From Functional Groups to Experimental Decision-Making: Protein Crosslinker Selection and Experimental Essentials

1. Define the Experimental Objective First

Crosslinkers are used to form covalent linkages between two molecules, or between adjacent structural elements within the same molecule. In protein experiments, crosslinkers are mainly used to address four types of tasks: capturing interactions, preparing conjugates, immobilizing proteins, and assisting structural analysis. When selecting a crosslinker, first determine the experimental objective, then evaluate the reactive functional groups, spacer arm, solubility, membrane permeability, and detection method. Crosslinker selection should take reaction specificity, spacer arm length, solubility, and the specific application scenario into account simultaneously.

Experimental objective	Preferred strategy	Key consideration
Capture protein-protein interactions	Homobifunctional crosslinkers	The two ends carry the same reactive group and are suitable for fixing spatial proximity relationships
Prepare antibody, enzyme, or peptide conjugates	Heterobifunctional crosslinkers	The two ends carry different reactive groups, allowing stepwise conjugation and reducing self-polymerization
Immobilize proteins or peptides on carboxylated surfaces	Zero-length coupling systems	Directly link surface carboxyl groups to protein primary amines without introducing an additional spacer arm
Crosslinking mass spectrometry analysis	MS-cleavable or cleavable crosslinkers	Control the degree of crosslinking and reduce sample complexity

Common selection logic:

Question	Corresponding choice
Is the study limited to cell-surface proteins?	Prefer water-soluble, membrane-impermeable crosslinkers
Is intracellular access required?	Choose membrane-permeable crosslinkers
Is subsequent dissociation of the complex required?	Choose a cleavable spacer arm
Is a long-term stable linkage required?	Choose a non-cleavable linkage
Is retention of antibody binding or enzyme activity required?	Reduce the degree of modification and prioritize stepwise conjugation or site-specific conjugation

2. Choose the Reaction Chemistry According to the Target Functional Group

Common reactive sites in protein crosslinking include primary amines, sulfhydryls, and carboxyl groups. Primary amines mainly come from lysine side chains and the protein N-terminus; sulfhydryls mainly come from free cysteine residues; carboxyl groups mainly come from aspartic acid, glutamic acid, the protein C-terminus, or the surface of carboxylated materials.

Target group	Common reaction system	Representative reagents	Common conditions	Components or conditions to avoid
Primary amines	N-hydroxysuccinimide esters (NHS esters) or sulfo-N-hydroxysuccinimide esters (sulfo-NHS esters)	Bis(sulfosuccinimidyl) suberate (BS3), disuccinimidyl suberate (DSS)	Commonly used at pH 7.2 to 8.5	Tris(hydroxymethyl)aminomethane (Tris), glycine, lysine, ethanolamine
Sulfhydryls	Maleimide chemistry	Sulfo-SMCC, SMCC, N,N′-bismaleimidohexane (BMH)	Commonly used at pH 6.5 to 7.5	Dithiothreitol (DTT), β-mercaptoethanol, and other thiol-containing reducing agents
Carboxyl groups plus primary amines	1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) combined with NHS or sulfo-NHS	EDC/NHS, EDC/sulfo-NHS	The activation step is commonly carried out at pH 4.5 to 7.2; after activation, reaction with primary amines is often performed around pH 7 to 8	Exogenous primary amines, exogenous carboxylic acids, and protecting agents containing interfering components
Primary amines	Imidoester chemistry	Dimethyl pimelimidate (DMP)	Higher efficiency at pH 8 to 9	Amine-containing buffers such as Tris and glycine

Supplementary notes

1. NHS ester reactions

NHS esters react with primary amines to form stable amide bonds and are commonly used for modification of lysine side chains and protein N-termini. NHS esters undergo hydrolysis in aqueous solution, and hydrolysis becomes faster as buffer pH increases. Tris and glycine can be used to quench the reaction after completion, but they should not be present at the start of the reaction.

2. Maleimide reactions

Maleimides react preferentially with sulfhydryls at pH 6.5 to 7.5 to form stable thioether bonds. Thiol-containing reducing agents such as DTT and β-mercaptoethanol will compete for reactive sites, so desalting or buffer exchange is usually required after reduction treatment.

