Amino acids have different chemical properties that determine whether they

are found on the surface of globular proteins. Which of the following series of

amino acids is ordered according to how likely it will be found on the surface

of a protein:

Arginine > Leucine > Aspartic acid> Phenylalanine

Threonine > Arginine > Phenylalanine > Asparagine

Arginine > Phenylalanine > Threonine > Glycine

Aspartic acid > Threonine > Leucine > Phenylalanine

In globular proteins, hydrophilic (polar or charged) amino acids such as arginine and aspartic acid are most likely found on the surface to interact with water, while hydrophobic ones like leucine and phenylalanine prefer the buried core. The correct series, ordered from most to least likely on the surface, is Aspartic acid > Threonine > Leucine > Phenylalanine. This reflects their hydrophilicity scales, where charged Asp (acidic) tops, followed by polar Thr, then mildly hydrophobic Leu, and highly hydrophobic Phe.

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Why Surface Preference Matters

Globular proteins fold to minimize energy, burying nonpolar residues inside away from aqueous solvent and exposing polar/charged ones outward. Hydrophobicity drives this: positive ΔG for hydrophobic burial stabilizes the fold. Studies of solvent-accessible surface areas (SASA) confirm charged residues like Arg and Asp have high exposure probabilities, while aromatics like Phe have low.

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Option Analysis

• Arginine > Leucine > Aspartic acid > Phenylalanine: Incorrect. Arg (basic, very hydrophilic) > Asp (acidic, hydrophilic), but Leu (hydrophobic) and Phe (very hydrophobic) should rank lower than both; order mixes charged with hydrophobics wrongly.

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• Threonine > Arginine > Phenylalanine > Asparagine: Incorrect. Thr (polar) and Asn (polar) are surface-prone, but Arg (charged) > Thr/Asn; Phe (hydrophobic) cannot precede Asn.

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• Arginine > Phenylalanine > Threonine > Glycine: Incorrect. Arg leads correctly, but Phe (buried) > Thr/Gly (surface/moderately exposed) reverses hydrophilicity.

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• Aspartic acid > Threonine > Leucine > Phenylalanine: Correct. Asp (charged, most exposed) > Thr (polar) > Leu (aliphatic hydrophobic, semi-buried) > Phe (aromatic hydrophobic, most buried), matching SASA distributions.

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