4-Chloromercuribenzoic acid (p-chloromercuribenzoic acid, PCMB) is an organomercury compound that is used as a protease inhibitor, especially in molecular biology applications.

4-Chloromercuribenzoic acid^[1]

Names
IUPAC name (4-Carboxyphenyl)chloromercury
Other names p-Chloromercurybenzoic acid; p-Chloromercuribenzoate; 4-Chloromercuribenzoate
Identifiers
CAS Number	59-85-8 Y
3D model (JSmol)	Interactive image
Abbreviations	PCMB
Beilstein Reference	3662892
ChEBI	CHEBI:28420 Y
ChEMBL	ChEMBL575867 Y
ChemSpider	1667 Y
ECHA InfoCard	100.000.402
EC Number	200-442-6
Gmelin Reference	261316
KEGG	C03444
PubChem CID	1730
UNII	E1LE0WZ4BO Y
CompTox Dashboard (EPA)	DTXSID8040725
InChI InChI=1S/C7H5O2.ClH.Hg/c8-7(9)6-4-2-1-3-5-6;;/h2-5H,(H,8,9);1H;/q;;+1/p-1 Y Key: YFZOUMNUDGGHIW-UHFFFAOYSA-M Y InChI=1/C7H5O2.ClH.Hg/c8-7(9)6-4-2-1-3-5-6;;/h2-5H,(H,8,9);1H;/q;;+1/p-1/rC7H5ClHgO2/c8-9-6-3-1-5(2-4-6)7(10)11/h1-4H,(H,10,11) Key: YFZOUMNUDGGHIW-PSWPUYSSAS
SMILES O=C(O)c1ccc([Hg]Cl)cc1
Properties
Chemical formula	C7H5ClHgO2
Molar mass	357.16 g·mol−1
Melting point	287 °C (549 °F; 560 K) (dec.)
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H300, H310, H330, H373, H410
Precautionary statements	P260, P262, P264, P270, P271, P273, P280, P284, P301+P310, P302+P350, P304+P340, P310, P314, P320, P321, P322, P330, P361, P363, P391, P403+P233, P405, P501
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

PCMB reacts with thiol groups in proteins and is therefore an inhibitor of enzymes that are dependent on thiol reactivity, including cysteine proteases such as papain and acetylcholinesterase. Because of this reactivity with thiols, PCMB is also used in titrimetric quantification of thiol groups in proteins.