Table III.

Comparison of Amino Acid Side-Chain Hydrophilicity/Hydrophobicity Scales

	Random coil peptides (pH 2)								α-Helical peptides (pH2)			Random coil peptides (pH 7)				α-Helical peptides (pH7)		Other approaches
Amino Acid	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21
Trp	32.4	32.3	18.1	15.1	8.8	16.3	2.29	5.91	7.1	1.6	4.88	33.0	33.7	14.9	9.5	7.6	5.1	2.25	1.88	−2010	2.60
Phe	29.1	29.1	13.9	12.6	8.1	19.2	4.80	9.24	7.9	4.0	5.00	30.1	30.8	13.2	9.0	7.8	5.8	1.79	1.87	−2330	2.30
Leu	23.3	23.4	10.0	9.6	8.1	20.0	3.50	6.57	8.5	5.0	4.76	24.6	25.1	8.8	9.0	7.6	7.8	1.70	1.81	−2460	1.90
Ile	21.4	21.3	11.8	7.0	7.4	6.6	3.48	4.38	8.5	5.4	4.41	22.8	23.0	13.9	8.3	7.7	7.9	1.80	1.81	−2260	1.90
Met	15.7	16.1	7.1	4.0	5.5	5.6	0.21	−3.12	6.3	3.0	3.23	17.3	16.8	4.8	6.0	5.8	4.3	1.23	0.81	−1470	2.40
Tyr	14.7	15.4	8.2	6.7	4.5	5.9	1.89	1.39	4.2	−0.9	2.00	16.0	15.1	6.1	4.6	4.9	−0.8	0.96	1.20	−2240	1.60
Val	13.4	13.8	3.3	4.6	5.0	3.5	1.59	2.30	6.7	4.9	3.02	15.0	14.6	2.7	5.7	5.9	4.9	1.22	1.27	−1560	1.50
Pro	9.0	9.4	8.0	3.1	2.0	5.1	0.71	−0.12	−3.9	−5.0	−4.92	10.4	9.9	6.1	2.2	−2.2	−7.2	0.72	0.95	−980	1.20
Cys	7.6	8.1	−2.2	4.6	2.6	−9.2	0.49	0.73	4.4	3.0	2.49	9.1	8.2	−6.8	2.6	3.8	---	1.54	0.43	−450	0.38
Lys	2.8	−7.0	−3.2	−3.0	−2.1	−3.7	−1.62	−2.78	−3.1	−9.2	−5.00	−2.0	3.4	0.1	−0.2	−1.8	−6.3	−0.99	−1.80	−350	−0.57
Glu	2.8	3.6	−7.5	1.1	1.1	−7.1	−0.10	−0.45	0.7	−3.9	−1.49	−0.4	−7.1	−16.9	−1.3	−2.4	−11.7	−0.64	−3.84	−300	−0.76
Ala	2.8	3.6	−0.1	1.0	2.0	7.3	−0.06	2.62	4.0	3.0	0.17	4.1	3.4	0.5	2.2	3.2	0	0.31	0.32	−200	0.67
Thr	2.3	2.8	1.5	−0.6	0.6	0.8	0.65	1.81	1.1	0.5	−1.08	4.1	2.5	2.7	0.3	1.0	−2.5	0.26	−0.30	−520	0.52
Asp	1.6	2.2	−2.8	−0.5	0.2	−2.9	−0.20	−2.84	−1.5	−4.4	−2.49	−0.8	−7.6	−8.2	−2.6	−4.3	−11.3	−0.77	−3.18	−200	−1.20
Arg	0.6	−5.0	−4.5	−2.0	−0.6	−3.6	−0.85	1.26	−2.2	−8.3	−2.77	4.1	6.4	0.8	0.9	−1.1	−6.5	−1.01	−3.04	−120	−2.10
Gln	0.6	0.5	−2.5	−2.0	0	−0.3	0.31	−1.69	−1.5	−5.8	−2.75	1.6	0	−4.8	0	−0.8	−7.3	−0.22	−1.15	160	−0.22
His	0	−7.0	0.8	−2.2	−2.1	−2.1	−2.24	−0.74	−3.6	−8.6	−4.63	4.7	3.4	−3.5	2.2	0.6	−6.3	0.13	0.01	−120	0.64
Ser	0	0	−3.7	−2.9	−0.2	−4.1	−0.62	−1.39	−0.6	−1.2	−2.84	1.2	−0.5	1.2	−0.5	−0.4	−5.6	−0.04	−0.62	−390	0.01
Gly	0	0	−0.5	0.2	−0.2	−1.2	0.21	−1.15	0	0	−3.31	0	0	0	−0.2	0	−6.6	0	0	0	0
Asn	−0.6	0	−1.6	−3.0	−0.6	−5.7	0.25	−1.27	−3.5	−5.8	−3.79	1.0	−0.8	0.8	−0.8	−2.2	−8.2	−0.60	−0.97	80	−0.60

