 salt bridges are known to play an 
 essential role in the thermodynamic 
 stability of the folded conformation of many proteins , but their influence 
 on the kinetics of folding remains largely unknown . 
 here , we investigate the effect of glu - arg salt bridges on the kinetics 
 of -helix folding using temperature - jump transient - infrared 
 spectroscopy and steady - state uv circular dichroism . 
 we find that 
 geometrically optimized salt bridges ( glu and arg+ are spaced four peptide units apart , and the glu / arg order 
 is such that the side - chain rotameric preferences favor salt - bridge 
 formation ) significantly speed up folding and slow down unfolding , 
 whereas salt bridges with unfavorable geometry slow down folding and 
 slightly speed up unfolding . 
 our observations suggest a possible explanation 
 for the surprising fact that many biologically active proteins contain 
 salt bridges that do not stabilize the native conformation : these 
 salt bridges might have a kinetic rather than a thermodynamic function . 
Supplementary Material