Small-angle X-ray scattering (SAXS) measurements reveal a striking
difference in intermolecular interactions between two short
highly charged peptides - deca-arginine (R10) and deca-lysine
(K10). Comparison of SAXS curves at high and low salt concentration
shows that R10 self-associates, while interactions between
K10 chains are purely repulsive. The self-association of R10 is
stronger at lower ionic strengths, indicating that the attraction
between R10 molecules has an important electrostatic component.
SAXS data are complemented by NMR measurements and
potentials of mean force between the peptides, calculated by
means of umbrella-sampling molecular dynamics (MD) simulations.
All-atom MD simulations elucidate the origin of the R10 -
R10 attraction by providing structural information on the dimeric
state. The last two C-terminal residues of R10 constitute an adhesive
patch formed by stacking of the side chains of two arginine
residues and by salt bridges formed between the like-charge ion
pair and the C-terminal carboxyl groups. A statistical analysis of
the Protein Data Bank reveals that this mode of interaction is a
common feature in proteins.