A reagent reacts with accessible native thiols and has no other targeting mechanism. What limits its use for labelling one protein?

• It must recognise only the intended protein even when others expose thiols
• It cannot react with any accessible thiol in an aqueous environment
• It can label other proteins with accessible thiols

You plan to label a protein in a living cell's near-neutral cytosol. Why choose chemistry that works under those conditions?

• To keep the assay compatible with living cells
• To make every fluorescent reporter emit the same colour
• To stop all native amines and thiols from reacting

A position-only fluorescent tag can show where labelled protein is detected, but its brightness alone does not establish catalytic activity.

Which comparison best tests whether fluorescence depends on an installed handle?

• Handle-bearing cells given extra dye, compared with cells imaged using a dimmer lamp
• Handle-bearing cells without washing, compared with cells given a different reporter
• Matched cells with and without the handle, labelled and imaged under the same conditions

A live-cell assay uses a low reagent concentration and a short labelling period. Which reaction property must you check?

• Slow reaction throughout the available labelling period
• Adequate rate at the chosen concentration and conditions
• High rate measured only in hot, strongly acidic solvent

Many proteins expose native thiols. You can install a unique handle on the target. Which plan offers a more selective route, subject to experimental checks?

• Increase the dose of a reagent that reacts with accessible thiols on many proteins
• Pair the target handle with a reagent that avoids native groups
• Use a brighter general thiol label without changing how it recognises proteins

In a hypothetical matched assay, handle-bearing cells give 8 fluorescence units and no-handle cells give 2. Dose, washing and imaging settings are the same. Which conclusion is supported?

• Background is present; the higher signal supports handle dependence
• Every fluorescent label is on the target because the no-handle control is dimmer
• The no-handle signal proves that neither sample contains any labelled target protein

You want to measure changes in a protein's catalytic activity over time. Which reporter directly addresses that question?

• An activity-sensitive reporter calibrated against that activity
• A position-only tag observed without any link between its signal and catalytic activity
• A brighter position-only tag assumed to measure catalysis from brightness alone