A-Z-RNA CONFORMATIONAL-CHANGES EFFECTED BY HIGH-PRESSURE

This paper reports evidence obtained by circular dichroism (CD) spectr oscopy measurements indicating that two oligoribonucleotide duplexes w ith the alternating purine-pyrimidine sequences r(GC)6 or r(AU)6 chang e their A-RNA conformation under high pressure. Under the high-pressur e conditions at which B-Z-DNA transition easily occurs, RNA acquires a conformation which only differs slightly from that of A-RNA. However, exposure of r(GC)6 or r(AU)6 to high pressure (6 kbar) in the presenc e of 5 m NaCl causes a conformation change of both oligoribonucleotide duplexes from their A- to their Z-RNA form. The departure of RNA or D NA duplexes from their original conformations under high pressure depe nds on the water structure itself and involves displacing an active (s tructural) water molecule outside the nucleic acid molecules. Experime nts carried out until now in many laboratories have shown that B-Z or A-Z transitions of DNA or RNA, respectively, do not depend on the cond itions applied, but the common mechanism for these processes seems to be dehydration. This same effect can be observed either at high salt c oncentrations or in the presence of an alcohol or at high pressure. Ou r results also support the view that the higher stability of RNA compa red with DNA duplexes is due to the strong interaction of the 2'-hydro xyl groups of RNA with water molecules.