Anisotropic Media

For a fully oriented molecule, the amide ¹H-¹⁵N dipolar coupling is over 20 KHz and the ¹H-¹H dipolar coupling of a proton-proton pair separated by 5 amstrongs can be as large as 1 KHz. Such large values affords highly complicated spectra and, therefore, new techniques should be required to achieve partial alignment and, in this way, slightly anisotropic solutions. Some pre-requisits are:

• Very weak alignment induced by steric and/or electrostatic forces.
• Aqueous medium is required
• No interactions between the orienting media and the solute
• Stability over a range of temperatures, pH, salt concentrations ....
• Minimal adverse effect on rotational diffusion.

Types of weak alignment media:

• Natural Alignment
  Residual one-bond ¹H-¹³C and ¹H-¹⁵N dipolar couplings can be measured for biomolecules having sufficient magnetic susceptibility anisotropy ( 95PNAS9279 , 96JACS6264 and 01JMR23-148 ).
  Using paramagnetic ion lanthanides such as Ytterbium³⁺ ( 99JACS8947 and 00JMR381-146 ) or Tb³⁺ ( 01JB41-21 and 02JACS374 ) .
  Combined use of lanthanide-substituted proteins and external orienting media ( 02JB365-22 )
• Artificial alignment:
  A dilute liquid crystalline medium, consisting of a solution containing magnetically aligned bicelles ( 94PROG421 ):

  Phospholipids dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in water ( 97SCI1111 and 97JB289 ) or with some analogs but shorter chain phospholipids ( 98JB443 , 98JB361 , and 99JB187 ).

  A bicelle system composed of 1,2-di-O-dodecyl-sn-glycero-3-phosphocholine (DIODPC) and 3-(chloramidopropyl)dimethylammonio-2-hydroxyl-1-propane (CHAPSO) very useful under acidic conditions. ( 99JB387 ).

  Charged bicelles that align by electrostatic forces ( 98JACS9107 and 98JB447 ).

  Phospholipid bicelles that align their normals parallel to the magnetic field ( 01JACS1537 and 02JACS11827 ).
  A filamentous phage ( 98NAT1065 , 99JB175 , 00METH220 and 01JB365-20 ).
  A nematic phase of rod-shaped viruses ( 98JACS10571 , 01JMR154-149 ).
  A suspension of planar purple membrane fragments, containing bacteriorhodopsin ( 99JACS1385 and 99JACS2047 ). Effect of lanthanides on the orientation of purple membrane has been described ( 02JACS374 ).
  A lyotropic liquid crystal consisting of an aqueous solution of a mixture of cetylpyridinium chloride and n-hexanol in salt (Nacl or NaBr) ( 98JACS11010 and 00JB329 ).
  Poly(ethylene glycol)-based systems ( 00JACS7793 )
  Dissolving the molecule in an anisotropic aqueous polymeric gel, the so-called SAG or strain-induced alignment in a gel ( 00JACS9340 ).In this case, the direction of molecular alignment is dictated by the direction of strain, not by the magnetic field.

  Use of polyacrylamide gels (00JB303-18 , 00JACS9340 , 01JB141-21 and 01JB377-21 ).

  Combined use of polyacrylamide and Pf1 phage ( 02JB83-22 ).

  Charged acrylamide copolymer gels ( 02JB351-24 ). Application to membrane proteins ( 04JACS16259 ).
  Use of chemical tags coordinating lanthanide ions (( 04JB339-29 ))
• Partial alignment can be also achieved in the presence of electric fields (99JPCA8604 , 00JPCA3326 , 00JMR104-147 and 00JMR361-147 ).
• Assessment of change in several alignment media using RDCs (02JB123-24 , 02JACS12020 and 03JACS5596 )

Miscellaneous

• Magnetic field dependence of RDCs in different liquid cristalline media (04JMR191-170 )
• Diffusion studies on several anisotropic media (01JACS12343 )