Adsorption-desorption
The backbone of DNA contains negatively charged phosphodiester groups
which play a key role in DNA adsorption to mineral and organic particle
surfaces. At circumneutral pH, DNA is electrostatically attracted to
positively charged mineral surfaces, such as those of iron
(oxyhydr-)oxides and aluminium (hydr-)oxides, resulting in strong
adsorption. Conversely, DNA is electrostatically repelled from
negatively charged surfaces, including silicon dioxide or the basal
planes of some clay minerals. Therefore, the importance of adsorbed DNA
in a water sample likely increases with increasing suspended amounts of
positively charged minerals. Electrostatic DNA-sorbent interactions can
be modulated by solution pH for sorbents that carry a variable charge:
increasing pH decreases the positive charges (and increases the negative
charges), thereby weakening electrostatic attraction. Thus, increasing
solution pH is expected to lower DNA adsorption and can facilitate DNA
desorption from variably charged surfaces.
DNA-sorbent electrostatic interactions are also modulated by solution
ionic strength and composition. Increases in solution ionic strength
attenuate both DNA electrostatic attraction to and repulsion from
positively and negatively charged surfaces, respectively. At very high
ionic strength, electrostatic repulsion from negatively charged surfaces
may be attenuated to an extent that close-contact DNA-surface attractive
interactions (see below) result in DNA adsorption. The presence of
divalent cations in solution may lead to increased adsorption to
negatively charged sorbents via ‘cation bridging’ between the
like-charged DNA and the
sorbent34–37.
Therefore, information on the solution ionic strength and concentrations
of Ca2+ and Mg2+ is important to
assess the extent of DNA adsorption. Besides electrostatic interactions,
DNA-surface van der Waals interactions and H-bonding may drive
adsorption. However, these energetic contributions are expected to be
small in comparison to electrostatic interactions.
All of the aforementioned interactions result in ‘physisorption’ - the
interaction of DNA with the sorbent surface without forming covalent
bonds. However, DNA may additionally bind to some surfaces through
‘chemisorption’, which involves the formation of covalent bonds between
the phosphodiester group of the DNA and hydroxyl groups on the mineral
surfaces. The resulting ‘inner sphere’ complexes are very stable and may
result in both DNA adsorption to mineral surfaces even at high pH
(despite net negative surface charges on the minerals) as well as
prevent DNA desorption from mineral surfaces even if changes in solution
conditions result in DNA-sorbent electrostatic repulsion. DNA may thus
be irreversibly adsorbed, which is clearly relevant for eDNA decay and
detection.
Finally, co-solutes may compete with DNA for adsorption sites on
particle surfaces and thereby suppress DNA adsorption. For instance,
both dissolved organic matter (DOM) and phosphate are expected to adsorb
to some mineral surfaces and may thus increase the fraction of eDNA
present in the dissolved
state24.