  Water and glycerol are well-known to facilitate the structural relaxation of
amorphous protein matrices. However, several studies evidenced that they may
also limit fast ($\sim$ pico-nanosecond, ps-ns) and small-amplitude ($\sim$ \AA
) motions of proteins, which govern their stability in freeze-dried sugar
mixtures. To determine how they interact with proteins and sugars in glassy
matrices and, thereby, modulate their fast dynamics, we performed molecular
dynamics (MD) simulations of lysozyme/trehalose/glycerol (LTG) and
trehalose/glycerol (TG) mixtures at low glycerol and water concentrations. Upon
addition of glycerol and/or water, the glass transition temperature,
T$_{\textrm{g}}$, of LTG and TG mixtures decreases, the molecular packing of
glasses is improved, and the mean-square displacements (MSDs) of lysozyme and
trehalose either decrease or increase, depending on the time scale and on the
temperature considered. A detailed analysis of the hydrogen bonds (HBs) formed
between species reveals that water and glycerol may antiplasticize the fast
dynamics of lysozyme and trehalose by increasing the total number and/or the
strength of the HBs they form in glassy matrices.