^{Scale 1}Kovacs et al, 2005. ⁴ RP-HPLC of model random-coil synthetic peptides (20 amino acid substitutions). Column: Kromasil C₁₈ (150 × 2.1 mm I.D., 5 μm particle size, 100 Å pore size). Conditions: linear AB gradient (0.25% CH₃CN/min, starting from 2% CH₃CN) at a flow-rate of 0.3 ml/min, where eluent A is 20 mM aq. TFA and eluent B is 20 mM TFA in CH₃CN; temperature, 25°C. Values denote change in peptide retention time relative to the Gly-substituted analog. Peptide sequences shown in Table IV.

^{Scale 2}Kovacs et al, 2005. ⁴ Details as Scale 1, except for 20 mM H₃PO₄ in place of TFA.

^{Scale 3}Meek, 1980. ⁸ RP-HPLC of random peptide mixture (25 peptides). Column: Bio-Rad ODS (C₁₈). Conditions: linear AB gradient (0.75% CH₃CN/min) at a flow-rate of 1.0 ml/min, where eluent A is 0.1% aq. phosphoric acid (pH 2.1) and eluent B is 0.1% phosphoric acid in CH₃CN, both eluents also containing 0.1 M NaClO₄; room temperature. Values obtained by repetitive regression analysis; refers to cystine in the publication.

^{Scale 4}Meek and Rossetti, 1981. ¹⁷ RP-HPLC of random peptide mixture (100 peptides). Column: Bio-Rad ODS (C₁₈) (250 × 4.0 mm I.D., 10 μm). Conditions: linear AB gradient (0.75% CH₃CN/min) at a flow-rate of 1.0 ml/min, where eluent A is 0.1 M aq. NaH₂PO₄ containing 0.2% H₃PO₄ and eluent B is 0.1 % H₃PO₄ in CH₃CN. Values obtained by repetitive regression analysis; refers to cystine in the publication.

^{Scale 5}Guo et al, 1986. ¹⁸ RP-HPLC of model random-coil synthetic peptides (20 amino acid substitutions). Column: SynChropak RP-P C₁₈ (250 × 4.1 mm I.D., 6.5 μm, 300 Å). Conditions: linear AB gradient (1% CH₃CN/min) at a flow-rate of 1 ml/min, where eluent A is 0.1% aq TFA and eluent B is 0.1% TFA in CH₃CN; temperature, 26 °C. Peptide sequences shown in Table IV. Values derived from average of simultaneous equation and “core” peptide approaches, as described in the text.

^{Scale 6}Browne et al, 1982. ²² RP-HPLC of 25 native peptides. Column: Waters μBondapak C₁₈. Conditions: linear AB gradient (20% CH₃CN/h), starting from 1.6% CH₃CN) at a flow-rate of 1.5 ml/min, where eluent A is 0.1% aq TFA and eluent B is 0.1% TFA in CH₃CN. Values obtained by repetitive regression analysis.

^{Scale 7}Wilce et al, 1995. ¹⁶ RP-HPLC of random peptide mixture (1738) from 2-50 amino acid residues. Column: Bakerbond WP n-octadecylsilica (C₁₈; 250 × 4.6 mm I.D., 5 μm, 300 Å). Conditions: linear AB gradient, where eluent A is 0.1% aq. TFA and eluent B is 0.1% TFA in CH₃CN (gradient rate and flow-rate not reported); temperature ca. 20°C. Values obtained by multiple linear regression analysis.

^{Scale 8}Wilce et al, 1995. ¹⁶ Details as Scale 7, except for a column of Bakerbond WP n-octylsilica (C₈).

^{Scale 9}Sereda et al, 1994. ²³ RP-HPLC of model amphipathic α-helical synthetic peptides (20 amino acid substitutions) (“Alα-face” helical net shown in Fig. 3). Column: Aquapore RP-300 C₈ (220 × 4.6 mm I.D., 7 μm, 300 Å). Conditions: linear AB gradient (1% CH₃CN/min) at a flow-rate of 1.0 ml/min, where eluent A is 0.1% aq. TFA and eluent B is 0.1% TFA in CH₃CN. Values denote change in peptide retention time relative to the Gly-substituted analog. Peptide sequences shown in Table IV.

^{Scale 10}Sereda et al, 1994. ²³ Details as Scale 9, except for the amino acid substitutions being made in the “Leu-face” (helical net shown in Figure 3) of the amphipathic α-helix. Peptide sequences shown in Table IV.

