Alternating d(GA.TC)n sequences are highly structurally polymorphic. Most of 
their conformational flexibility is likely to reside in the structural 
properties of the individual strands themselves. In this paper the 
conformational behaviour of the d(GA)20 and d(TC)20 oligonucleotides was 
analysed. Formation of d(GA)20 intramolecular duplexes is observed at any pH 
value, from 8.3 to 4.6. On the other hand, intramolecular d(TC)20 duplexes are 
formed only under acidic conditions. The acid d(TC)20 intramolecular duplex is 
likely to be stabilized through the formation of C+C pairs, the thymine residues 
remaining unpaired. The d(GA)20 oligonucleotide also forms intermolecular 
duplexes which coexist with the intramolecular forms at any pH, from 8.3 to 4.6. 
The structural conformation adopted by the d(TC)20 oligonucleotide at neutral pH 
is uncertain. Under these conditions, this oligonucleotide shows an 
electrophoretic apparent molecular weight consistent with the formation of a 
bimolecular complex. However, no hydrogen bonding was observed to occur under 
these conditions. Implications of these results for an understanding of the 
molecular principles behind the conformational flexibility of alternating 
d(GA.TC)n sequences are discussed. The possible biological significance of these 
results is also discussed.