Poly A. 2poly

The extra hydrogen-bonding sites can, however, also form further base pairs, as observed for poly(A) poly(U). In the Watson-Crick A-U pair, one of the adenine N(6)-H donors and the N(7) acceptor are still free and can form another A-U pair of the Hoogsteen type (Fig. 16.16). This is in fact observed if the Watson-Crick duplex poly(A) poly(U) is exposed to high salt conditions, it disproportionates to form a triplex poly(A)-2poly(U) and a single strand poly(A) [539] ... 

Triplexes are usually less stable than related double-strand structures because of the stronger electrostatic repulsion of the third chain. However, certain metal cations at appropriate concentrations can favor the formation of such structures by neutralization of the repulsive phosphate negative charges by the metal cations. For example, atNa+ concentrations about or less than 0.2 M, duplex poly(A)-poly(U) is predominant, but at higher concentrations of monovalent cations or in the presence of certain divalent metal cations (e.g. Mg + and Mn + at concentrations > 10 mM), the duplex disproportionates to form poly(A)-2poly(U) triplex and a free poly (A) strand. 

In an attempt to estimate the energetics and kinetics of nonequilibrium transformations in metastable biopolymers the physical chemical behavior of a model system exhibiting pronounced hysteresis loops was investigated.The model hysteresis to be briefly discussed results from the acid-base titration of the polyelectrolyte complex poly(A) 2poly(U). The overall process underlying the hysteresis loop is the cyclic transition between two helical structures the triple helix poly(A) -2poly(U) and the protonated double helix poly(A) poly(A). 

Fig. 16.16. The U-A-U base triple observed in poly(U) 2poly(A) is composed of a Watson-Crick AU62 and a Hoogsteen base pair AU32. A corresponding C-G-C triple is not observed. It would only be possible if the C of the Hoogsteen base pair were protonated...

Hoffmann,S.,Witkowski,W.,Ruttinger,H.-H.,"Hystere-tic Metastabilities in the 2Poly(I) Poly(A)-Tri-plex" Conf.Proc.IUPAC 4th Disc.Conf."Heterogenities in Polymers ,1974,Marianske Lazne,Czechoslovakia... 

The adenosine dimer ApA forms an ApA 2poly(U) triple-helical complex in the presence of 1 M NaCl [95, 96]. We performed mixing experiments to investigate the stoichiometry of the interaction between Ap A and poly(U). The mixing curves for Ap A 7a or 7b and poly(U) in the presence of 1.0 M NaCl revealed a break at Ap A poly(U) = 1 2, indicating the formation of an Ap A 2poly(U) triple-helical complex. We then measured the values of the dimer 2poly(U) triple-helical... 

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