The presence of a single red-shifted band points out that the amide nitrogens are equivalent and that they are weakly hydrogen bonded to the nitrogen atoms of pyridine rings.
The evolution of infrared features for interfacial and weakly hydrogen bonded water were followed upon exposure of initially vacuum dried membranes, exchanged by either Na+ or H+, to low humidity atmospheres.
Two broad peaks were observed in OH stretching region at ca. 3200 cm−1 and ca. 3400 cm−1, which are known to be due to the symmetric OH stretching (υ1) of tetrahedrally coordinated, i.e., strongly hydrogen bonded "ice-like" water, and the asymmetric OH stretching (υ3) of water molecules in a more random arrangement, i.e., weakly hydrogen bonded "liquid-like" water, respectively.
Under approximately 10% relative humidity, the time dependent increases in absorbance for a mode of interfacial water near 3674 cm−1 and a mode of more bulk-like, weakly hydrogen bonded water at 3525 cm−1 in Na+ exchanged Nafion 112 could be fit by a pore diffusion model.
This indicates that the weakly hydrogen bonded waters do not strongly interact with Glu169 or Asp299 (see Discussion and Conclusions).
The OH stretching mode of weakly hydrogen bonded waters is normally found in the region from 3550 to 3700 cm 1.
All of these bands can be assigned to one or more weakly hydrogen bonded internal waters on the basis of a H2O-induced downshift in frequency.
Bands in the region of weakly hydrogen bonded D2O from 2650 to 2700 cm 1 (OD stretch mode) have also been identified in the C1C2 chimera (whose three-dimensional structure has been determined at 2.3 Å resolution).
In contrast, in BR it has been shown that a weakly hydrogen bonded water (W401) that is part of a hydrogen-bonded pentagonal cluster located near the retinal Schiff base undergoes an increase in its strength of hydrogen bonding.
Interestingly, the bands assigned to weakly hydrogen bonded water molecules appear not to be significantly altered by these mutations as most clearly seen in the region of the OH stretch.
A decrease in the hydrogen bond strength of a weakly hydrogen bonded internal water is detected in both CaChR1 and CrChR2, but the bands are much broader in CrChR2, indicating a more heterogeneous environment.
