## 3 Results and Discussion
Assuming that the Gly-Xaa-Yaa triplet contains hydroxproline in the Yaa position and any of the amino acids listed in [37] in the Xaa position, all the nitrogen atoms will be in the peptide bond. Then, each peptide bond will also involve one carbon atom and one oxygen atom, i.e., 2 at.% carbon and 2 at.% oxygen. Assuming the maximum possible content of carbon and oxygen atoms in the side members of amino acids in positions Xaa and Yaa to be 3 and 2 atoms, respectively, one can estimate the upper limit of atoms in the spongin as 2 at.% (nitrogen) + 5 at.% (carbon) + 4 at.% (oxygen). Atoms in the NH\({}_{2}\) and COOH groups at the beginning and end of the spongin fibers were not taken into account. As a result, it was found that at least -70 at.% of carbon and -11 at.% of oxygen are located in the sponge volume between the spongin fibrils. The sponge C 1s NEXAFS spectrum (Figure 3) was measured to elucidate the chemical composition of the sponge substance. Due to the high oscillator strength of C 1s \(\rightarrow\)\(\pi^{\star}\) electron transitions in linear and planar atomic groups, which appear in absorption spectra as narrow and intense peaks with characteristic photon energies, these groups were identified in the studied sample. Figure 3 shows the partial C 1s NEXAFS spectra of the native sponge obtained by subtracting the contributions from the overlying electron levels extrapolated by the power dependence of the absorption cross-section from the low energy region before the C 1s absorption edge. In the fine structure region of the C 1s spectrum, there was an intense peak with a photon energy of 285.0 eV related to C 1s \(\rightarrow\)\(\pi^{\star}\) transitions in aromatic structures. At the same time, the spectrum also contained four additional less intense narrow peaks and a broad band, which were due to C 1s \(\rightarrow\)\(\pi^{\star}\) and C 1s \(\rightarrow\)\(\sigma^{\star}\) electron transitions in corresponding atomic groups, respectively. The energy positions of these narrow peaks agreed well with the energies of 1s \(\rightarrow\)\(\pi^{\star}\) transitions in the C-O (286.7 eV), C-O-C (287.6 eV), and C=O (288.6 eV) planar oxygen-containing groups (carboxyl and carbonyl), which was confirmed by numerous studies of carbon oxides [38; 39; 40; 41] and various oxygen-carbon  
molecular groups [42; 43; 44; 45; 46]. However, the 287.6 eV peak can also be attributed to transitions in amide (N-C=O) or imide (C=N) groups, while the 288.1 eV peak can be attributed to transitions in the CON peptide group [47]. The broad absorption band at the photon energy of 291.9 eV can be attributed to C 1s \(\rightarrow\)\(\sigma^{*}\) electron transitions in various linear and planar atomic groups. As for the structures at the energies of 297.5 eV and 300.0 eV, they referred to the 2p\({}_{3/2,\,1/2}\)\(\rightarrow\) 3d electron transitions in potassium compounds, which were present in the form of impurities in the sponge. Normally, the C 1s spectra of proteins are dominated by a narrow and intense \(\pi^{*}\) peak of the peptide group, and thus, the low intensity of this transition in the sponge spectrum confirmed the low spongin content in the native sponge. The apparent dominance in the sponge C 1s NEXAFS spectrum of peaks associated with electron transitions in the aromatic and C=O atomic groups indicated the high content of aromatic compounds and carbon oxides in the substance filling the space between the spongin fibrils in the sponge. N 1s, C 1s, and O 1s XPS spectra (Figure 2b-d) were measured to quantify the atomic concentration of nitrogen, carbon, and oxygen atoms within atomic groups. The photoelectron (PE) spectrum of nitrogen contained a symmetric peak at a binding energy (BE) of 399.7 eV, which corresponded to the BE of N 1s electrons in the peptide group.