H-bonding side-chains

Frank R. Gorga fgorga at bridgew.edu
Thu Jun 3 14:46:32 EST 1999

colby wrote:
> Can the side chain of a tryptophan residue accept a H-bond? What about
> an arginine residue?
> According to http://swift.embl-heidelberg.de/future/aainfo/hbonds.html,
> the only site on the web I found that discusses amino acid side chain
> H-bonding, these two residues do not seem to accept H-bonds. Is it
> because of steric hindrance by the atoms that the N-atoms are bonded to?
> If so, why does it not affect the H-bond accepting capability of
> histidine?
> I appreciate any replies.

The simple answer is "yes"... the side chains of all of these residues
(Tyr, Arg & His) are chemically capable of both donating to and
accepting hydrogen bonds. Wether they will do so in a specific
situtation in a protein is, of course, much more complicated.

Here is the longer answer...

Hydrogen bonding requirements

 Donor --  a polar X-H bond

    i.e. X is significantly more electronegative than H;
    for proteins X is almost always N or O

 Acceptor -- -Y:

    i.e. an electronegative atom with a nonbonded (lone) pair
of            electrons, again for proteins this is almost always N or O

Bottom line --

The hydroxyl (OH) of tyrosine is capable of both donating to and
accepting H bonds The same is true for both the guanidium group of arg
and the imidazole group of histidine.

As for resonance (suggested in other replies)... it has nothing to do
with hydrogen bonding.

Biochemically --

The situtation in proteins is much more complicated... in order to form
a H-bond in proteins both donor and acceptor have to be positioned
properly with respect to one another (both angle and distance are
critical). The polarity of environment is also critical... the more
polar the enviroment the less likely a residue will be H-boned to
anything other that water.

Hope this helps,

-- FRG

Frank R. Gorga, Ph.D.
Department of Chemical Sciences
Bridgewater State College
Bridgewater, MA 02325
(508) 697-1200 x2827
fgorga at bridgew.edu

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