Cryosections of X-ray monitored collagen fibrils provide support for quasi-hexagonal molecular packing

Quantification of the second-order nonlinear susceptibility of collagen I using a laser scanning microscope

Characteristic changes in the organization of fibrillar collagen can potentially serve as an early diagnostic marker in various pathological processes. Tissue types containing collagen I can be probed by pulsed high-intensity laser radiation, thereby generating second harmonic light that provides information about the composition and structure at a microscopic level. A technique was developed to determine the essential second harmonic generation (SHG) parameters in a laser scanning microscope setup. A rat-tail tendon frozen section was rotated in the xy-plane with the pulsed laser light propagating along the z-axis. By analyzing the generated second harmonic light in the forward direction with parallel and crossed polarizer relative to the polarization of the excitation laser beam, the second-order nonlinear optical susceptibilities of the collagen fiber were determined. Systematic variations in SHG response between ordered and less ordered structures were recorded and evaluated. A 500 microm-thick z-cut lithiumniobate (LiNbO(3)) was used as reference. The method was applied on frozen sections of malignant melanoma and normal skin tissue. Significant differences were found in the values of d(22), indicating that this parameter has a potential role in differentiating between normal and pathological processes.

Collagen structure and functional implications

The bio-mechanical requirements to which the connective tissue is subjected suggest that a causal correlation exist between the substructure and the collagen fibril function. We discuss the relationship between the inner structure of collagen fibrils, their diameter, their spatial layout and the functional requirements they have to withstand, and suggest that collagen fibrils may belong to two different forms indicated as "T-type" and "C-type". The first class, consisting of large, heterogeneous fibrils, parallely tightly packed, subjected to tensile stress along their axis is found in highly tensile structures such as tendons, ligaments and bone. The other class, consisting of small, homogeneous fibrils, helically arranged, resisting multidirectional stresses, is mostly present within highly compliant tissues such as blood vessel walls, skin and nerve sheaths. What causes these architectures to appear is discussed in detail in this review.

An energetic evaluation of a "Smith" collagen microfibril model

An energy minimized three-dimensional structure of a collagen microfibril template was constructed based on the five-stranded model of Smith (1968), using molecular modeling methods and Kollman force fields (Weiner and Kollman, 1981). For this model, individual molecules were constructed with three identical polypeptide chains [Gly-Pro-Pro)n, (Gly-Prop-Hyp)n, or (Gly-Ala-Ala)n, where n = 4, 12, and 16) coiled into a right-handed triple-helical structure. The axial distance between adjacent amino acid residues is about 0.29 nm per polypeptide chain, and the pitch of each chain is approximately 3.3 residues. The microfibril model consists of five parallel triple helices packed so that a left-handed superhelical twist exists. The structural characteristics of the computed microfibril are consistent with those obtained for collagen by X-ray diffraction and electron microscopy. The energy minimized Smith microfibril model for (Gly-Pro-Pro)12 has an axial length of about 10.2 nm (for a 36 amino acid residue chain), which gives an estimated D-spacing (234 amino acids per chain) of approximately 66.2 nm. Studies of the microfibril models (Gly-Pro-Pro)12, (Gly-Pro-Hyp)12, and (Gly-Ala-Ala)12 show that nonbonded van der Waals interactions are important for microfibril formation, while electrostatic interactions contribute to the stability of the microfibril structure and determine the specificity by which collagen molecules pack within the microfibril.

Different architectures of the collagen fibril: morphological aspects and functional implications

Several tissues known to contain collagen fibrils with a 'helical' arrangement were studied by t.e.m. and freeze-fracture. In all the tissues examined, the diameter of the collagen fibrils appeared to be tissue-specific and fairly constant within the same tissue. No statistical differences, on the contrary, were detectable in the coiling angle which appeared similar in all the tissues and independent of both diameter and age of the fibril. Rat tail tendon was also examined under the same technical conditions and showed collagen fibrils of large and very heterogeneous diameter and with a consistent 'straight' arrangement. These data seem to suggest that the 'helical' and 'straight' arrangements may actually identify different types of collagen fibrils. The authors discuss the possible functional significance of these arrangements and present two hypotheses on the three-dimensional structure of the 'helical' fibril.

Glycation induces expansion of the molecular packing of collagen

Exposure of rat tail tendon to a reducing sugar results in covalent attachment of the sugar to collagen, a process termed glycation, and leads to the formation of stable intermolecular cross-links. We have used X-ray diffraction to study the changes in the crystalline unit cell of rat tail tendon collagen brought about by glycation. Ribose was selected as a model compound for most of the study because its reaction with proteins is faster than that of glucose, and therefore more convenient for laboratory studies, but glucose and glyceraldehyde were used as well. A kinetic model describing the process of glycation by ribose and subsequent cross-link formation has been developed. Glycation resulted in an expansion by more than 12% of the unit cell that describes the three-dimensional structure of rat tail tendon collagen. The expansion was in a direction perpendicular to the axes of the rod-shaped molecules, indicating that the intermolecular spacing of the collagen increased. Thus, the structure of collagen in rat tail tendon is significantly altered by glycation in vitro. The expansion was not isotropic, but was directed parallel to the (120) planes, one of the three major planes of the quasi-hexagonal structure that is densely populated by collagen molecules. It is hypothesized that this expansion is brought about by the formation of one, or at most a few, specific intermolecular cross-links in the overlap zone that act to push the molecules apart. It is likely that similar structural changes in collagenous tissues are caused by glycation in vivo during the natural course of aging, and that these changes are accelerated in chronic hyperglycemia such as that associated with diabetes. Analysis of the structure of glycated rat tail tendon potentially can give us new insight into the detailed molecular structure of collagen.