Peptide Purity Versus Net Peptide Content

Two questions, two numbers

Purity asks: of the peptide-related material detected in a sample, how much is the intended molecule? It usually comes from a chromatogram as area percent; see reading an HPLC chromatogram.

Net peptide content asks: of the whole powder on the balance, how much is peptide? The rest is water, counter-ions, and sometimes other salts or residual solvent. The mass on the balance is the gross weight; the peptide part of it is the net.

A powder can be 98 % pure and still be only about three quarters peptide by mass. Neither number is wrong. They answer different questions.

What is in the rest of the powder

• Counter-ions. A synthetic peptide is a salt, and its anions add real mass; see counter-ions in synthetic peptides.
• Water. Freeze-dried cakes hold residual moisture and many take up more from the air; see lyophilization.
• Other solids. Buffer salts or bulking agents, if any were used.
• Related peptides. The impurities counted in the purity figure are themselves not the target.

How net peptide content is measured

Several methods are used, and they do not always agree exactly.

• Amino acid analysis. The peptide bonds are broken by hydrolysis, usually with hot hydrochloric acid, and the liberated amino acids are separated and quantified. Because the amino acid composition of the peptide is known, the amount of peptide can be worked out. The method needs care for amino acids that do not survive hydrolysis well, such as tryptophan, and for methionine and cysteine, which need special handling.^[1]
• Ultraviolet absorbance. If the peptide contains tryptophan, tyrosine or cystine, absorbance near 280 nm can be converted to concentration using an extinction coefficient calculated from the composition.^{[2, 3]} At 214 nm the peptide bonds themselves absorb, and coefficients can be predicted for sequences without aromatic residues.^[4] Both approaches rest on the extinction coefficient being right, and one of the references recommends measuring it where possible.^[3]
• Separate component measurements. Water by Karl Fischer titration and counter-ions by ion chromatography, subtracted from 100 %.^[5]

Why purity and content interact

A mass-balance approach to purity subtracts every kind of impurity from 100 %, including related peptides, and a metrology study on a model peptide hormone showed why this matters: if related impurities are not corrected for, the amount of peptide worked out by amino acid analysis is off, in that case by about one percent.^[6] In day-to-day work the two numbers are often combined to estimate the mass of the target peptide in a powder.

Why content cannot be assumed

Net peptide content depends on the sequence and the history of the material, which is why a number from a different peptide does not transfer. Counter-ions are bound to basic sites, so a peptide with many lysine and arginine residues carries more counter-ion mass per molecule than one with none. A made-up comparison, both as TFA salts with every basic site paired with one trifluoroacetic acid (114.02 g/mol), for a 1500 g/mol peptide: with one basic site the peptide is 1500 ÷ (1500 + 114.02) = 92.9 % of the salt; with five it is 1500 ÷ (1500 + 5 × 114.02) = 72.5 %. Water comes on top of that.

Content also changes with time. A hygroscopic cake that has been opened and exposed to humid air holds more water than it did when it was measured, so a measured content belongs to the material at the moment it was measured.

Worked arithmetic

The numbers below are made up to show the calculation. Suppose a vial of powder gives a gross mass of 5.00 mg, a net peptide content of 78 %, and an area-percent purity of 96 %.

• Peptide in the powder: 5.00 mg × 0.78 = 3.90 mg.
• Target peptide, if purity is treated as a mass fraction: 3.90 mg × 0.96 = 3.744 mg.
• Gross mass needed for 2.00 mg of target peptide: 2.00 ÷ (0.78 × 0.96) = 2.671 mg.
• Effect on a stock solution: 5.00 mg in 2.00 mL is 2.5 mg/mL by gross weight, but the peptide concentration is 5.00 × 0.78 ÷ 2.00 = 1.95 mg/mL.

Treating an area-percent purity as a mass fraction is an approximation, because impurities and the main peptide do not give the same detector signal per gram. Use it as an estimate and say so in your records. The purity and content calculator does this arithmetic for numbers you type and shows its working.

Reading specifications with this in mind

Sealed Peptides product pages state purity as supplied. Where a specification gives only a purity and no net peptide content, the content is unknown. If your experiment depends on the exact amount of peptide, you have three options: measure the content yourself, treat the gross weight as a ceiling and report concentrations as nominal, or ask the supplier what is known; questions can go to support.

A short checklist

1. Write “gross weight” and “net weight” as separate terms in your notebook.
2. Record where each percentage came from: which method, which date, which sample.
3. Do not multiply a purity by a content if the two were measured on different material.
4. When a concentration matters, say whether it is nominal (from gross weight) or corrected (from net peptide content).