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“And it just stays there forever.”
“These do.” Lucy pushes one of the cooked
mussels around with her spoon. “I think these
are blue mussels, like you’d get at the grocery
store.”
“Stuck in one place could be nice, if you’re
with the right person.” Alex decides he can
use this line of thought to turn the conversa-
tion in the direction of romance and get to the
surprise for the prom.
“Glue’s pretty cool,” Lucy says, prying apart
another mussel and not noticing Alex’s sigh.
“Glues are sticky because they make bonds
with the stuff they are holding together. And
when you try to pull apart the stuff, the glue
just passes the force across the bond.”
Bonds can work, too, Alex thinks. “Those
covalent bonds we were talking about in
chemistry the other day, sharing electrons
between two atoms—that can be fun if you’re
sharing electrons with the right person.”
Please don’t let Lucy go off on one of her
geek lectures about electrons, Alex thinks.
Usually it’s cute, she gets so excited, but he
needs to do something to stay on this topic,
Mussels can bind strongly to hard surfaces underwater thanks to proteins that contain a large amount of the
amino acid 3,4-dihydroxyphenylalanine (DOPA). Part of the DOPA protein, called a catechol group (C6H4(OH)
2), is a benzene ring with two
hydroxyl (–OH) functional groups attached. This group interacts directly
with the surface material through two types of intermolecular forces,
called hydrogen bonding and London dispersion force attractions (Fig. 1a
and Fig. 1c). If metals are present in the surface materials, the catechol
group can also form covalent bonds with the metals (Fig. 1b).
The catechol molecules
displace water molecules that
were bound to the surface and
adhere directly to the surface,
because that is the lowest
energy configuration among
possible combinations of catechol groups, water, and surface molecules. So the catechol
group—and thus the entire
adhesive protein—will bond
with the surface rather than with
water molecules.
The mussel binds to a surface through a structure called a byssus (Fig. 2), which consists of a
byssal thread and a cuff-like root.
The deposited adhesive contains proteins
of different molecular weights. Molecular
weight is the sum of the masses of each
atom in a molecule. Changes in molecular
weight affect many properties of substances,
including their strength, stiffness, and toughness. In the case of the mussel adhesive, a
combination of proteins with various molecular weights makes the adhesive stronger
than if it were made of individual proteins
with the same molecular weight.
When a mussel deposits its adhesive, also called a plaque, each type
of protein plays a different role. Low-molecular-weight proteins make the
bond between the surface and the plaque; middle-molecular-weight proteins make up the main body of the plaque, and high-molecular-weight
proteins coat the outside of the plaque. This combination of molecular
weights increases the strength of the adhesive.
—Mary Alexandra Agner
so he can segue to his surprise. “When I
visited my brother at the college over spring
break, he was talking about how he is working
in this lab studying glues,” he says.
Lucy looks doubtful. “I thought your brother
was an urban planner?”
“Civil engineer,” he says.
Finally! Alex can get to the romantic bit. He
is not expecting Lucy to be interested in his
brother’s school stuff but she asks, “Well?
What glue do civil engineers use?”
Now, Alex feels that he is directing the
conversation. “So, do you know how railroads
work? I bet you don’t.”
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ChemMatters | FEBRUARY/MARCH 2016 9
HO
HO
(a) (b) (c)
MM M
Protein
Protein
Hydrogen
Bonds Covalent
Bonds
HO
Protein
Protein
HO Protein
HO
HO Protein
HO
MM M
OH HO OH
O
OO
+2H2O
H
OH
O
H
HO HO
HO
London
Dispersion-
Force
Attractions
Figure 2. Mussel’s byssus structure
Byssal
Plaque
Byssal
Retractor
Muscle
Byssal
Threads
Byssus
Stem
Foot
Figure 1. Catechol molecules (structures shown above) can bind to various
surfaces, as follows: (a) Catechols can bind to hydrophilic surfaces by
making hydrogen bonds to –OH groups; (b) many mineral surfaces have
metal ions exposed, and catechols can form covalent bonds with these
metals; and (c) on hydrophobic surfaces, the benzene ring of a catechol
molecule can interact with surfaces by London dispersion force attractions.
How Mussels’ Bioglue Works
Blue mussels use byssal
threads to anchor to rocks.
