Leucine zippers constitute a widely observed structural motif which serves to promote both homo- and heterodimerization in a number of DNA-binding proteins. As part of our ongoing efforts to characterize both the structure and the dynamical properties of this dimerization domain as they relate to biological function, we report here the secondary structure in solution of a recombinant dimeric peptide (rJunLZ) comprising residues Arg276-Asn314 of the leucine zipper domain of c-Jun. Two- and three-dimensional homo- and heteronuclear NMR experiments have allowed definition of the secondary structure of rJunLZ and have provided a total of approximately 1500 interproton distance and 62 phi dihedral angle constraints for tertiary structure calculations. Amide proton protection factors, calculated from hydrogen-deuterium exchange experiments, have identified 62 hydrogen bonds in the rJunLZ dimer. We have also examined the role of Asn22, the only polar residue situated at the hydrophobic dimer interface. Virtually all leucine zipper sequences contain such a polar residue (usually Asn) near the center of the motif. X-ray crystallographic studies showed that, in the case of the GCN4 homodimer, the polar residue (Asn) adopts an asymmetric conformation in an otherwise essentially symmetric structure. In contrast, all NMR studies of leucine zipper homodimers to date have suggested that the dimers are completely symmetric in solution. We present evidence that the side-chain amide protons of Asn22 are hydrogen-bonded in solution and that this side chain exchanges rapidly between two distinct conformations. On the basis of these observations, we propose a dynamic model which can explain the apparent differences in symmetry observed in NMR and X-ray crystallographic studies of leucine zipper homodimers. We show that mutation of Asn22 to a hydrophobic Leu residue markedly increases the thermal stability of the rJunLZ homodimer, consistent with a destabilizing role for this residue. However, at temperatures below 30 degrees C, the Asn22-->Leu mutant rearranges to form oligomers larger than the dimer, as was previously observed for the corresponding Asn-->Val mutation in the GCN4 leucine zipper. These results are consistent with the hypothesis that the polar Asn residue commonly observed at the interface of leucine zippers imposes specificity for the dimer structure at the expense of stability [Harbury, P.B., Zhang, T., Kim, P.S., & Alber, T. (1993) Science 262, 1401-1407].