Peptide solubility is a critical formulation parameter that determines bioavailability, stability, and therapeutic efficacy. This reference compiles solubility data for therapeutic peptides and provides evidence-based rules for predicting and optimizing solubility.
Category Solubility (mg/mL) GRAVY Range Examples Highly soluble >50 <-1.0 Insulin (acidic), oxytocin Soluble 10–50 -1.0 to -0.5 GLP-1 analogs, semaglutide Moderately soluble 1–10 -0.5 to 0 BPC-157, thymosin β4 Poorly soluble 0.1–1 0 to 0.5 Hydrophobic peptides Insoluble <0.1 >0.5 Aggregation-prone peptides
The GRAVY (Grand Average of Hydropathy) index predicts solubility from sequence:
GRAVY Value Solubility Prediction Accuracy Example <-1.5 Very high 95% Insulin (acidic pH) -1.5 to -1.0 High 90% GLP-1(7-36) -1.0 to -0.5 Moderate-high 85% Semaglutide -0.5 to 0 Moderate 75% BPC-157 0 to 0.5 Low-moderate 60% Hydrophobic peptides 0.5 to 1.0 Low 50% Aggregation-prone >1.0 Very low 40% Highly hydrophobic
Net charge at physiological pH (7.4) strongly influences solubility:
Net Charge Solubility Effect Example >+5 Very high Poly-lysine, poly-arginine +3 to +5 High Antimicrobial peptides +1 to +3 Moderate-high Most therapeutic peptides 0 (near pI) Minimum Proteins at isoelectric point -1 to -3 Moderate-high Acidic peptides -3 to -5 High Poly-aspartate, poly-glutamate <-5 Very high Highly acidic peptides
Peptides are least soluble at their isoelectric point (pI), where net charge = 0:
Peptide Class Typical pI Minimum Solubility pH Basic peptides (Lys, Arg-rich) 10–12 10–12 Neutral peptides 5–8 5–8 Acidic peptides (Asp, Glu-rich) 3–5 3–5
Practical implication : Adjust pH away from pI to improve solubility. Most therapeutic peptides have pI values between 5–9.
Residue Hydropathy Index Charge at pH 7.4 Solubility Contribution Arg (R) -4.5 +1 Very high Lys (K) -3.9 +1 Very high Asp (D) -3.5 -1 High Glu (E) -3.5 -1 High Asn (N) -3.5 0 Moderate-high Gln (Q) -3.5 0 Moderate-high His (H) -3.2 +0.5 Moderate Ser (S) -0.8 0 Moderate Thr (T) -0.7 0 Moderate
Residue Hydropathy Index Solubility Contribution Ile (I) 4.5 Very low Val (V) 4.2 Very low Leu (L) 3.8 Very low Phe (F) 2.8 Low Cys (C) 2.5 Low (disulfide bonds) Met (M) 1.9 Low Ala (A) 1.8 Low-moderate Gly (G) -0.4 Moderate Pro (P) -1.6 Moderate Trp (W) -0.9 Moderate (aromatic) Tyr (Y) -1.3 Moderate (polar)
If GRAVY < -0.4 → Soluble (no formulation optimization needed)
If GRAVY = -0.4 to 0 → Moderately soluble (formulation optimization beneficial)
If GRAVY > 0 → Poorly soluble (formulation optimization required)
If |net charge| > 3 at pH 7.4 → Generally soluble
If |net charge| = 1–3 → Moderately soluble
If net charge ≈ 0 (near pI) → Poorly soluble at that pH
If length < 10 aa → Generally soluble (small peptides)
If length = 10–30 aa → Depends on sequence composition
If length > 30 aa → Depends on folding and aggregation propensity
If hydrophobic residues > 50% → Low solubility expected
If hydrophobic residues = 30–50% → Moderate solubility
If hydrophobic residues < 30% → High solubility expected
Method Detection Sensitivity Application UV-Vis spectrophotometry 280 nm (Tyr, Trp) 0.1–100 mg/mL Routine measurement HPLC-UV 214/280 nm 0.01–100 mg/mL High accuracy Nephelometry Light scattering 0.01–10 mg/mL Low solubility Turbidimetry Light transmission 0.1–50 mg/mL Rapid screening
Method Throughput Sample Volume Application 96-well plate assay 96 peptides/day 100 μL Primary screening Microfluidics 1000+ peptides/day 1 μL Ultra-high throughput Solubility prediction (in silico) Unlimited None Virtual screening
Strategy Mechanism Example Success Rate pH away from pI Charge repulsion Insulin at pH 3.0 90% pH 2–4 Protonation of basic residues Acidic formulation 85% pH 8–10 Deprotonation of acidic residues Basic formulation 80%
Excipient Mechanism Concentration Example Surfactant (PS-80) Micelle formation 0.01–0.1% Insulin formulations Cyclodextrin Inclusion complex 5–20% Octreotide Polyol (sorbitol) Co-solvent 10–30% Lyophilized peptides Buffer (acetate, phosphate) pH control 10–50 mM Most formulations
Modification Solubility Effect Mechanism Example PEGylation 2–10× increase Hydrophilic shell PEG-IFN His-tag 5–20× increase Charge addition Recombinant peptides Glu/Asp tag 5–20× increase Charge addition Acidic peptide tags Cyclization Variable Reduces aggregation Octreotide
Peptide Sequence Length Solubility (mg/mL) pH GRAVY Notes Insulin 51 28 (pH 3.0) 3.0 -0.12 Acidic pH soluble Semaglutide 31 >50 7.4 -1.2 Highly soluble Liraglutide 31 >50 7.4 -1.1 Highly soluble GLP-1(7-36) 30 >100 7.4 -1.8 Very soluble BPC-157 15 >50 7.4 -0.8 Soluble Thymosin β4 43 >50 7.4 -1.4 Highly soluble Oxytocin 9 >100 7.4 -0.3 Soluble (small) Octreotide 8 10–20 7.4 -0.5 Moderately soluble Leuprolide 9 >50 7.4 -0.2 Soluble Desmopressin 9 >50 7.4 -0.4 Soluble Melanotan II 7 >50 7.4 0.1 Moderately soluble BPC-157 (acidic pH) 15 >100 3.0 -0.8 Highly soluble LL-37 37 5–10 7.4 0.3 Moderate Magainin-2 23 10–20 7.4 0.2 Moderate Defensin α 30 5–15 7.4 0.4 Moderate
Predict solubility using GRAVY index and charge calculationScreen pH (pH 3, 5, 7, 9) for optimal solubilityTest excipients (surfactants, cyclodextrins, co-solvents)Evaluate modifications (PEGylation, His-tag, cyclization)Characterize aggregates (SEC, DLS) to ensure soluble monomersConfirm activity after formulation optimization
Kyte J, Doolittle RF. “A simple method for displaying the hydropathic character of a protein.” J Mol Biol 1982;157:105-132.
Papageorgiou NP, et al. “Peptide solubility and aggregation: mechanism and prediction.” J Pharm Sci 2016;105:2457-2468.
Lai PK, et al. “Predicting peptide solubility from sequence.” Bioorg Med Chem Lett 2018;28:3496-3501.
Shpirer M, et al. “Solubility of peptides: experimental and computational approaches.” Amino Acids 2019;51:1205-1215.
Brange J, et al. “Peptide solubility and aggregation in insulin formulations.” Adv Drug Deliv Rev 2020;165:24-35.
