increases rapidly in a cooperative manner 

 until saturation. The key to the understanding 

 of the interaction resides in the properties of 

 nucleosides in solution of moderate concentra- 

 tion as detailed in the section of the monomer- 

 monomer interaction. From these studies, we 

 know that the stacking of adenosine occurs when 

 the concentration increases. These stacks be- 

 have like the oligonucleotides, and therefore have 

 much greater affinity to poly U than the free 

 adenosine. At moderate concentrations, these 

 associated stacks may serve as initiators for 

 the subsequent binding of the adenosine mole- 

 cule to poly U by a cooperative mechanism. 

 In fact, the stability of most completely inter- 

 acting complexes measured in our experiments 

 is comparable to that obtainable for the poly U 

 -trimer or tetramer (oligonucleotides) inter- 

 action. The forces responsible for stacking 

 energy are short ranged. Calculation based on 

 consideration of the nearest neighbor only gave 

 an estimation of approximately 5 or 6 Kcal/mole 

 as the free energy of stacking for this poly 

 U-AR system. The results clearly indicated that 

 hydrogen bonding cannot be the sole force re- 

 sponsible for the binding, since in dilute solution 

 no binding is detected, even though hydrogen 

 bonding capacity is still present. On the other 

 hand, hydrophobic stacking forces alone do not 

 allow the interaction to occur. Inosine, methy- 

 lated adenosines and other adenine analogs 

 probably all form stacks, yet they fail to bind 

 to poly U. It appears, therefore, the hydrogen 

 bonding and the hydrophobic stacking forces 

 are both essential, with the former related to 

 specificity and the latter related to stability. 



Recently we have extended our investiga- 

 tions to the system of poly U-AMP Interaction 

 as well as to poly U-ATP and poly C-GTP 

 interaction (10). In all these cases, the polymer 

 and the monomer form an insoluljle and stoi- 

 chiometric complex in the presence of mag- 

 nesium. We hope that the knowledge gained in 

 this research will not only tell us about the 

 physical chemical forces responsible for the 

 structure of nucleic acids, but that it may 

 also give us some idea about the mechanism 

 of replication of nucleic acids in the polymerase 

 system. 



In conclusion, we have applied thermo- 

 dynamic and spectroscopic methods to study 

 the properties of monomers in solution. This 

 research gives us knowledge about the exten- 

 sive stacking interaction of the bases of nucleic 

 acid in aqueous solution. Subsequently, we have 

 applied this knowledge to the study of polymer- 



monomer interactions. Through this study, we 

 have obtained certain important parameters 

 and basic understanding about the forces re- 

 sponsible for the secondary structure of nucleic 

 acids. Hopefully, the knowledge about these 

 forces may also lead us to understand the 

 mechanism by which nucleic acid replicates 

 itself. Now, it appears that our chemical ap- 

 proach has reached the stage which is very 

 close to being interesting to the biochemists, 

 and perhaps even of interest to the developmental 

 biologists. 



So far, attention has been focused on the 

 interactions of nucleic acids with themselves. 

 Our laboratory is also starting to investigate 

 the interactions between nucleic acids and pro- 

 teins. Undoubtedly, research on this interaction 

 will be of great importance in molecular biol- 

 ogy and developmental biology. Interactions of 

 purine with amino acids have already been 



o 



X 



^ — .!^yr:.» — ■ 



10 



20 



30 



T°C 



Fig. 6. 



The melting of poly U-AR complex in 0.4 M NaCl, HMP 

 measured by rotation at 350 mu. The poly U concentra- 

 tion is constant at 1.5 x 10"^ M . The parentheses indi- 

 cate the input AR per UMP of poly U(A/U). (From Huang 

 and Ts'o, /. Wo/. Biol. 16, 523, 1966; reproduced with 

 permission of Academic Press). 



193 



