Hydrogen bonds involving a carbon donor are very common in protein structures, and energy calculations suggest that Calpha-H...O hydrogen bonds could be about one-half the strength of traditional hydrogen bonds. It has therefore been proposed that these nontraditional hydrogen bonds could be a significant factor in stabilizing proteins, particularly membrane proteins as there is a low dielectric and no competition from water in the bilayer core. Nevertheless, this proposition has never been tested experimentally. Here, we report an experimental test of the significance of Calpha-H...O bonds for protein stability. Thr24 in bacteriorhodopsin, which makes an interhelical Calpha-H...O hydrogen bond to the Calpha of Ala51, was changed to Ala, Val, and Ser, and the thermodynamic stability of the mutants was measured. None of the mutants had significantly reduced stability. In fact, T24A was more stable than the wild-type protein by 0.6 kcal/mol. Crystal structures were determined for each of the mutants, and, while some structural changes were seen for T24S and T24V, T24A showed essentially no apparent structural alteration that could account for the increased stability. Thus, Thr24 appears to destabilize the protein rather than stabilize. Our results suggest that Calpha-H...O bonds are not a major contributor to protein stability.

A combination of oligomeric porphyrin copolymers and bidentate Lewis bases has been used to construct 2-dimensional multilayer molecular architectures on siloxane-derivatized glass substrates. The key to this supramolecular chemistry is coordinate covalent bonding between zinc-metalated porphyrins and bipyridyl "spacer" ligands, generating layers in an alternating, iterative fashion. UV-vis, XPS, AFM, and X-ray diffraction data support the formation of ordered thin film arrays. The stepwise deposition of each porphyrin layer is tracked by the linear increase in the absorbance of the porphyrin Soret band. XPS results demonstrate that the pyridinium coupling in the first monolayer is susceptible to X-ray decomposition during prolonged exposure. Extremely weak emission in the 600-800 nm region is attributed to the low chromophore concentration in the multilayer films. These results contrast sharply with those of spin-coated polymer thin films and represent an encouraging first step toward the design of new multilayer architectures.