Characterising the structural dynamics of proteins and the effects of excipients are critical for optimising the
design of formulations. In this work we investigated four lyophilised formulations containing bovine serum
albumin (BSA) and three formulations containing a monoclonal antibody (mAb, here mAb1), and explored the
role of the excipients polysorbate 80, sucrose, trehalose, and arginine on stabilising proteins. By performing
temperature variable terahertz time-domain spectroscopy (THz-TDS) experiments it is possible to study the
vibrational dynamics of these formulations. The THz-TDS measurements reveal two distinct glass transition
processes in all tested formulations. The lower temperature transition,
Tg, , is associated with the onset of local
motion due to the secondary relaxation whilst the higher temperature transition,
Tg, , marks the onset of the
-relaxation. For some of the formulations, containing globular BSA as well as mAb1, the absorption at terahertz
frequencies does not increase further at temperatures above
Tg, . Such behaviour is in contrast to our previous
observations for small organic molecules as well as linear polymers where absorption is always observed to
steadily increase with temperature due to the stronger absorption of terahertz radiation by more mobile dipoles.
The absence of such further increase in absorption with higher temperatures therefore suggests a localised
confinement of the protein/excipient matrix at high temperatures that hinders any further increase in mobility.
We found that subtle changes in excipient composition had an effect on the transition temperatures
Tg, and
Tg,
as well as the vibrational confinement in the solid state. Further work is required to establish the potential
significance of the vibrational confinement in the solid state on formulation stability and chemical degradation
as well as what role the excipients play in achieving such confinement.