Collagen optical properties

The proposed procedure allows a rather accurate assessment of the size and optical properties of large molecular formations. In a recent example (Shcheslavskiy et al. 2006b), we attempted to interrogate the size of molecular assemblies of collagen... 

Shcheslavskiy, V., Petrov, G. I., and Yakovlev, V. V. 2005h. Nonhnear optical properties of collagen in solution. Chem. Phys. Lett. 402 170-74. 

The last word has probably not been written about the complex optical properties of collagen. The situation is undoubtedly so difficult that it will be clarified only after more detailed models for fibril structure appear from other directions. At the moment one should conclude from the optical studies that in collagen fibers there are oriented long particles (fibrils), that within these are also regularly arranged thinner elements (protofibrils or molecular chains), and that the particles are penetrable in optically indeterminable ways by chemical agents whose molecules can be of some size. 

Beside the large changes in the mechanical properties the sol-gel transition is accompanied by an increase of the amount of the specific optical rotation. The changes of the optical properties can be explained by at least a part of the recovering of the original helical structure of the collagen molecules. 

J. Y. Huang and his coworkers used SHG to probe the nonlinear optical properties of purple membrane-poly(vinyl alcohol) films [10]. The SHG is attributed to the naturally oriented dipole layers. Recently Alfano and coworkers have also reported SHG from animal tissues [21]. Even though they attributed the SHG only from the surface term that is due to the broken symmetry at the boundary, they also found some SHG intensity dependence on the tissue constitutes with asymmetric structures such as collagen. Combining this effect, they demonstrated SHG tomography for mapping the structure of animal tissues by use of 100-fs laser pulses at 625 nm [33]. 

The 1/33=0 result implies that it is difficult to produce an axial second-order polarization with an optical field parallel to the myosin/actin filaments and that the radiated SHG are contributed mainly from d =dacx and dis=dxzx- Note that the non-zero d i and dis suggest the chirahty in myosin/actin filaments inside a myofibril. This suggestion comphes with a previous conjecture, which states that the nonlinear optical properties in several biological materials (e.g. collagen matrix and muscle) are induced or enhanced by the chirality in their structure [68,69]. 

The vitreous situates behind the lens and occupies 80% of the volume of the eyeball. It is a gel that is 99% water, with collagen fiber, sodium hyaluronic acid, and a small amount of soluble protein and glycoproteins. The proposed structure is a composite of collagen fibers and coiled hyaluronic acid in a network. This stmcture operates as a stable hydrogel to maintain the mechanical and optical properties of the vitreous (humor) [18]. A stmcture in which hyaluronic acid is entangled with collagen fibers was proposed based on electron microscopy as shown in Fig. 2 [19]. This fiber is a one-dimensional (ID) rather than two-dimensional (2D) stmcture for the cornea and is fiuee-dimensional (3D) for the lens. 

In the last few years, attention has focused primarily on theoretical and experimental optical rotatory studies of the a-helix and the /3-structiu es in proteins and polypeptides (see article in this volume by Urnes and Doty). (For recent reviews, see Schellman and Schellman, 1958,1961 Blout, I960 Yang, 1961.) Collagen, when considered at all, has been quickly put aside as a rather unpleasant exception to a number of otherwise generally applicable empirical generalizations. Yet the rotatory properties of the poly-L-proline Il-type helices of collagen are at least as striking and as characteristic as those of the a-helix. A brief comparative consideration of the rotatory parameters of the two systems makes this clear ... 

Certain other features of the optical rotatory properties are worthy of comment. First, as mentioned previously and as might be expected on X-ray grounds, the structural portion of the specific rotation of collagen is very close to that obtained with poly-L-proline II. Thus (see Table IX) at the sodium D-line the specific rotation of collagen is —400°, while the residue rotation is about —125°, yielding a configurational contribution... 

The reversion of gelatins to collagen has also been discussed with refer ence to cross-links. The denaturation of collagen results in marked changes of viscosity, optical rotation, molecular weight, volume, kinetics of proteolysis, and other properties. Under certain conditions a partial reversal of these changes can be achieved (Flory and Garrett, 1958 von Hippel... 

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