Fig. 3.

Collagen is less hydrated in D₂O than in H₂O, but retains the same helicity. (A) Crystal structure of the (Gly-Pro-Hyp) nonamer PDB ID: 3B0S(33). (B) IR spectra of heavy water and water solutions containing Type I full-length collagen at a concentration of 2 mg/ml and 10 mg/ml, respectively, recorded at 23°C. Full IR spectra are shown in SI Appendix, Fig. S3. The IR spectrum in D₂O was obtained using FTIR in transmission mode, in H₂O it was obtained by using FTIR in reflection mode (ATR-FTIR). In the latter case, because of the shorter optical path length, we used a higher collagen concentration to obtain a sufficient signal-to-noise ratio. The IR spectrum of collagen in water is not concentration dependent (ref. 34 and SI Appendix, Fig. S4). (C) 2D–IR spectrum of a heavy water solution containing Type I full-length collagen at a concentration of 2 mg/ml recorded at a waiting time between pump and probe pulses of 1 ps. The blue contours represent a decrease in absorption ($\mathrm{\Delta }$A $<$ 0) due to depletion of the $v$ = 0 state, and the red contours an increase in absorption ($\mathrm{\Delta }$A $>$ 0) due to the induced absorption of the v = 1 $\to$ 2 transition. Colored lines in the 2D-IR spectrum represent the calculated central lines (See SI Appendix for more details). (D and E). CD spectra and melting curves extracted from temperature-dependent CD measurements of Type I full-length collagen dissolved in water and heavy water at a concentration of 0.1 mg/ml, respectively (see SI Appendix for more details). (F) Interaction energies between the peptide and the solvent (D₂O, H₂O) molecules and schematic of collagen hydration in D₂O and H₂O. (G) Intramolecular energies and energies between the peptide and the ions in D₂O and H₂O.