3. EDC-mediated carboxyl activation

EDC is a zero-length coupling reagent. It first activates carboxyl groups to form reactive intermediates, which then react with primary amines to form amide bonds. EDC itself is not retained in the final linkage structure, so it is suitable for carboxyl-to-amine coupling and surface immobilization when a short linkage distance is desired.

4. DMP imidoester reactions

DMP is an imidoester crosslinker rather than an NHS ester. It reacts with primary amines and is commonly used for protein amine-to-amine crosslinking and for immobilizing antibodies onto protein A- or protein G-based supports.

3. Control Crosslinking Outcomes with the Spacer Arm

The spacer arm determines the distance between the two linked groups and also affects the flexibility, hydrophilicity, membrane permeability, and downstream analytical difficulty of the crosslinked product.

Selection point	Experimental impact	Typical use case
Short spacer arm	Stricter spatial restriction; crosslinking efficiency may be lower	Assessing close-range interactions
Long spacer arm	Makes linkage to target groups easier, but lowers spatial interpretation precision	When the distance between target groups is uncertain
Polyethylene glycol spacer arm	Improves hydrophilicity and reduces hydrophobic aggregation	When conjugates precipitate easily or improved solubility is needed
Disulfide-containing spacer arm	Can be cleaved by reducing agents	When complexes need to be compared under reducing and non-reducing conditions
Non-cleavable spacer arm	Gives more stable linkage	Preparation of protein conjugates intended for long-term use

Common reagent differences:

Reagent	Reaction type	Solubility and membrane permeability	Suitable use
BS3	Amine-amine	Water-soluble, membrane-impermeable	Cell-surface protein crosslinking
DSS	Amine-amine	Water-insoluble, membrane-permeable	Crosslinking of intracellular proteins or membrane-associated proteins
DTSSP	Amine-amine	Water-soluble; central disulfide bond can be reductively cleaved	Protein complexes that need to be dissociated under reducing conditions
Sulfo-SMCC	Amine-sulfhydryl	Water-soluble	Stepwise conjugation of antibodies, enzymes, and peptides
SMCC	Amine-sulfhydryl	Water-insoluble and must first be dissolved in an organic solvent	Conjugation systems that can tolerate a small amount of organic solvent
DMP	Amine-amine	Water-soluble, membrane-permeable	Immobilization of antibodies onto protein A/G supports and crosslinking of protein complexes

4. Key Operating Conditions

Crosslinking failure is often caused by buffer incompatibility, crosslinker hydrolysis, inaccessibility of the target group, excessive reagent loading, or insufficient purification. Before the reaction, samples should first be exchanged into a compatible buffer, and conditions should be determined using small-scale gradients.

Reaction system	Recommended buffer	pH	Quenching or purification
NHS ester-primary amine	Phosphate, carbonate, borate, or 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer	Commonly used at 7.2 to 8.5	Quench with Tris or glycine; remove small molecules by desalting or dialysis
Maleimide-sulfhydryl	Phosphate or HEPES buffer; ethylenediaminetetraacetic acid (EDTA) may be added to reduce metal-catalyzed oxidation	Commonly used at 6.5 to 7.5	Remove free crosslinker; if necessary, quench with free sulfhydryl compounds
EDC/NHS carboxyl activation	2-(N-morpholino)ethanesulfonic acid (MES) buffer is commonly used	Commonly used at 4.5 to 7.2	Add the amine-containing molecule after activation; cap residual active sites after the reaction
DMP imidoester reaction	Phosphate, borate, carbonate, or HEPES buffer	Higher efficiency at 8 to 9	Can be quenched with acid or primary amines, followed by removal of excess small molecules

Operational control points:

Step	Key point
Crosslinker preparation	NHS esters, maleimides, and imidoester reagents should be prepared fresh whenever possible
Reagent ratio	Set low, medium, and high molar ratio gradients; avoid using a highly excessive condition from the start
Reaction time	Start screening with short reaction times; optimization is often performed within the range of 30 minutes to 2 hours
Protein concentration	Excessively high protein concentration can lead to multimer formation; excessively low concentration may reduce crosslinking efficiency
Organic solvents	Water-insoluble reagents such as DSS and SMCC should first be dissolved in dimethyl sulfoxide (DMSO) or dimethylformamide (DMF), with the final solvent concentration controlled
Post-treatment	Desalting, ultrafiltration, dialysis, or size-exclusion chromatography can be used to remove free crosslinker and by-products

