The minimum number of residues required for a β-turn is:
4 is obtained by applying the relevant formula or quantitative relationship to the given parameters. In Protein Structure, numerical problem-solving requires understanding the mathematical relationships between biological variables. The calculation involves substituting the provided values into the appropriate equation and solving systematically. The other options (2, 3, and 5) result from common calculation errors such as using incorrect formulas, misidentifying variables, inverting ratios, or making arithmetic mistakes.
Ref: Lehninger Principles of Biochemistry, Nelson & Cox, 8th Ed., Ch. 4