Applied Radiation and Isotopes 54 (2001) 887–891 EPR investigation of the gamma-ray irradiated natural and tanned collagen O.G. Duliu a, *, M. Epuras a , V. Trandafir b a Department of Atomic and Nuclear Physics, University of Bucharest, Ma ˜ gurele, P.O. Box MG-11, RO-76900 Bucharest, Romania b National Research Institute for Textiles and Leather, Division: Leather Research Institute, 93, Ion Minulescu str. RO-74259 Bucharest, Romania Received 29 April 2000; received in revised form 30 September 2000; accepted 2 October 2000 Abstract Free radicals produced in natural and tanned collagen by gamma-ray irradiation within 1–15 kGy absorbed dose ranges were investigated by EPR spectroscopy. Tanned collagen was prepared using formaldehyde as well as aluminum basic salts [Al(OH)SO 4 ] tanning processes. Both natural and formaldehyde-tanned irradiated collagen show the same kind of EPR spectrum consisting of a single broad, slightly asymmetric line. Irradiated collagen tanned by aluminum basic salts process displayed a complex EPR spectrum consisting of a superposition of broad and narrow lines. A computer simulation of this spectrum allowed to evidence the presence of seven different kinds of paramagnetic centers, including those observed in the irradiated natural collagen. Corresponding Spin Hamiltonian parameters (g-factor, hyperfine splitting constant) as well as relative concentrations of these centers were calculated. Experimentally determined relative concentrations display a positive correlation with the absorbed dose described by a linear-type dependence. After three weeks of storage at room temperature, the concentration of some centers diminished by about 50%. The possible nature of these centers is discussed in connection with the local structure of the tanned collagen. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: EPR; Collagen; Irradiation; Free radicals 1. Introduction Collagen represents the structural proteins of bones, skin, tendons and ligaments. Currently, investigations have shown that at least 19 different types of collagen are present in various tissues of both vertebrates and invertebrates. All types of collagen are formed with the same structural features consisting of right-handed triple helix polypeptide chains, each chain representing a repeating tripeptide left-handed coil, glycine occupying every third residue. The triple helix is stabilized by a hydrogen bond from the glycine to the peptides of an adjacent chain. At the molecular level, the collagen structure is highly ordered, resembling a monocrystal (Bailey, 1997). The most common is fibrous type I collagen which is the dominant protein in the vertebrates (Kielty et al., 1993). Type I collagen is widely used as a raw material for medical devices owing to its low antigenicity and its ability to support cell adhesion. Due to its chemical and biological properties, collagen is also the biomaterial closest to living tissues (Bailey, 1992). The tanning process consists of blocking amino acid polar groups (–NH 2 , –COOH and –OH) mainly on lateral polypeptide chains. In this way new covalent as well as hydrogen bonds are created. As a result, the amount of water molecules bound on polar groups is diminished considerably. Therefore the total humidity of collagen decreases up to 10–15% which considerably rises physico-chemical as well as biological stability. The tanning process can be controlled by choosing proper tanning agents. The most common tanning agents are basic salts of chromium, aluminum or zirconium, *Corresponding author. Fax: +401-420-8625. E-mail address: odlu@scut.fizica.unibuc.ro (O.G. Duliu). 0969-8043/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII:S0969-8043(00)00357-2