5. Three Common Experimental Scenarios

5.1 Capturing cell-surface receptor-ligand interactions

Item	Selection
Recommended reagent	BS3; choose DTSSP if reductive cleavage is required
Reason for selection	Water-soluble and membrane-impermeable, making it suitable for preferentially fixing neighboring proteins on the cell surface
Reaction buffer	Phosphate or HEPES buffer without primary amines
Key controls	No-crosslinker control, competing ligand control, reducing/non-reducing condition control
Detection methods	Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), immunoblotting, immunoprecipitation, liquid chromatography-tandem mass spectrometry (LC-MS/MS)

Common problems:

Observation	Cause	Solution
High-molecular-weight smearing	Excess crosslinker, overly long reaction time, or excessively high protein concentration	Reduce the crosslinker ratio and shorten the reaction time
No obvious complex formation	Surface primary amines are inaccessible, reaction conditions are unsuitable, or the crosslinker has hydrolyzed	Change the spacer length, prepare the reagent fresh, and optimize the pH
High background signal	Intracellular proteins are involved, lysis conditions are too harsh, or washing is insufficient	Use membrane-impermeable reagents and increase washing and negative controls

5.2 Preparing antibody-peptide or antibody-enzyme conjugates

Item	Selection
Recommended reagent	Sulfo-SMCC
Reason for selection	One end reacts with primary amines on the antibody or carrier protein, and the other end reacts with sulfhydryls on the peptide or enzyme
Reaction mode	Stepwise conjugation
Intermediate treatment	Free crosslinker must be removed after the first step
Result verification	SDS-PAGE, size-exclusion chromatography, antigen-binding assay, enzyme activity assay

Recommended workflow:

Step	Operational focus
First step	Modify primary amines on the antibody or carrier protein with sulfo-SMCC
Purification	Remove free sulfo-SMCC by desalting or size-exclusion chromatography
Second step	Add a peptide, enzyme, or small molecule containing a free sulfhydryl; if the target itself does not have an available free sulfhydryl, a sulfhydryl can first be introduced using 2-iminothiolane hydrochloride (Traut’s Reagent) or SATA
Optimization	Set input gradients for the peptide or enzyme
Verification	Evaluate both conjugation efficiency and retention of function

Common problems:

Observation	Cause	Solution
Low conjugation efficiency	Sulfhydryl oxidation, interference from free reducing agents, or maleimide hydrolysis	Reduce immediately before use, desalt, and proceed rapidly to conjugation
Decreased antibody binding ability	Random lysine modification affects the binding region	Reduce the degree of modification or switch to site-specific sulfhydryl conjugation
Aggregation	Multisite crosslinking or excessive reagent loading	Reduce the ratio of crosslinker and peptide, and increase purification steps

5.3 Immobilizing proteins on carboxylated magnetic beads or carboxylated surfaces

Item	Selection
Recommended system	EDC combined with NHS or sulfo-NHS
Reason for selection	After carboxyl activation, stable amide bonds are formed with protein primary amines
Suitable objects	Carboxylated magnetic beads, carboxylated microspheres, carboxylated chips, carboxylated plate surfaces
Key controls	No-EDC control, no-protein control, blank surface control after blocking
Result verification	Immobilized amount, retained amount after washing, binding capability, enzyme activity, or antigen recognition ability

Recommended workflow:

Step	Operational focus
Surface activation	React the carboxylated surface with EDC/NHS
Protein addition	Use a protein solution free of exogenous primary amines
Blocking	Block residual active sites with ethanolamine or similar agents
Washing	Remove non-covalently adsorbed protein
Verification	Compare immobilization amount and retention of function

Common problems:

Observation	Cause	Solution
Low immobilization level	Insufficient carboxyl activation or incompatible protein buffer	Optimize activation time and buffer conditions
High immobilization level but low activity	Random conjugation affects active sites	Reduce the degree of surface activation and adjust protein orientation
High background adsorption	Insufficient surface blocking	Optimize the blocking agent and washing conditions

6. Result Verification and Troubleshooting

Successful crosslinking should not be judged only by the appearance of high-molecular-weight bands. It is also necessary to determine whether the product matches the experimental objective. For structural analysis, attention should be paid to whether the crosslinking sites are interpretable; for conjugate preparation, whether function is retained; for immobilization, whether stability and activity are maintained after washing. Crosslinking combined with mass spectrometry can be used to analyze protein conformation and protein-protein interactions, but excessive crosslinking, high sample complexity, and insufficient data analysis can all reduce result reliability.

