Raman spectral evidence for hydration forces between collagen triple helices

Abstract

Hydration forces are thought to result from the energetic cost of water rearrangement near macromolecular surfaces. Raman spectra, collected on the same collagen samples on which these forces were measured, reveal a continuous change in water hydrogen-bonding structure as a function of separation between collagen triple helices. The varying spectral parameters track the force-distance curve. The energetic cost of water ''restructuring,'' estimated from the spectra, is consistent with the measured energy of intermolecular interaction. These correlations support the idea that the change in water structure underlies the exponentially varying forces seen in this system at least over the 13-18-Å range of interaxial separations.The structure of water surrounding collagen has been studied by a wide variety of techniques and is considered a paradigm for protein hydration. It is commonly accepted that at least some water in collagen fibers differs from bulk, ambient water. Within the most simple scheme, this interstitial water is separated into two distinct classes-tightly bound and ''free'' or bulk-like (1, 2). More elaborate models imagine three or even more fractions (3-8). Tightly bound waters are believed to stabilize the triple helix by participating in the H-bond backbone (see ref. 9 for a review). In a larger context, however, the specific role of water in collagen assembly, structure and stability is still an open question.New insights into the structure and the role of water near collagen come from the high-resolution x-ray structure of a collagen-like peptide (10, 11). Malleable clusters and chains of H-bonded waters form a complex network connected to polar side chains and to backbone carbonyls on the peptide triple helices. The uncharged helices do not show any direct contacts; water directly mediates the interaction between them.This interpretation of the x-ray results correlates well with the picture that has emerged from direct measurement of force-distance curves between triple helices. It has been argued that the measured short-range repulsion, which prevents collagen molecules from coming too close to each other, originates from the energetic cost of removal of the interstitial water (12-14). The longer-range attraction, which prevents the molecules from coming too far apart and which induces self-assembly of collagen fibers from solution, appears to be associated with formation of hydrogen bonded water bridges at specific recognition sites (13). The arguments, however, have been based so far on the process of elimination of other explanations for the observed forces, rather than on direct observation of changes in water structure.The present study establishes more explicit connection between forces and water structure by observing Raman spectra from the same samples on which osmotic stress force measurements have been made. We analyze the changes in the OOH and NOH vibrational bands measured in the 3,100-3,800-cm Ϫ1 range as a function of separation between collagen triple helices and correlat...

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