacent residues for attach- 

 ment and proper orientation for reaction (Fig. 14) . In some of the 

 nucleotides only the ionized phosphoric acid group may attach to 

 the protein (106) ; while nucleic acid may require four adjacent, 

 positively ionized residues, such as those of arginine, for attach- 

 ment to its four negatively charged phosphoric acid groups. Evidence 

 (107) is accumulating which indicates that diphosphothiamine may 

 require two points of attachment, i. e., its phosphoric acid group and 

 the amino group of its pyrimidine ring. A somewhat similar attach- 

 ment was suggested for lactoflavinphosphate (108, 109) . 



Degrees of specificity seem still more evident when it is noted 

 that in certain instances where two adjacent residues are required, 

 any two of seven different polar endings may be satisfactory; while 

 in other cases a more restricted combination is necessary. The 

 relative lengths of the adjacent residues may further restrict or 



