Quick ! Can you form a hydrogen bond from a carbon hybridized sp3 to an oxygen atom?
I didn’t think so, but you can. This, in spite of reading about proteins for over half a century. [ Proc. Natl. Acad. Sci. vol. 111 pp. E888 – E895 ’14 ] describes this (along with lots of references backing up the statements which follow) to such bonds forming between the transmembrane segments of membrane proteins (estimated to be 30% of all our proteins).
Whether or not they contribute to membrane stability isn’t known. Consider the alpha carbon of an amino acid. It is adjacent to a carbonyl group of an amide (electron hungry, but less so than a pure carbonyl because of resonance) and the nitrogen atom of an amide (slightly more electronegative than carbon, and probably more electron hungry because it loses part of its lone pair to resonance).
They are usually found from the alpha carbon of glycine on one helix to the carbonyl of an adjacent transmembrane helix. Glycine zippers (e.g. the G X X X G motif) have long been known in transmembrane helices. Since glycine is the smallest amino acid, having them on the same side of the helix was thought to be a way to pack adjacent helices together.
What would you consider good evidence for such a bond? Spectroscopy of model compounds with deuterium for the alpha hydrogen would be one way (it’s been done). The best evidence would be a shortened distance between the hydrogen and the carbonyl and this has been found as well.
Humbling ! !