Detection method	Question answered
SDS-PAGE	Whether dimers, multimers, or an upward molecular-weight shift appear
Immunoblotting	Whether the target protein is present in the crosslinked complex
Size-exclusion chromatography	Whether aggregates, free protein, and the target conjugate are present
LC-MS/MS	Whether the crosslinking sites support spatial proximity
Enzyme activity assay	Whether the enzyme retains activity after conjugation or immobilization
Antigen-binding assay	Whether the antibody retains recognition ability after conjugation
Surface binding assay	Whether the immobilized protein remains stably retained and can bind the target molecule

Failure mode	Common cause	Solution
Insufficient crosslinking	Inaccessible target groups, reagent hydrolysis, or unsuitable pH	Switch to a longer spacer arm, prepare reagents fresh, and optimize the pH
Over-crosslinking	Excess crosslinker or overly long reaction time	Reduce the reagent ratio and shorten the reaction time
Protein precipitation	Too much hydrophobic crosslinker, overly high organic solvent content, or excessive protein modification	Switch to a water-soluble crosslinker and reduce the final concentration of organic solvent
Loss of function after conjugation	Modified sites affect the active region or binding interface	Reduce the degree of modification or switch to site-specific conjugation or a longer spacer arm
Maleimide reaction failure	Sulfhydryl oxidation or competing reaction from reducing agents	Reduce immediately before use, desalt, and conjugate rapidly
High background in EDC immobilization	Insufficient surface blocking or nonspecific adsorption	Add blank surface controls and optimize blocking and washing
Complex mass spectrometry results	Over-crosslinking, excessive sample complexity, or lack of controls	Reduce the degree of crosslinking and first validate in a purified system

7. Classification, Features, and Applications of Representative Chemicals Related to Protein Crosslinker Selection and Experimental Essentials (Tables 1-6)

Table 1. Buffers, Dilution Media, and Organic Solvents for Crosslinking Reactions

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Carboxyl activation buffer	4432-31-9	M108952	MES	For plant cell culture, ≥99.5%	Used to prepare reaction systems free of primary amines; commonly used for carbodiimide-mediated carboxyl activation and immobilization on carboxylated materials
Borate buffer component	10043-35-3	B111604	Boric acid	Molecular biology grade, ≥99.5%(T)	Used to prepare borate buffer; applicable to imidoester crosslinking and some primary amine modification systems
Bicarbonate buffer component	144-55-8	S112338	Sodium bicarbonate	Molecular biology grade, ≥99.7%	Used to prepare bicarbonate buffer; commonly used for reactions between active esters and protein primary amines
Phosphate buffer component	7558-79-4	S118443	Sodium phosphate dibasic	Molecular biology grade, ≥99.5%(T)	Used with sodium dihydrogen phosphate to prepare phosphate buffer for protein crosslinking, sulfhydryl conjugation, and elution systems
Tris buffer system component	77-86-1	T110602	Tris(hydroxymethyl)aminomethane (Tris base)	For cell culture, ≥99.9%(T)	Commonly used as a quenching agent or buffer component; because it contains a primary amine, it is usually avoided at the start of active ester reactions
Organic cosolvent	67-68-5	D103280	Dimethyl sulfoxide (DMSO)	Pharmaceutical grade, PharmPure™	Used to dissolve water-insoluble crosslinkers and hydrophobic modification reagents, allowing small-volume addition into protein reaction solutions
Carbonate buffer component	497-19-8	S774703	Sodium carbonate	Anhydrous grade, PharmPure™, JP, BP, European Pharmacopoeia (Ph. Eur.), NF	Used to prepare carbonate buffer for active ester conjugation under conditions that favor deprotonation of primary amines
Organic cosolvent	68-12-2	D119450	N,N-Dimethylformamide (DMF)	Anhydrous grade, ≥99.8%	Used to dissolve hydrophobic crosslinkers, protected thiolation reagents, and some photo-crosslinking reagents
HEPES buffer system component	7365-45-9	H774625	HEPES	Animal-free, for cell culture, ≥99.5%, ultra-low endotoxin	Suitable for mild protein reaction conditions; commonly used in sulfhydryl conjugation and cell-surface crosslinking buffer systems
Phosphate buffer component	10049-21-5	S431210	Sodium dihydrogen phosphate monohydrate	European Pharmacopoeia (Ph. Eur.), suitable for analysis, ACS, premium grade	Used with sodium phosphate dibasic to prepare phosphate buffer for protein crosslinking, washing, and solution preparation before desalting

