What molecular weight range do plant peptides have?

Beyond Biopharma | Plant Protein Peptide FAQ

Short answer: Most commercial plant protein peptides are supplied with a molecular weight distribution centred on the 1,000-3,000 dalton range, with a substantial fraction below 1,000 Da and a small tail above 5,000 Da. The figure on a certificate of analysis describes a distribution, not a single value, and it is method-dependent: a result is only comparable with another result obtained by the same technique, column set and calibration standards.

Why the distribution matters more than an average

Hydrolysis produces a mixture: free amino acids, dipeptides and tripeptides, short chains, medium chains and a residual fraction of larger fragments. Each part of that mixture behaves differently in a formulation. The very small fraction dissolves instantly but contributes to taste and to osmolality, the 1,000-3,000 Da band usually combines good solubility with a clean mouthfeel, and the larger tail can add viscosity, haze or sediment after heat treatment. Two products with the same average molecular weight can behave quite differently if the spread around that average differs, which is why specifications list fractions rather than one mean value.

A typical distribution in commercial grades

The actual shares differ between sources and between grades of the same source, so the distribution should be read from the certificate of analysis for the lot in question rather than assumed from a typical profile.

How the distribution is measured

Molecular weight distribution is normally determined by gel permeation chromatography (GPC), also called size exclusion chromatography (SEC). The sample is separated by size on a column and the elution profile is converted into a distribution using a calibration curve built from standards of known molecular weight. Column choice, mobile phase, calibration standards and detection all shift the apparent numbers, so a distribution obtained on one system cannot simply be subtracted from one obtained on another. This is why a specification should state the method, not just the result.

Molecular weight distribution and the TCA solubility index are not the same thing

A simpler and older approach reports the share of nitrogen that stays soluble in trichloroacetic acid, which is used as an indicator of how extensively a protein has been broken down. It is quick and useful for in-process control, but it is not a molecular weight distribution and cannot be converted into one. A product described as 80 percent TCA soluble nitrogen is not thereby a product with 80 percent of its material below a particular molecular weight. When two suppliers report different kinds of number, ask for the underlying method before comparing.

Relationship with degree of hydrolysis

Degree of hydrolysis measures how many peptide bonds were broken, while molecular weight distribution measures how large the resulting chains are. They usually move together - more hydrolysis generally means smaller fragments - but they are not interchangeable. Enzyme specificity and any fractionation step can leave two products with the same degree of hydrolysis but different distributions. For a clear beverage, specify both.

What to ask for on a specification

• The reported distribution bands and the percentage in each band, for example below 1,000 Da, 1,000-3,000 Da and above 3,000 Da.
• The analytical method, including the standards used for calibration.
• The degree of hydrolysis with its measurement basis, read alongside the distribution.
• Protein content with the nitrogen conversion factor, since it determines how much of the powder is actually peptide-bound material.
• A solubility and clarity check at your own use level, pH and process conditions rather than only in water.

Related reading

See what degree of hydrolysis means, which specifications matter on a plant peptide COA and how plant peptides are produced.