Chemistry Reference
In-Depth Information
3.1.2 Dodecylphosphocholine
Dodecylphosphocholine (DPC) has a zwitterionic headgroup (Table
1
) with a
reduced tendency to participate in interactions that would disrupt protein structure
[
139
]. It has proven to be particularly useful for NMR structure elucidation of TM
helix interactions, as was first demonstrated with the GpA dimer [
140
]. It has since
been used for some of the largest multi-spanning helical membrane protein systems
determined to date (e.g., DAGK [
38
], phospholamban [
141
], and UCP2 [
41
]). The
ready availability of deuterated DPC, its stable lipid-like headgroup structure, and
its relatively small micelle aggregation number and narrow micelle size distribution
over a range of conditions [
142
] are all favorable features contributing to the
general utility of this detergent. For these reasons DPC has become one of the
most popular detergents for solution NMR of membrane proteins, with more than
one-third of the integral membrane protein NMR structures being determined in
this detergent (Table
2
).
3.1.3 Short-Chain Phosphatidylcholines
For those looking for closer mimics of a native phospholipid bilayer, short chain
phosphatidylcholines, namely 1,2-diheptanoyl- and 1,2-dihexanoyl-
sn
-glycero-3-
phosphocholine (C
7
-DHPC and C
6
-DHPC, respectively), have provided an attractive
alternative [
143
]. These phospholipids differ from those found in biological
membranes in the length of the acyl chains, giving rise to a tendency to form micelles
instead of bilayer structures. Meanwhile the motionally averaged conformational
properties of these lipids resemble those of the long-chain phosphatidylcholine bilayers
at higher temperatures [
143
]. Particularly useful for solution NMR is the low polydis-
persity and stable size of the C
6
-DHPC micelle over a range of concentrations [
144
].
However, the best compromise between sample stability and spectral quality was
actually found with C
7
-DHPC for the seven TM-helix GPCR pSRII [
145
]. Although
the molecular mass and polydispersity of protein-free C
7
-DHPC micelles is highly
dependent on its concentration [
144
], the complexes formed with pSRII were spectrally
homogeneous, illustrating the strong influence of the protein on the properties of the
detergent-protein complex as has been previously observed for other proteins [
146
].
3.1.4 Lysolipids
Lysolipids have also been gaining attention as effective, relatively mild solubiliza-
tion agents [
88
,
147
-
150
]. The polar glycerol spacer between the headgroup and
alkyl chain provides a gentler transition between hydrophobic and charge-
containing phases of the micelle, reducing potentially denaturing effects of the
headgroup charges. This was illustrated with DAGK, which was shown to retain
full activity in lyso-myristoyl phosphatidylglycerol
(LMPG) micelles while
