Table 1.

Estimated Frequency and Energy of Secondary Forces in Protein Folding

Interaction	Approximate Frequency per 100 Residues	Approximate Energy (kcal/mol)a
n→π* Interactions	3357	0.2569
C–H···O Hydrogen bonds	1031	0–141,42
π–π Interactionsb	5111	0.5–1.5117–119
C5 Hydrogen bonds	583	0.25–1.583
Cation–π interactions	1–294	0.5–285
Sulfur–arene interactionsb	2–3130	0.3–0.5133,134
Anion–π interactions	1–2125	0.5124
Chalcogen bonds	<1140	0.64140
X–H∙∙∙π Interactions	1102	0.35143

^aPreference is given to experimental measurements in proteins and peptides. Computational values are used in the absence of experimental data.

^bFrequency per 100 residues was estimated by multiplying the frequency of relevant residues⁹⁵ by the fraction of those residues that engage in the interaction.