Section 22.2.7. Non-conventional hydrogen bonds

Sutor (1962) first summarized evidence for C—H···O hydrogen bonds following earlier suggestions by Pauling (1960), and current evidence has been nicely summarized in several recent articles (Derewenda et al., 1995; Wahl & Sundaralingam, 1997). The energy of C—H···O hydrogen bonds has been generally estimated as ∼0.5 kcal mol⁻¹ (about 10% of an N—H···O interaction) but may be higher, especially in hydrophobic environments. It also depends on the acidity of the C—H proton, with methylene (CH₂) and methyne (CH) groups being most favourable.

A number of examples of C—H···O hydrogen bonds can be found in nucleic acid structures (Wahl & Sundaralingam, 1997). The best known is that between the backbone O5′ oxygen and a purine C(8)—H or pyrimidine C(6)—H, when the bases are in the anti conformation. Another example is given by a U–U base pair, in which the two bases form a conventional N(3)—H···O(4) hydrogen bond and a C(5)—H···O hydrogen bond.

In proteins, two groups are regarded as being particularly significant (Derewenda et al., 1995). These are the C^ɛH of His side chains and the methylene H atoms of the main-chain α-carbon atoms. C—H···O hydrogen bonds involving His side chains have been found for the active-site His residues of proteins of the lipase/esterase family and in other proteins (Derewenda et al., 1994). The C^αH atoms appear to provide much more widespread C—H···O hydrogen bonding, however, especially in β-sheets, where they are directed towards the `free' lone pairs of the main-chain C=O groups. C—H···O hydrogen bonds may thus play a previously unrecognised role in satisfying the hydrogen-bond potential of C=O groups. In general, Derewenda et al. (1995) find a significant number of C···O contacts that meet the criteria for C—H···O hydrogen bonds; the H···O distance peaks at 2.45 Å (C···O 3.5 Å), which is less than the van der Waals distance of 2.7 Å, and the angles indicate that the H atoms are directed at the acceptor lone-pair orbitals.

Sulfur atoms are larger and have a more diffuse electron cloud than oxygen or nitrogen, but are nevertheless capable of participating in hydrogen bonds. Given that the radius of sulfur is ∼0.4 Å greater than that of oxygen, hydrogen bonds can be assumed if the distance H···S is less than ∼2.9 Å, or S···O(N) is less than ∼3.9 Å, providing the angular geometry is right. In proteins, the SH group of cysteine can be a hydrogen-bond acceptor or donor, whereas the sulfur atoms in disulfide bonds and in Met side chains can act only as acceptors.

The clearest example of hydrogen bonding involving Cys residues is given by the NH···S hydrogen bonds in Fe-S proteins (Adman et al., 1975); here, peptide NH groups are oriented to point directly at the S atoms of metal-bound Cys residues, with H···S distances of 2.4–2.9 Å. Similar NH···S hydrogen bonds are found in blue copper proteins, involving the Cys ligands. In these cases, the cysteine sulfur is deprotonated and therefore more negative, making it a stronger hydrogen-bond acceptor, and it is likely that hydrogen bonding to cysteine S⁻ atoms is common. A large survey of Cys and Met side chains in proteins has given evidence of both N—H···S and S—H···O hydrogen bonds involving the SH groups of Cys side chains (Gregoret et al., 1991). In particular, Cys residues in helices frequently hydrogen bond to the main-chain C=O group four residues back in the helix in interactions analogous to those seen for Ser and Thr residues in helices. On the other hand, O—H···S or N—H···S hydrogen bonds to the S atoms of Met or half-cystine side chains, although they do exist, are rare (Gregoret et al., 1991; Ippolito et al., 1990).

Surveys of protein structures have shown that aromatic rings (of Trp, Tyr, or Phe) are frequently in close association with side-chain NH groups of Lys, Arg, Asn, Gln, or His (Burley & Petsko, 1986). Energy calculations further suggest that where an N—H group, as donor, is directed towards the centre of an aromatic ring, as acceptor, a hydrogen-bonded interaction with an energy of ∼3 kcal mol⁻¹ (about half that of a normal N—H···O or O—H···O hydrogen bond) can result (Levitt & Perutz, 1988). Whether the close associations observed by Burley & Petsko can truly be regarded as hydrogen bonds has been controversial, however. Mitchell et al. (1994) have analysed amino–aromatic interactions and shown that by far the most common form of association between sp² nitrogen atoms and aromatic rings involves approximately plane-to-plane stacking, which cannot represent hydrogen bonding. There is still, however, a significant number of cases where the H atoms of N—H groups are directed towards aromatic rings, and these represent genuine hydrogen bonds (Mitchell et al., 1994). It is clearly essential to consider the donor–acceptor geometry, both distances and angles, before assuming an amino–aromatic hydrogen bond; the N···ring distance should be less than ∼3.8 Å, and N—H··· C angle greater than 120°, where C is the ring centre (Mitchell et al., 1994).