Detergents are amphiphilic compounds usually having a well-defined hydrophilic domain often referred to as the "hydrophilic head" and a separate hydrophobic domain often referred to as the "hydrophobic tail". Stable isotope-labeled detergents are that the one or more atoms of detergents are replaced by their stable isotope. Usually, the stable isotope-labeled detergents that currently commercially available are deuterium detergents. Deuteriumdetergents include three major categories of ionic detergents, non-ionic detergents and amphoteric detergents. Deuterium detergents are indispensable for structural and functional studies of integral membrane proteins in solution-state NMR spectroscopy technology. That is the primary use of deuterium detergents.
Whether as a monomer or as a micelle, or in combination with other molecules, the high aqueous solubility of deuterium detergents makes they play a crucial role in the extraction, purification and stabilization of integral membrane proteins, then achieving the best quality NMR spectra.
Fig. 1 Illustration of a protein-detergent micelle complex.
Typical deuterium detergents and their applications
Deuterated detergents eliminate interfering proton signals coming from the detergent itself and also eliminate potential proton relaxation pathways and strong dipole-dipole interactions that contribute line broadening effects. Using deuterated detergents therefore provides better resolution and sensitivity of NMR and can access to overlapped areas of the protein spectrum, which make determination of the structure and dynamics of membrane proteins easier compared to using unlabeled agents . Here are some typical deuterated detergents that used in research of membrane proteins. The details are as follows.
Sodium dodecyl sulphate
Sodium dodecyl sulphate (SDS), also known as sodium lauryl sulphate, is an anionic detergent with an aliphatic 12-carbonchain and a small negatively charged head group. d25-SDS has been
one of the most commonly used deuterated detergents for solubilising membrane proteins, principally for solution-state NMR measurements of structure, dynamics and ligand binding interactions. For example, early NMR studies with the ion channel-forming pentadecapeptide gramicidin A achieved high-resolution 2D spectra for the peptide solubilized in d25-SDS micelles. These were used were used to confirm the structure of ion-channel state of gramicidin A. In addition to NMR, d25-SDS has also found use in small angle neutron scattering (SANS) studies with membrane proteins as a contrasting reagent, especially for investigation of association states and conformational changes.
Lauryldimethylamine-oxide (LDAO), also known as N, N dimethyldodecylamine-N-oxide (DDAO), is a non-denaturing zwitterionic detergent with an aliphatic 12-carbon chain. Uniformly deuterated LDAO (d31-LDAO) is used to solubilize a small number of membrane proteins for determination of their 3D structure by solution-state NMR. For example, the solution-state NMR structure determination of human voltage-dependent anion channel (VDAC-1) used d31-LDAO for solubilization revealing a novel β-barrel fold with 19 transmembrane strands and with the first and last strands parallel with each other giving a closed structure.
Fig. 2 Structure of human VDAC-1 shown as a side view (left) and from above (right) with the N-terminus in blue and C-terminus in red.
n-Octyl-β-D-glucoside (β-OG) is a non-ionic detergent with an aliphatic eight-carbon chain. The commercially available deuterated β-OG is usually in aliphatic chain deuterated (d17-β-OG) and in perdeuterated form (d24-β-OG). The best-known NMR structure for a membrane protein solubilized in d24-b-OG micelles is that for the bacterial outer membrane enzyme PagP, which transfers a palmitate chain from a phospholipid to lipid A. d17-β-OG has also been used for solubilization of membrane proteins in small angle neutron scattering (SANS) experiments.
Fig. 3 Structure of PagP with the N-terminus in blue and C-terminus in red.
n-Dodecyl-b-D-maltoside (DDM) is a non-ionic detergent with an aliphatic 12-carbon chain, which tends to disrupt lipid-lipid and lipid-protein interactions but not protein-protein interactions. d25-DDM is common in the market but a perdeuterated form (d39-DDM) is rare. d25-DDM has been used in solution-state NMR studies with bacteriorhodopsin and with the potassium ion channel KcsA. It has also been used in investigations of the composition of supported model membranes determined by neutron reflection, in studies of hydrogen oxidation by a membrane-bound hydrogenase immobilized on gold electrodes and in small angle neutron scattering (SANS) studies of solubilized membrane proteins.
The fos-cholines are zwitterionic detergents with a range in lengths of their aliphatic carbon chain. By far the most commonly used of these is foscholine-12, also known as dodecylphosphocholine or DPC. Perdeuterated DPC (d38-DPC), semi-deuterated head (d9-DPC), a perdeuterated head (d13-DPC) and with just the aliphatic chain deuterated (d25-DPC) are common in market. However, approximately a half of the NMR membrane protein structures used d38-DPC. One of the first NMR structures of a β-barrel membrane protein, the transmembrane domain of OmpA, was determined with the protein solubilized in d38-DPC micelles. And the NMR structures of a-helical membrane proteins are determined in d38-DPC micelles include amyloid precursor protein transmembrane domains.
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- Hiruma-Shimizu, K., et al. Deuterated detergents for structural and functional studies of membrane proteins: Properties, chemical synthesis and applications[J]. Molecular Membrane Biology, 2015, (5-8):139-155
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