Molecular Weight
Definition
The mass of one molecule, in daltons or g/mol. For a peptide it follows arithmetically from the sequence, so a stated weight that does not match the stated sequence is a contradiction on the page and a reason to look harder at the rest of the document.
In more detail
Molecular weight is fixed by the molecular formula, so a formula and a weight that disagree mean one of them is wrong rather than that the compound is unusual. Checking the two against each other is a quick test of whether a specification sheet was transcribed carefully.
There are two figures in common use and they are not interchangeable for peptides. The average mass uses the natural mixture of isotopes and is what a specification sheet usually quotes; the monoisotopic mass uses the lightest isotope of each element and is what a high-resolution mass spectrometer reports. On a molecule of a few thousand daltons the two differ by several units.
For a salt the question of what is being weighed arises as well. A trifluoroacetate salt weighs more than the free peptide, which is a further reason a mass on a certificate should say what form it refers to.
Where this term comes up
Entries whose definition mentions Molecular Weight.
What this means for research sourcing
Structure and identity terms are what a certificate of analysis is written in. Every Volta compound ships with its sequence, molecular weight and measured purity, so these terms map directly onto the specification on the product page.
Related Terms in Chemistry & Structure
Kilodalton (kDa)
One thousand daltons, the unit used once a molecule is large enough that daltons become unwieldy. Short peptides are quoted in daltons; proteins and fusion constructs in kDa.
Isoelectric Point (pI)
The pH at which a peptide carries no net charge. Solubility is usually at its worst there, so a material that will not dissolve in a neutral buffer often dissolves once the pH is moved away from its pI.
Zwitterion
A molecule carrying a positive and a negative charge at the same time while being neutral overall. Free amino acids exist as zwitterions at physiological pH, which is why they are far more water-soluble than their structures suggest.
Hydrophilic and Hydrophobic
Whether a region of a molecule associates with water or avoids it. The balance across a sequence sets solubility, how a peptide behaves on a reversed-phase column, and how readily it crosses a lipid membrane.
Solubility
How much of a peptide a given solvent will take into solution. It is sequence-specific rather than general: some materials need a trace of acid or a small percentage of organic solvent before water will carry them, and forcing the issue with agitation causes aggregation instead.
Denaturation
Loss of folded structure through heat, extreme pH or mechanical shear, without the backbone necessarily being cut. The mass can be unchanged while the activity is gone, which is why an identity test alone does not establish that a material is intact.
Counter-Ion
The small ion supplied alongside a charged peptide to balance it, usually acetate and sometimes trifluoroacetate. It is part of the powder's mass, so gross vial weight and peptide content are two different numbers.
Acetate Salt
The form most synthetic peptides are supplied in, with acetate as the counter-ion. Acetate is preferred over trifluoroacetate for work in cell culture, where residual TFA can be toxic at low concentrations.