Nonpolar amino acid residues are typically found in the interior of globular proteins like chymotrypsin. which chemical force is most directly responsible?

The chemical force most directly responsible in the given case is hydrophobic interactions.  

The hydrophobic interactions illustrate the associations between the hydrophobes, that is, low water-soluble molecules, and water. Hydrophobes are nonpolar molecules and generally exhibit a lengthened chain of carbons, which do not associate with the molecules of water.  

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