Core Hydrophobic Amino Acids
The amino acids with the strongest hydrophobic forces are those whose side chains are predominantly nonpolar and aliphatic or aromatic, minimizing water interactions. These are alanine (Ala, A), valine (Val, V), leucine (Leu, L), isoleucine (Ile, I), proline (Pro, P), phenylalanine (Phe, F), and methionine (Met, M). Their side chains—such as -CH3 (Ala), -CH(CH3)2 (Val), and benzyl (Phe)—drive burial in protein cores via van der Waals interactions and entropy gains from water release.1
Why These Show Strongest Hydrophobicity
Hydrophobicity stems from side chain partitioning into nonpolar environments, measured by scales like Kyte-Doolittle (positive values indicate hydrophobicity). Top scorers: Ile (4.5), Val (4.2), Leu (3.8), Phe (2.8), Cys (2.5), Met (1.9), Ala (1.8). Proline's ring enforces rigidity, enhancing packing. Tryptophan (Trp, W; -0.9 KD score) is borderline due to its indole ring's partial polarity but often clusters hydrophobically.1
How Hydrophobicity Scales Rank Them
| Amino Acid | 3-Letter | 1-Letter | Kyte-Doolittle Score |
|------------|----------|----------|----------------------|
| Isoleucine | Ile | I | 4.5 |
| Valine | Val | V | 4.2 |
| Leucine | Leu | L | 3.8 |
| Phenylalanine | Phe | F | 2.8 |
| Cysteine | Cys | C | 2.5 |
| Methionine | Met | M | 1.9 |
| Alanine | Ala | A | 1.8 |1
Scales like Eisenberg consensus normalize for size, confirming Ile, Leu, Val, Phe as extremes. Glycine (Gly, G; -0.4) is neutral due to its H side chain.3
Borderline and Weak Hydrophobics
Tryptophan (Trp) and tyrosine (Tyr, Y) have hydrophobic rings but polar groups (indole NH, phenol OH), yielding mixed behavior—Y ranks low ( -1.3 KD). Histidine (His, H) is weakly hydrophobic when neutral. These participate in hydrophobic cores but form hydrogen bonds more readily.2
Hydrophilic Counterparts for Comparison
Polar/charged amino acids exhibit weak or repulsive hydrophobic forces: aspartic acid (Asp, D; -3.5), asparagine (Asn, N; -3.5), glutamic acid (Glu, E; -3.5), glutamine (Gln, Q; -3.5), serine (Ser, S; -0.8), threonine (Thr, T; -0.7). They prefer solvent exposure.1
Role in Protein Folding and Mutations
Strong hydrophobics cluster in folding cores, stabilizing structures—mutations here (e.g., Leu to Asp) cause misfolding diseases like cystic fibrosis. In design, pairing Ile/Leu maximizes core packing density.2