Table 2. Broad-Spectrum Fixatives, Reducing Agents, Quenching/Blocking Reagents, and Detection-Auxiliary Reagents

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Aldehyde broad-spectrum fixative/crosslinker	111-30-8	G105908	Glutaraldehyde	Photographic grade, 50% in H2O	Strong aldehyde crosslinking fixative; can be used for protein fixation, enzyme immobilization, and surface activation of carriers
Metal ion chelating additive	6381-92-6	E118596	Ethylenediaminetetraacetic acid disodium salt dihydrate	For plant cell culture, ≥99%	Chelates metal ions and slows sulfhydryl oxidation; commonly used in sulfhydryl conjugation and protein preservation systems
Sulfhydryl reducing agent	60-24-2	M301574	2-Mercaptoethanol	For cell culture, suitable for electrophoresis, molecular biology grade, ≥99%	Reduces disulfide bonds and is commonly used in sample pretreatment; usually needs to be removed before sulfhydryl conjugation
Reaction quenching/blocking reagent	141-43-5	E103809	Ethanolamine	For cell culture, ≥99%	Used to cap residual active esters or activated surfaces; commonly used in post-treatment after immobilization and affinity carrier conjugation
Sulfhydryl reducing agent	3483-12-3	D104861	DL-Dithiothreitol	Suitable for electrophoresis, ≥99%	Reduces disulfide bonds and can also cleave disulfide-containing linkages; usually needs to be removed by desalting before sulfhydryl conjugation
Aldehyde broad-spectrum fixative/crosslinker	50-00-0	F111941	Formaldehyde solution	Molecular biology grade, ≥36.0% in H2O(T), contains 10-15% methanol as stabilizer	Commonly used for cell fixation and capture of protein interactions; suitable for systems requiring relatively mild fixation conditions
Sulfhydryl reducing agent	51805-45-9	T755610	Tris(2-carboxyethyl)phosphine hydrochloride (TCEP HCl)	UltraBio™, suitable for electrophoresis, SDS-PAGE tested	A non-thiol reducing agent suitable for liberating free cysteine residues; usually does not need to be removed in advance before maleimide conjugation, while for other sulfhydryl conjugation systems, desalting should be determined according to the reagent instructions
Reaction quenching/blocking reagent	56-40-6	G432934	Glycine	UltraBio™, molecular biology grade, ultrapure grade, ≥99%(NT)	Commonly used to quench active ester reactions and to quench residual aldehyde groups after fixation
Sulfhydryl quantification reagent	69-78-3	D105559	5,5'-Dithio bis-(2-nitrobenzoic acid)	≥98%	Used to quantify free sulfhydryl groups and assess reduction efficiency and the degree of thiolation
Deprotection/cleavage auxiliary reagent	5470-11-1	H112477	Hydroxylammonium chloride	PrimorTrace™, ≥99.99% metals basis	Used to remove acetyl protecting groups and release sulfhydryl groups; can also be used in the post-treatment of some cleavable linkages
Reaction quenching/blocking reagent	56-87-1	L424803	L-Lysine	Moligand™, 10 mM in Water	Contains a primary amine; can be used for active ester quenching, surface blocking, and competitive blocking
Aldehyde broad-spectrum fixative/crosslinker	30525-89-4	C104188	Paraformaldehyde	AR	Polymerized formaldehyde commonly used for preparing fixation solutions; suitable for cell and tissue fixation

Table 3. Auxiliary Reagents for Carboxyl Activation, Sulfhydryl Introduction, and Protected Thiolation

