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 behavior, and more is not better: over-labeling 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 labeled 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.
An anti-his antibody and the tag it detects
An anti-his antibody detects a short polyhistidine tag, which is the smallest of the common tags and usually the least disruptive, and the tag's accessibility rather than the antibody decides whether a construct is detected. An untagged lysate is the specificity control. Metal affinity purification competes with the antibody for the same tag, so elution buffers matter in a pulldown.
An anti yfp antibody and the fluorescent protein family
An anti yfp antibody detects a yellow variant derived from the same ancestor as the green protein, so cross reactivity with related variants is usual rather than exceptional and is useful when a construct's color is not the point. Where two variants must be told apart, the fluorescence spectrum rather than the antibody is the discriminator, and the datasheet should state what it was tested against.
A yfp antibody in a blot and in a section
A yfp antibody is bought when the reporter has to be detected in a fixed sample, in a pulldown, or where the protein is present but not fluorescent because it has not matured or has been denatured. Those cases are exactly where fluorescence and antibody readouts disagree, which is the informative situation rather than a problem with either.
A firefly luciferase antibody and the enzyme as an antigen
A firefly luciferase antibody detects the beetle enzyme rather than the marine ones, and the two families share no useful sequence, so a reagent for one does not detect the other. It is used to confirm expression of a reporter construct where the light signal is low, which separates a transfection problem from a promoter one, and an untransfected lane is the control.
An anti phosphoserine antibody and its breadth
An anti phosphoserine antibody recognizes the modified residue in many sequence contexts, so it reports a global phosphorylation state rather than a site and is used for screening and for enrichment rather than for a specific claim. Sequence context still biases it, which is why enrichment with it is not unbiased. Phosphatase treatment of a duplicate sample is the specificity control.
A dsrna antibody and the structure it recognizes
A dsrna antibody binds a double stranded RNA helix regardless of sequence, which is what makes it the reagent for detecting viral replication intermediates and for checking an in vitro transcribed RNA for the by-product that triggers innate sensing. Length dependence differs between clones, so the datasheet's stated minimum duplex length decides what it can see.
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-labeling 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.