Elastin is a major component of tissues such as lung and blood vessels, and endows them with the long-range elasticity necessary for their physiological functions. Recent research has revealed the complexity of these elastin structures and drawn attention to the existence of extensive networks of fine elastin fibres in tissues such as articular cartilage and the intervertebral disc. Nonlinear microscopy, allowing the visualization of these structures in living tissues, is informing analysis of their mechanical properties. Elastic fibres are complex in composition and structure containing, in addition to elastin, an array of microfibrillar proteins, principally fibrillin. Raman microspectrometry and X-ray scattering have provided new insights into the mechanisms of elasticity of the individual component proteins at the molecular and fibrillar levels, but more remains to be done in understanding their mechanical interactions in composite matrices. Elastic tissue is one of the most stable components of the extracellular matrix, but impaired mechanical function is associated with ageing and diseases such as atherosclerosis and diabetes. Efforts to understand these associations through studying the effects of processes such as calcium and lipid binding and glycation on the mechanical properties of elastin preparations in vitro have produced a confusing picture, and further efforts are required to determine the molecular basis of such effects.

The cyanogen bromide-resistant proteins of lamprey cartilage are biochemically related to the mammalian elastic protein, elastin. This study investigates their mechanical properties and enquires whether, like elastin, long-range elasticity arises in them from a combination of entropic and hydrophobic mechanisms. Branchial and pericardial proteins resembled elastin mechanically, with elastic moduli of 0.13-0.35 MPa, breaking strains of 50%, and low hysteresis. Annular and piston proteins had higher elastic moduli (0.27-0.75 MPa) and larger hysteresis. Exchanging solvent water for trifluoroethanol increased the elastic moduli, whereas increasing temperature lowered the elastic moduli. Raman microspectrometry showed small differences in side-chain modes consistent with reported biochemical differences. Decomposition of the amide I band indicated that the secondary structures were like those of elastin, preponderantly unordered, which probably confer the conformational flexibility necessary for entropy elasticity. Piston and annular proteins showed the strongest interactions with water, suggesting, together with the mechanical testing data, a greater role of hydrophobic interactions in their mechanics. Two-photon imaging of intrinsic fluorescence and dye injection experiments showed that annular and piston proteins formed closed-cell honeycomb structures, whereas the branchial and pericardial proteins formed open-cell structures, which may account for the differences in mechanical properties.

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