Bioconjugation is the attachment of something to a protein: a dye, an enzyme, a polymer, a drug, an oligonucleotide or a surface. The chemistry is the easy part to read about and the ratio is the part that decides whether the conjugate works, and most disappointing conjugates are the result of a specification that named a label and not a number. This page is about what to specify, in what order, and what evidence to ask for.
Which chemistry, and what it costs you
Amine-directed chemistry is simple, works on any protein and attaches wherever lysines are, including near the binding site. Thiol-directed chemistry is more controlled but needs a free cysteine, which usually means reducing a disulphide or engineering one in. Site-specific approaches, enzymatic or through an unnatural amino acid, give a defined product and cost more and take longer. Click chemistry joins two parts that were each prepared separately, which is what makes it attractive for difficult payloads. The choice is a trade between control and effort, and it should be made from what the conjugate has to do rather than from what is convenient.
The ratio is the specification
How many labels per protein decides brightness, activity and behaviour, and more is not better: over-labelling quenches fluorescence, blocks binding sites and drives aggregation. The useful specification names a target ratio and a range, and the useful certificate reports the ratio actually achieved and its distribution rather than an average alone. A distribution matters because a mean of three can be a uniform population or a mixture of unlabelled protein and heavily labelled aggregate, and those two behave differently in every assay.
What to prove after the reaction
Three things: how much label is attached, how much free label remains, and whether the protein survived. Absorbance or mass measures the first, a separation step and a purity assay the second, and a binding or activity assay the third. Size exclusion reports the aggregate the reaction created. A conjugate delivered with a ratio and no activity result has answered the easy question only.
Buying it, or doing it in house
A kit and an afternoon is the right answer for a dye on a plentiful antibody. A service is the right answer when the payload is precious, the ratio has to be controlled tightly, the chemistry is site-specific, or the material is heading somewhere regulated, where the documentation of the process becomes part of what you are buying. Ask a provider for the ratio distribution and the activity data on a comparable project, rather than for a list of chemistries offered.
Questions people ask about bioconjugation
Which bioconjugation chemistry should I choose?
Amine chemistry for a straightforward dye on a robust protein, thiol chemistry when a free cysteine is available and placement matters, and a site-specific method when the product has to be defined. Choose from what the conjugate must do, not from convenience.
What substitution ratio should I ask for?
A target with a range, plus the distribution rather than the mean alone. Over-labelling quenches signal, blocks binding and drives aggregation, so the highest achievable ratio is rarely the best one.
What should a conjugate's certificate show?
The ratio achieved and its distribution, free label remaining, aggregate by size exclusion, and an activity or binding result. Without the last one the conjugate has not been shown to work.