Circular dichroism spectroscopy
المؤلف:
Peter Atkins، Julio de Paula
المصدر:
ATKINS PHYSICAL CHEMISTRY
الجزء والصفحة:
ص490-492
2025-12-07
43
Circular dichroism spectroscopy
Electronic spectra can reveal additional details of molecular structure when experiments are conducted with polarized light, electromagnetic radiation with electric and magnetic fields that oscillate only in certain directions. Light is plane polarized when the electric and magnetic fields each oscillate in a single plane (Fig. 14.16). The plane of polarization may be oriented in any direction around the direction of propagation (the x-direction in Fig. 14.16), with the electric and magnetic fields perpendicular to that direction (and perpendicular to each other). An alternative mode of polarization is circular polarization, in which the electric and magnetic fields rotate around the direction of propagation in either a clockwise or a counter-clockwise sense but remain perpendicular to it and each other. When plane-polarized radiation passes through samples of certain kinds of matter, the plane of polarization is rotated around the direction of propagation. This rotation is the familiar phenomenon of optical activity, observed when the molecules in the sample are chiral (Section 12.3b). Chiral molecules have a second characteristic: they absorb left and right circularly polarized light to different extents. In a circularly polarized ray of light, the electric field describes a helical path as the wave travels through space (Fig. 14.17), and the rotation may be either clockwise or counterclock wise. The differential absorption of left- and right-circularly polarized light is called circular dichroism. In terms of the absorbances for the two components, AL and AR, the circular dichroism of a sample of molar concentration [J] is reported as
where l is the path length of the sample. (14.6) Circular dichroism is a useful adjunct to visible and UV spectroscopy. For example, the CD spectra of the enantiomeric pairs of chiral d-metal complexes are distinctly different, whereas there is little difference between their absorption spectra (Fig. 14.18). Moreover, CD spectra can be used to assign the absolute configuration of complexes by comparing the observed spectrum with the CD spectrum of a similar complex of known handedness. We shall see in Chapter 19 that the CD spectra of biological polymers, such as proteins and nucleic acids, give similar structural information. In these cases the spectrum of the polymer chain arises from the chirality of individual monomer units and, in addition, a contribution from the three-dimensional structure of the polymer itself.
The eye is an exquisite photochemical organ that acts as a transducer, converting radiant energy into electrical signals that travel along neurons. Here we concentrate on the events taking place in the human eye, but similar processes occur in all animals. Indeed, a single type of protein, rhodopsin, is the primary receptor for light throughout the animal kingdom, which indicates that vision emerged very early in evolution ary history, no doubt because of its enormous value for survival. Photons enter the eye through the cornea, pass through the ocular fluid that fills the eye, and fall on the retina. The ocular fluid is principally water, and passage of light through this medium is largely responsible for the chromatic aberration of the eye, the blurring of the image as a result of different frequencies being brought to slightly different focuses. The chromatic aberration is reduced to some extent by the tinted region called the macular pigment that covers part of the retina. The pigments in this region are the carotene-like xanthophylls (3), which absorb some of the blue light and
Fig. 14.18 (a) The absorption spectra of two isomers, denoted mer and fac, of [Co(ala)3], where ala is the conjugate base of alanine, and (b) the corresponding CD spectra. The left- and right-handed forms of these isomers give identical absorption spectra. However, the CD spectra are distinctly different, and the absolute configurations (denoted Λ and ∆) have been assigned by comparison with the CD spectra of a complex of known absolute configuration.
hence help to sharpen the image. They also protect the photoreceptor molecules from too great a flux of potentially dangerous high energy photons. The xanthophylls have delocalized electrons that spread along the chain of conjugated double bonds, and the π*←πtransition lies in the visible. About 57 per cent of the photons that enter the eye reach the retina; the rest are scattered or absorbed by the ocular fluid. Here the primary act of vision takes place, in which the chromophore of a rhodopsin molecule absorbs a photon in another π* ←πtransition. A rhodopsin molecule consists of an opsin protein molecule to which is attached a 11-cis-retinal molecule (4). The latter resembles half a carotene molecule, showing Nature’s economy in its use of available materials. The attachment is by the formation of a protonated Schiff’s base, utilizing the -CHO group of the chromophore and the terminal NH2 group of the sidechain, a lysine residue from opsin. The free 11-cis-retinal molecule absorbs in the ultraviolet, but attachment to the opsin protein molecule shifts the absorption into the visible region. The rhodopsin molecules are situated in the membranes of special cells (the ‘rods’ and the ‘cones’) that cover the retina. The opsin molecule is anchored into the cell membrane by two hydrophobic groups and largely surrounds the chromophore (Fig. 14.19). Immediately after the absorption of a photon, the 11-cis-retinal molecule under goes photoisomerization into all-trans-retinal (5). Photoisomerization takes about 200 fs and about 67 pigment molecules isomerize for every 100 photons that are absorbed. The process occurs because the π* ← π excitation of an electron loosens one of the π-bonds (the one indicated by the arrow in 5), its torsional rigidity is lost, and one part of the molecule swings round into its new position. At that point, the molecule returns to its ground state, but is now trapped in its new conformation. The straightened tail of all-trans-retinal results in the molecule taking up more space than 11-cis-retinal did, so the molecule presses against the coils of the opsin molecule that surrounds it. In about 0.25–0.50 ms from the initial absorption event, the rhodopsin molecule is activated both by the isomerization of retinal and deprotonation of its Schiff’s base tether to opsin, forming an intermediate known as meta rhodopsin II. In a sequence of biochemical events known as the biochemical cascade, meta rhodopsin II activates the protein transducin, which in turn activates a phosphodiesterase enzyme that hydrolyses cyclic guanine monophosphate (cGMP) to GMP. The reduction in the concentration of cGMP causes ion channels, proteins that mediate the movement of ions across biological membranes, to close and the result is a sizable change in the transmembrane potential (see Impact I7.2 for a discussion of transmembrane potentials). The pulse of electric potential travels through the optical nerve and into the optical cortex, where it is interpreted as a signal and incorporated into the web of events we call ‘vision’.
The resting state of the rhodopsin molecule is restored by a series of nonradiative chemical events powered by ATP. The process involves the escape of all-trans-retinal as all-trans-retinol (in which -CHO has been reduced to -CH2OH) from the opsin molecule by a process catalysed by the enzyme rhodopsin kinase and the attachment of another protein molecule, arrestin. The free all-trans-retinol molecule now under goes enzyme-catalysed isomerization into 11-cis-retinol followed by dehydrogenation to form 11-cis-retinal, which is then delivered back into an opsin molecule. At this point, the cycle of excitation, photoisomerization, and regeneration is ready to begin again.
Fig. 14.17 In circularly polarized light, the electric field at different points along the direction of propagation rotates. The arrays of arrows in these illustrations show the view of the electric field when looking toward the oncoming ray: (a) right circularly polarized, (b) left-circularly polarized light.
Fig. 14.19 The structure of the rhodopsin molecule, consisting of an opsin protein to which is attached an 11-cis-retinal molecule embedded in the space surrounded by the helical regions.
الاكثر قراءة في مواضيع عامة في الكيمياء الفيزيائية
اخر الاخبار
اخبار العتبة العباسية المقدسة
