Reading a Spec Sheet: Sequence Notation and Molecular Weight

A specification sheet compresses a great deal into a few lines. Each element is checkable, and cross-checking them is a fast way to catch errors.

Sequence notation

Sequences are written N-terminus to C-terminus using three-letter or single-letter amino acid codes. Modifications appear as annotations: Ac- at the start indicates N-terminal acetylation, -NH2 at the end indicates C-terminal amidation, and D- before a residue indicates the D-stereoisomer rather than the naturally predominant L-form. Cyclisation and disulfide connectivity are noted explicitly, since they change the molecule substantially without changing its residue composition.

Molecular weight

Two figures are commonly reported. Average molecular weight uses the weighted average of natural isotopic abundances and is the figure used for routine mass calculations. Monoisotopic mass uses only the most abundant isotope of each element and is the value compared against high-resolution mass spectrometry data. For small peptides the two are close; for larger molecules they diverge measurably, and comparing the wrong one to an MS result produces an apparent mismatch that is really a unit error.

Cross-checking

The stated molecular weight should be consistent with the stated sequence and modifications. A C-terminal amide reduces mass by approximately one unit relative to the free acid; acetylation adds roughly forty-two; each disulfide bond removes about two. These relationships make internal consistency easy to verify.

Salt form and content

Finally, the sheet should state the salt form and, ideally, net peptide content. The molecular weight refers to the peptide molecule; the material in the vial is the salt, with counterion and residual moisture included in its weighed mass. Keeping those two quantities distinct is what makes concentration calculations correct.

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