The ability of serine phosphate (SerP) or alpha-amino-gamma-phosphonobutyric acid (AbuP) and arginine to form a salt bridge between their side chains appears to be much greater when they are spaced i/i+4 than when they are spaced i/i+3. The side chain-side chain interaction between SerP/Arg and AbuP/Arg, positioned i/i+4, contribute 0.45 and 0.62 kcal mol(-1), respectively, toward stabilizing the alpha-helical conformation of a peptide.