^{Scale 11}Liu and Deber, 1998. ²⁵ RP-HPLC of model α-helical synthetic peptides (20 amino acid substitutions) (helical net shown in Figure 3). Column: Primesphere C₄ (250 × 4.6 mm I.D. 10 μm, 300 Å). Conditions: linear AB gradient (2% CH₃CN/min) at a flow-rate of 1 ml/min, where eluent A is 0.1% aq TFA and eluent B is 0.1% TFA in CH₃CN. Scale obtained by assigning a value of +5 and −5 to the most hydrophobic (Phe) and most hydrophilic (Lys) amino acid residues. Peptide sequences shown in Table IV.

^{Scale 12}Kovacs et al, 2005. ⁴ RP-HPLC of model random-coil synthetic peptides (20 amino acid substitutions). Column: Zorbax Eclipse XDB C₈ (150 × 2.1 mm I.D., 5 μm, 80 Å). Conditions: linear AB gradient (0.25% CH₃CN/min) at a flow-rate of 0.3 ml/min, where eluent A is 10 mM aq. NaH₂PO₄ (pH 7) and eluent B is 10 mM NaH₂PO₄ in 50% aq. CH₃CN, both eluents also containing 50 mM NaCl ; temperature, 25°C. Values denote change in retention time relative to the Gly-substituted analog. Peptide sequences shown in Table IV.

^{Scale 13}Kovacs et al, 2005. ⁴ Details as Scale 12, except for 50 mM NaClO₄ in both eluents in place of NaCl.

^{Scale 14}Meek, 1980. ⁸ Details as Scale 3, except for buffer conditions where eluent A is 5 mM aq. phosphate buffer (pH 7.4) and eluent B is 5 mM phosphate buffer in 60% aq. CH₃CN, each eluent also containing 0.1 M NaClO₄

^{Scale 15}Guo et al, 1986. ¹⁸ Details as Scale 5, except for the buffer conditions where eluent A is 10mM aq. (NH₄)₂HPO₄ (pH 7) and eluent B is 10mM (NH₄)₂HPO₄ in 60% aq. CH₃CN, both eluents also containing 0.1 M NaClO₄. Peptide sequences shown in Table IV.

^{Scale 16}Monera et al, 1995. ²⁴ Details as Scale 9, except for the buffer conditions where eluent A is 100 mM aq. triethylammonium phosphate (TEAP) (pH 7) and eluent B is 100 mM TEAP in 50% aq. CH₃CN. Peptide sequences shown in Table IV.

^{Scale 17}Tripet et al, 2000. ²⁶ RP-HPLC of model amphipathic α-helical synthetic peptides (20 amino acid substitutions) (helical net shown in Figure 3). Column: Zorbax Eclipse XDB-C₈ (150 × 4.6 mm I.D., 5 μm, 300 Å). Conditions: linear AB gradient (1% CH₃CN/min) at a flow-rate of 1 ml/min, where eluent A is 50 mM aq KH₂PO₄ (pH 7) and eluent B is 50 mM KH₂PO₄ in 50% aq. CH₃CN, both eluents also containing 100 mM NaClO₄; temperature 70 °C. Values denote change in peptide retention time relative to the Ala-substituted analog. Peptide sequences shown in Table IV.

^{Scale 18}Fauchere and Pliska, 1983. ⁴⁸ Water/octanol partitioning of N^α-acetyl-amino-acid amides at pH 7.0 - 7.2 and room temperature. The values shown are derived from the equation: side-chain hydrophobicity = log distribution coefficient (D) (acetyl-amino-acid amide) − (acetyl-glycine amide).

^{Scale 19}Abraham and Leo, 1987. ⁴⁹ Calculation of amino acid side-chain partition coefficients relative to glycine by the fragment method. Calculations are based on N^α-acetyl-amino-acid amide analogs. The values for His, Asp and Glu are based on the assumption that their side-chains are deprotonated, i.e., His is classed as polar uncharged; Asp and Glu are negatively charged.

^{Scale 20}Bull and Breese, 1974. ⁵⁰ Values represent free energies of transfer of amino acids relative to glycine derived from their surface tension in 0.1 M NaCl at 30°C. Amino acid solutions were at or near the isoelectric point of the acid. Due to solubility problems, the value for Tyr is only an estimate. The greater the surface tension lowering by an amino acid, the greater its hydrophobicity and, hence, the more negative its value in this hydrophilicity/hydrophobicity scale.

^{Scale 21}Eisenberg and McLachlan, 1986. ⁵¹ Calculated values of free energy of transfer to water of amino acid residues immersed in protein (relative to glycine). The value for Cys was calculated for half-cystine.