Distance requirement for hydrogen bonding to occur I am trying to understand the distance requirement for hydrogen bonding to occur (more specifically in the context of protein folding). There needs to be a donor and acceptor atom for bonding to occur. What is the range of distances between these atoms where one can say that a hydrogen bonding is likely to occur when the polypeptide is in an aqueous environment? Is it element pair specific?
You will probably find all this information in biochemistry textbook. To give you an idea in proteins, typically; alpha helices H..O distance is 2.06 Angstrom, and N..O, 2.99. In beta sheets (parallel) H--O 1.97, N--O 2.29 and in anti parallel sheets, H--O 1.96 and N--OI 2.91. These H bonds are not linear, e.g C=O..H-N typically 150 to 160 degree for COH or NHO angle.

In Watson-Crick base pairing in DNA, (C-G pairs) the linear H-bond distances  are  $\mathrm{N_4-(H)-O_6}$ 2.91, $\mathrm{N_3-(H)-N_1}$ 2.95, and $\mathrm{O_2-(H)-N_2}$ 2.86.