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Zero-length carboxyl activation reagent	25952-53-8	E106172	N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride	≥98%	Zero-length carboxyl activation reagent used for direct conjugation of carboxyl groups with primary amines and for surface immobilization
Primary amine thiolation reagent	4781-83-3	I106793	2-Iminothiolane hydrochloride	≥98%	Converts primary amines into sulfhydryl groups, facilitating subsequent maleimide, iodoacetyl, or pyridyldisulfide conjugation
Carboxyl activation auxiliary reagent	6066-82-6	H109330	N-Hydroxysuccinimide (NHS)	≥98%	Used with carbodiimides to form active esters and improve conjugation efficiency between carboxyl groups and primary amines
Water-soluble carboxyl activation auxiliary reagent	106627-54-7	H109337	N-Hydroxysulfosuccinimide sodium salt	≥98%	Improves the stability of activated esters in aqueous phase and is suitable for conjugation of proteins with carboxylated materials
Protected sulfhydryl introduction reagent	84271-78-3	N134552	N-Succinimidyl-S-acetylthiopropionate	≥95%	Introduces protected sulfhydryl groups into proteins or peptides; after deprotection, site-directed sulfhydryl conjugation can be performed
Protected sulfhydryl introduction reagent	76931-93-6	N159207	N-Succinimidyl S-Acetylthioglycolate	≥94%	Introduces short-chain protected sulfhydryl groups into primary amine-containing molecules for subsequent sulfhydryl crosslinking and labeling

Table 4. Homobifunctional Amine-Reactive Crosslinkers (NHS Ester Type, PEG Spacer Type, Cleavable Type, and MS-Cleavable Type)

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
MS-cleavable amine-amine crosslinker	1351828-03-9	D1455113	Disuccinimidyl sulfoxide	≥99%	MS-cleavable amine-amine crosslinker suitable for crosslinking mass spectrometry analysis of protein interactions and conformational restraints
PEG spacer amine-amine crosslinker	1008402-79-6	B595372	Bis-PEG17-NHS ester	≥98%	Long PEG spacer active ester used to improve conjugate hydrophilicity and reduce aggregation
Non-cleavable NHS ester amine-amine crosslinker	68528-80-3	D155694	Di(N-succinimidyl) Suberate	≥98%	Hydrophobic homobifunctional amine-amine crosslinker suitable for crosslinking soluble proteins and membrane-related systems
PEG spacer amine-amine crosslinker	756526-03-1	B595340	Bis-PEG5-NHS ester	≥98%	Medium-length PEG spacer active ester that improves water solubility and post-conjugation dispersibility
Non-cleavable NHS ester amine-amine crosslinker	79642-50-5	D304655	Disuccinimidyl glutarate	≥98%	Shorter spacer amine-amine crosslinker suitable for capturing close-range spatial proximity in proteins
Reducibly cleavable NHS ester amine-amine crosslinker	57757-57-0	D155705	Di(N-succinimidyl) 3,3'-Dithiodipropionate [Cross-linking Reagent]	≥97%	Amine-amine crosslinker containing a disulfide bond that can be dissociated under reducing conditions after crosslinking
Ester-cleavable NHS ester amine-amine crosslinker	70539-42-3	E595347	EGS Crosslinker (EGNHS)	≥96%	Ester bond-containing amine-amine crosslinker used in conjugation systems requiring subsequent chemical cleavage
MS-cleavable amine-amine crosslinker	1240387-33-0	U486584	Urea crosslinker – C4-arm, NHS ester (DSBU, BuUrBU)	≥95%(NMR)	MS-cleavable urea-based crosslinker suitable for studies of protein complexes and conformation
Water-soluble sulfo-NHS ester amine-amine crosslinker	82436-77-9	B596966	BS3 Crosslinker	≥90%	Water-soluble homobifunctional amine-amine crosslinker commonly used for cell-surface protein crosslinking
Water-soluble ester-cleavable amine-amine crosslinker	167410-92-6	S1452932	Sulfo-EGS	——	Water-soluble ester bond-containing amine-amine crosslinker suitable for aqueous protein conjugation and subsequent cleavage analysis

Table 5. Imidoester Amine-Amine Crosslinkers and Sulfhydryl-Sulfhydryl Crosslinkers

