Two conserved glutamines interact across the V_H-V_L dimer interface. Mutations affecting one of these Gln residues generally have a destabilizing effects. Efforts to stabilize the dimer interface by replacing the two glutamines with hydrophobic residues were unsuccessful, as were attempts to replace them by a pair of residues with opposite charges

For VL-Dimer (Len):
Substitution of Gln by Ala reduced the domain association constants (also true for Asp-Arg, Asp-Lys, Glu-Arg and Glu-Lys pairs, Glu-Glu leads to slightly higher association constant than Gln-Gln - but with upside - down domain pairing! ).