A crystallization adjutant is a material used to promote

crystallization Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...

, normally in a context where a material does not crystallize naturally from a pure solution.

Additives in Macromolecular Crystallization

In macromolecular crystallography, the term additive is used instead of adjutant. An additive can either interact directly with the

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

, and become incorporated at a fixed position in the resulting crystal or have a role within the disordered solvent, that in protein crystals constitute roughly 50% of the lattice volume.

Polyethylene glycol Polyethylene glycol (PEG; ) is a polyether compound derived from petroleum with many applications, from industrial manufacturing to medicine. PEG is also known as polyethylene oxide (PEO) or polyoxyethylene (POE), depending on its molecular wei ...

s of various

molecular weight A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

s and high-ionic strength salts such as

ammonium sulfate Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...

and sodium

citrate Citric acid is an organic compound with the formula . It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry Biochemistry, or biological chemistry, is the study of chemical processes within and relati ...

that induce protein precipitation when used in high concentrations are classified as precipitants, while certain other salts such as

zinc sulfate Zinc sulfate is an inorganic compound with the formula ZnSO4. It forms hydrates ZnSO4·''n''H2O, where ''n'' can range from 0 to 7. All are colorless solids. The most common form includes water of crystallization as the heptahydrate, with the che ...

calcium sulfate Calcium sulfate (or calcium sulphate) is an inorganic salt with the chemical formula . It occurs in several hydrated forms; the anhydrous state (known as anhydrite) is a white crystalline solid often found in evaporite deposits. Its dihydrate ...

that may cause a protein to precipitate vigorously even when used in small amounts are considered adjutants. Crystallization adjutants are considered additives when they are effective at relatively low concentrations. The distinction between

buffer Buffer may refer to: Science * Buffer gas, an inert or nonflammable gas * Buffer solution, a solution used to prevent changes in pH * Lysis buffer, in cell biology * Metal ion buffer * Mineral redox buffer, in geology Technology and engineeri ...

s and adjutants is also fuzzy. Buffer molecules can become part of the lattice (for example

HEPES HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) is a zwitterionic sulfonic acid buffering agent. It is one of the twenty Good's buffers. HEPES is widely used in cell culture, largely because it is better at maintaining physiological pH ...

in Quantitative structure-activity relationship of human neutrophil collagenase (MMP-8) inhibitors using comparative molecular field analysis and X-ray structure analysis.
becomes incorporated in crystals of human neutrophil collagenase) but their main use is to maintain the rather precise pH requirements for crystallization that many proteins have. Commonly used buffers such as

have a high ionic strength and at the typical buffer concentrations they also act as precipitants. Various species such as Ca²⁺ and Zn²⁺ are a biological requirement for certain proteins to fold correctly and certain co-factors are needed to maintain a well defined conformation. Certain strategies, like replacing precipitants and buffers with others intended to have a similar effect, have been used to differentiate between the roles played in protein crystallization by the various components in the crystallization solution.

Additives for Membrane Protein Crystallization

For

membrane protein Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane ...

s, the situation is more complicated because the system that is being crystallized is not the membrane protein itself but the micellar system in which the membrane protein is embedded. The size of the protein-detergent mixed

micelles A micelle () or micella () ( or micellae, respectively) is an aggregate (or supramolecular assembly) of surfactant amphipathic lipid molecules dispersed in a liquid, forming a colloidal suspension (also known as associated colloidal system). ...

are affected by both additives and detergents which will strongly influence the crystals obtained. In addition to varying the concentration of primary detergents, additives (

lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...

and

alcohols In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...

) and secondary detergents can be used to modulate the size and shape of the detergent micelles. By reducing the size of the mixed micelles lattice forming protein-protein contacts are encouraged. Lipid cubic phases, spontaneous self-assembling liquid crystals or lipid mesophases have been used successfully in the crystallization of integral membrane proteins. Temperature, salts, detergents, various additives are used in this system to tailor the cubic phase to suit the target protein. Typical detergents used are n-dodecyl-β-d-maltopyranoside, n-decyl-β-d-glucopyranoside, lauryldimethylamine oxide LDAO, n-hexyl-β-d-glucopyranoside, n-nonyl-β-d-glucopyranoside and n-octyl-β-d-glucopyranoside; the various lipids are dioleoyl

phosphatidylcholine Phosphatidylcholines (PC) are a class of phospholipids that incorporate choline as a headgroup. They are a major component of biological membranes and can easily be obtained from a variety of readily available sources, such as egg yolk or soyb ...

, dioleoyl

phosphatidylethanolamine Phosphatidylethanolamine (PE) is a class of phospholipids found in biological membranes. They are synthesized by the addition of cytidine diphosphate-ethanolamine to diglycerides, releasing cytidine monophosphate. S-Adenosyl methionine, ''S''-Ade ...

and monoolein.

References

External links

A list of adjutants from a German Crystallography laboratoryThe 'Jeffamine' group of compounds, a number of which are commonly used adjutants
{{Webarchive, url=https://web.archive.org/web/20061013144746/http://www.huntsman.com/performance_products/index.cfm?PageID=2136 , date=2006-10-13 Crystallography