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Imidoester amine-amine crosslinker	34490-86-3	D154729	Dimethyl Suberimidate Dihydrochloride	≥98%(T)	Imidoester amine-amine crosslinker that reacts with primary amines under mildly alkaline conditions and is suitable for protein complex crosslinking
Imidoester amine-amine crosslinker	58537-94-3	D155696	Dimethyl Pimelimidate Dihydrochloride	≥98%	Commonly used to immobilize antibodies onto protein A or protein G supports, reducing antibody contamination during immunoprecipitation elution
Reducibly cleavable imidoester amine-amine crosslinker	38285-78-8	D333975	Dimethyl 3,3′-dithiopropionimidate dihydrochloride	≥95%	Imidoester crosslinker containing a disulfide bond that can be cleaved under reducing conditions
Short-chain sulfhydryl-sulfhydryl crosslinker	5132-30-9	E134182	1,2-Bis(maleimido)ethane	≥98%	Short spacer sulfhydryl-sulfhydryl crosslinker suitable for close-range linkage between two cysteine sites
Medium-chain sulfhydryl-sulfhydryl crosslinker	28537-70-4	B137157	1,4-Bis(maleimido)butane	≥96%	Medium spacer sulfhydryl-sulfhydryl crosslinker used for bridging cysteine sites
Long-chain sulfhydryl-sulfhydryl crosslinker	4856-87-5	B136336	1,6-Bis(maleimido)hexane	≥97%	Longer spacer sulfhydryl-sulfhydryl crosslinker that can be used for protein sulfhydryl bridging and conformational restriction

Table 6. Amine-Sulfhydryl Heterobifunctional Crosslinkers and Photo-Reactive Crosslinkers

