Structural Differences between Type I and Type IV Collagen in Biological Tissues Studied in vivo by Attenuated Total Reflection/Fourier Transform Infrared Spectroscopy

Attenuated total reflection/Fourier transform infrared (ATR/FT-IR) spectra have been obtained in a nondestructive manner for the anterior surface, interior part, and posterior surface of the sclera, for the epithelium, Bowman's membrane, stroma, and endothelium of the cornea, and for the inner section of the Achilles' tendon of a rabbit. The corresponding spectra have been remeasured for the rabbit anterior and posterior lens capsule for purposes of comparison. The spectra of the three parts of the sclera and of the Bowman's membrane and stroma of the cornea are very close to the spectrum of purified type I collagen, confirming that their major components are type I collagen. The spectrum of the tendon is also very similar to that of purified type I collagen, but it contains a small contribution from hyaluronic acid in the 1100-1000 cm⁻¹ region. The amide I bands of the type I collagen-containing tissues are sharp and symmetrical, and their frequencies (1642 cm⁻¹) are almost identical to that (1640 cm⁻¹) of polyglycine II, which takes a 3, helix formation, indicating that the secondary structure of type I collagen in the tissues examined is for practical purposes a slightly modified 3₁ helix. A comparison of the spectra of the type I collagen-containing tissues and those of the type IV collagen-containing tissues reveals that there are two major differences between them; one is the spectral features in the 1100-1000 cm¹ region, where C-O stretching modes of polysaccharide are observed, and the other is the shape and frequency of the amide I band. Besides the peak at 1637 cm⁻¹, the amide I bands of the type IV collagen-containing tissues have shoulders near 1650 and 1655 cm⁻¹. This observation indicates that type IV collagen in the tissues examined assumes primarily a slightly modified 3₁ helix formation, but the percentages of α-helix and random coil structures are not negligible.