Myelin is the insulating sheath surrounding myelinated axons and is composed of ∼ 30 % protein and ∼ 70 % lipid by dry weight.
The CH 2 symmetric and asymmetric vibrational modes are particularly abundant in the long acyl chains of biological cell membranes, and especially in myelin that consists of multiple compacted wraps of extended glial cell membranes. This leads to a large frequency mismatch with other modes that, combined with long CH 2 acyl chains, creates a very distinct C ─ H stretch in lipids. The relatively stationary nature of the carbon limits vibrational coupling to any other adjacent modes. The vibrational resonance of a hydrocarbon chain is primarily due to the motion of the hydrogen atoms, as they are much lighter than carbon. Lipids typically contain long acyl chains with many C ─ H bonds in each molecule, providing a high concentration of Raman-active oscillators. In Raman spectroscopy nomenclature, the “ C ─ H region” contains strong CH 2 and CH 3 stretching modes ranging from ∼ 2800 to 3100 cm − 1. In two-beam CARS configurations, ω pr = ω p and the pump also act as the probe source. A photon from a third beam, the “probe” ( ω pr), then interrogates the excited vibrational mode, generating a blueshifted photon, the “anti-Stokes” ( ω as). 1 – 3ĬARS requires two laser sources, termed “pump” ( ω p) and “Stokes” ( ω s), which are tuned such that the energy difference between the frequencies of the two beams matches the potential energy in the vibrational resonance of a molecule. CARS can, therefore, be used to probe the nanostructural orientation of vibrational bonds within ordered samples. CARS signal generation is strongly dependent on the orientation and concentration of vibrational modes within the sample. Coherent anti-Stokes Raman scattering (CARS) is a label-free nonlinear imaging technique that probes vibrational resonances in molecular bonds to generate contrast.
0 kommentar(er)