Classification	CAS No.	Aladdin Cat. No.	Name	Specification or Purity	Product Features and Applications
Aromatic maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	58626-38-3	N159715	N-Succinimidyl 3-Maleimidobenzoate [Cross-linking Reagent]	≥98%(HPLC)	Aromatic amine-sulfhydryl heterobifunctional crosslinker used for conjugation between primary amine-containing molecules and sulfhydryl-containing molecules
Cyclohexane maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	64987-85-5	N159712	N-Succinimidyl 4-(N-Maleimidomethyl) cyclohexanecarboxylate (SMCC)	≥98%(HPLC)	Cyclohexane spacer amine-sulfhydryl heterobifunctional crosslinker commonly used for conjugation of antibodies, enzymes, proteins, and peptides
Aliphatic maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	80307-12-6	N159716	N-Succinimidyl 4-Maleimidobutyrate [Cross-linking Reagent]	≥98%(HPLC)	Shorter aliphatic-chain amine-sulfhydryl heterobifunctional crosslinker suitable for conjugation of proteins with small molecules or peptides
Long-chain cyclohexane maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	125559-00-4	N159126	N-Succinimidyl 6-[[4-(N-Maleimidomethyl)cyclohexyl]carboxamido]hexanoate	≥98%(HPLC)	Long-chain amine-sulfhydryl heterobifunctional crosslinker suitable for conjugation systems requiring a certain spacer distance
PEG spacer maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	756525-99-2	O463252	Maleimide-PEG4-NHS Ester	≥98%	PEG spacer amine-sulfhydryl heterobifunctional crosslinker used to improve conjugate hydrophilicity and reduce aggregation
Aromatic maleimide-NHS ester amine-sulfhydryl heterobifunctional crosslinker	79886-55-8	N137694	N-Succinimidyl 4-(p-Maleimidophenyl)butyrate	≥97%(HPLC)	Aromatic spacer amine-sulfhydryl heterobifunctional crosslinker suitable for directional protein-protein conjugation
Photo-reactive heterobifunctional crosslinker	1239017-80-1	N404336	NHS-Diazirine(SDA)	≥97%	One end reacts with primary amines, while the other captures neighboring molecules upon light activation; suitable for studies of transient interactions
Haloacetyl amine-sulfhydryl heterobifunctional crosslinker	72252-96-1	S487253	SIAB (N-succinimidyl (4-iodoacetyl)aminobenzoate)	≥97%	Iodoacetyl amine-sulfhydryl heterobifunctional crosslinker that forms stable thioether bonds with sulfhydryl groups
Haloacetyl amine-sulfhydryl heterobifunctional crosslinker	57159-62-3	S487952	SBAP (succinimidyl 3-(bromoacetamido)propionate)	≥95%	Bromoacetyl amine-sulfhydryl heterobifunctional crosslinker used for covalent linkage between primary amines and sulfhydryl groups
Pyridyldisulfide amine-sulfhydryl heterobifunctional crosslinker	1334177-95-5	N404799	NHS-PEG4-SPDP	≥95%	PEGylated pyridyldisulfide crosslinker that forms reducible disulfide linkages and is suitable for dissociable conjugates
Pyridyldisulfide amine-sulfhydryl heterobifunctional crosslinker	68181-17-9	S164298	SPDP (Succinimidyl 3-[2-pyridyldithio] propionate)	≥95%	Classic pyridyldisulfide amine-sulfhydryl heterobifunctional crosslinker that forms reducible disulfide linkages
Haloacetyl amine-sulfhydryl heterobifunctional crosslinker	39028-27-8	N133327	N-Succinimidyl Iodoacetate	≥95%	Short-chain iodoacetyl amine-sulfhydryl heterobifunctional crosslinker suitable for low-spacer sulfhydryl conjugation
Long-chain pyridyldisulfide amine-sulfhydryl heterobifunctional crosslinker	158913-22-5	S595683	SPDP-C6-NHS ester	≥90%	Long-chain pyridyldisulfide crosslinker suitable for cleavable conjugation requiring a certain spacer distance
Water-soluble long-chain pyridyldisulfide amine-sulfhydryl heterobifunctional crosslinker	169751-10-4	S595682	SPDP-C6-Sulfo-NHS ester	≥90%	Water-soluble long-chain pyridyldisulfide crosslinker suitable for aqueous conjugation between proteins and sulfhydryl-containing molecules
Water-soluble maleimide-sulfo-NHS ester amine-sulfhydryl heterobifunctional crosslinker	185332-92-7	S168124	Sulfo-N-succinimidyl 4-maleimidobutyrate sodium salt	≥90%	Water-soluble maleimide active ester crosslinker suitable for conjugation of proteins with sulfhydryl-containing molecules
Water-soluble aromatic maleimide-sulfo-NHS ester amine-sulfhydryl heterobifunctional crosslinker	92921-25-0	M293201	m-Maleimidobenzoyl-N-hydroxysulfosuccinimide ester (Sulfo-MBS)	≥80%	Water-soluble aromatic maleimide crosslinker suitable for aqueous protein conjugation and surface modification
Water-soluble cyclohexane maleimide-sulfo-NHS ester amine-sulfhydryl heterobifunctional crosslinker	92921-24-9	M123456	4-(N-Maleimidomethyl)cyclohexane-1-carboxylic acid 3-sulfo-N-hydroxysuccinimide ester sodium salt	——	Water-soluble cyclohexane spacer amine-sulfhydryl heterobifunctional crosslinker commonly used for conjugation of antibodies, enzymes, and proteins

Note: The above are representative Aladdin products. For more product specifications, search by “product name/CAS/catalog number” on the Aladdin official website.

References

[1] Hermanson GT. Bioconjugate Techniques. 3rd ed. Amsterdam: Academic Press; 2013.

[2] Mattson G, Conklin E, Desai S, Nielander G, Savage MD, Morgensen S. A practical approach to crosslinking. Molecular Biology Reports. 1993;17(3):167-183. doi:10.1007/BF00986726.

[3] Thermo Fisher Scientific. Amine-Reactive Crosslinker Chemistry.

[4] Thermo Fisher Scientific. Sulfhydryl-Reactive Crosslinker Chemistry.

[5] Thermo Fisher Scientific. Carbodiimide Crosslinker Chemistry.

[6] Thermo Fisher Scientific. Imidoester Crosslinkers: DMA, DMP, DMS, DTBP.

[7] Chavez JD, Bruce JE. Chemical cross-linking with mass spectrometry: a tool for systems structural biology. Current Opinion in Chemical Biology. 2019;48:8-18. doi:10.1016/j.cbpa.2018.08.006.

[8] Piersimoni L, Kastritis PL, Arlt C, Sinz A. Cross-Linking Mass Spectrometry for Investigating Protein Conformations and Protein-Protein Interactions. Chemical Reviews. 2022;122(7):7500-7531. doi:10.1021/acs.chemrev.1c00786.

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