The immunoglobulin-binding domain B1 of streptococcal protein G (GB1), a very stable, small, single-domain protein, is one of the most extensively used models in the area of protein folding and design. Variants derived from a library of randomized hydrophobic core residues previously revealed alternative folds, namely a completely intertwined tetramer (Frank et al., Nat Struct Biol 2002;9:877-885) and a domain-swapped dimer (Byeon et al., J Mol Biol 2003;333:141-152). Here, we report the NMR structure of the single amino acid mutant Ala-34-Phe which exists as side-by-side dimer. The dimer dissociation constant is 27 +/- 4 microM. The dimer interface comprises two structural elements: First, the beta-sheets of the two monomers pair in an antiparallel arrangement, thereby forming an eight-stranded beta-sheet. Second, the alpha-helix is shortened, ending in a loop that engages in intermolecular contacts. The largest difference between the monomer unit in the A34F dimer and the monomeric wild-type GB1 is the dissolution of the C-terminal half of the alpha-helix associated with a pronounced slow conformational motion of the interface loop. This involves a large movement of the Tyr-33 side chain that swings out from the monomer to engage in dimer contacts. 2007 Wiley-Liss, Inc.