Ion Spectroscopy of
Non-Covalent Complexes
Noncovalent
interactions,
including hydrogen bonds, salt
bridges, and metal cation binding, play important roles in the
structure, function, and reactivity of biomolecules. For example, salt
bridge formation is known to be an important feature in high-order
protein structure and in the interfaces of many enzyme-substrate
complexes, and the presence of metal cations is crucial for the proper
folding and function of many proteins. Water molecules can have complex
hydrogen-bonding interactions with biomolecules and play an important
role in their structure. The interaction of biomolecules with water and
cosolutes in vivo thus involves an intricate cooperation and
competition of many noncovalent interactions. By isolating model
systems in the gas phase, the effects of specific noncovalent
interactions on biomolecule structure can be investigated individually
or in combination. One of our goals is to relate these effects to
intrinsic properties of the components of the biomolecule complex, such
as gas-phase basicities and acidities, ion size and charge state, and
sequence. Infrared photodissociation (IRPD) spectroscopy is a sensitive
probe of structure and has been particularly useful in studying effects
of noncovalent interactions in gas-phase ions and complexes. In this
technique, the rate of dissociation of an isolated ion or complex due
to irradiation with a tunable laser is measured as a function of laser
photon frequency, resulting in an infrared “action”
spectrum
of the ion or complex, which can provide valuable structural
information that can be related to the physical properties of the
complex substituents.
Contacts: Terry
Chang and Richard
Cooper
Ion Spectroscopy of Hydrated
Ions and Large Droplet Surfaces
Many ions that occur as
cosolutes with biomolecules in vivo
play a crucial role in biomolecule structure, function, and reactivity.
The Hofmeister series, which ranks various ions according to their
effects on proteins in aqueous solution, has been studied for over a
hundred years, although the precise origins of these effects of ions on
the hydrogen-bond network of water and on biomolecules structure remain
poorly understood. In addition to studying direct effects of Hofmeister
series ions on biomolecules (see above), our group is interested in how
water molecules interact with different ions and how the identity of
the ion affects the development of hydrogen bonding structure in
successively larger solvation shells. In small gas-phase hydrates, many
ions tend to organize the water molecules nearest them in a manner that
differs substantially from the hydrogen-bonding network of bulk water.
As the number of water molecules in the hydrated ion complex increases,
it is interesting to ask at what point a transition from gas-phase to
solution-phase properties occurs. IRPD spectroscopy in the
hydrogen-stretch region (~2500–4000 cm-1) is sensitive to the
hydrogen-bonding environment of both the ion and water molecules and
can provide insight into the structure of the hydrated ion complex. In
this way, hydrated ion complexes serve as a bridge between the gas
phase and solution. Our efforts focus on characterizing the interior
and surface structure of hydrated ion complexes as well as relating
these results to known solution-phase properties, including ion size,
solution-phase coordination number, hydrolysis constants, Jahn-Teller
distortion, surface activity, and Hofmeister series effects.
Contacts: Terry
Chang and Richard
Cooper
Nanocalorimetry
Determining the energy
deposition during a gas-phase
dissociative
reaction is difficult due to various factors, such as multiple reaction
pathways and broad ranges of incident energies. To accurately measure
the energy distribution, not just the average energy deposition,
hydrated cluster ions can be used as chemical thermometers. Even at
relatively small sizes, these ‘nanocalorimeters’
provide a
ladder of discrete energy values based on the binding energy of each
water molecule to the cluster. When energy is deposited as the result
of a gas-phase reaction, water molecules evaporate to cool the heated
cluster. By using mass spectrometry, the number of water molecules lost
from a cluster is readily determined, and the incident energy of the
reaction may be obtained. This technique has been applied to various
hydrated cluster systems, from metal ions to peptides, and has provided
detailed information for ion-neutral, ion-electron, and ion-photon
reactions. These measurements have broad applicability to proteomics,
electrochemistry, and the characterization of gas-phase ion structure.
Standardless Quantitation
Electrospray mass spectrometry
methods excel at identifying
ion mass, composition and bond connectivity. However, a multitude of
ionization, transmission, and detection efficiency effects can mask the
presence of analytes in mixtures, making the determination of mixture
molar fractions directly from electrospray ion abundances extremely
unreliable. To counter these effects, we have developed a series of
techniques that use nonspecific clusters as probes of solute
composition in mixtures. At sufficiently large cluster sizes,
nonspecific cluster ions contain an accurate estimate of the solution
mole fraction for each analyte that is present, and are largely
unaffected by differences in efficiency. These
“standard-free” quantitation methods have broad
application to mixtures found in pharmaceuticals, proteomics, and other
biological fluids. Studies looking at the effects of complex mixture
components, dramatic changes in ionization efficiency, and improving
the dynamic range and sensitivity of this technique are
ongoing
Contacts: Terrence
Chang
Biophysical Mass Spectrometry
In solution, the proximity of
two residues predicts whether
the
effect of removing them both equals the sum of the effects from
removing them singly, or whether they are coupled.
We will apply the same logic to gas phase protein-protein
complexes, using the coupling between residue relative free energies to
produce a
distance map of contacts in the interface: a first approximation of the
local gas-phase structure and an experimental probe of water effects.
We have alanine-scanned the binding interface of two proteins,
barnase and barstar, and have simple enzymological means to see how
mutations affect binding energy in solution. The mass
spectrum of a solution of competing monomers gives an instant and
high-throughput readout
of their relative solution affinities for heterodimer formation.
Contact: Catherine
Cassou