Glossary
Freeze–Thaw Cycle
One freeze–thaw cycle is a full trip from frozen to liquid and back again. Why every cycle erodes peptide integrity, and how aliquoting solves the problem.
A freeze–thaw cycle means a solution goes once from the frozen state to liquid and back again. Stability data express tolerance as a count of these repetitions, because the harm accumulates with the number of transitions rather than with the total time spent in the freezer.
What happens during each cycle
Three mechanisms act while the phase changes. When ice forms, dissolved molecules are pushed out of the crystal lattice and pile up in the ever-smaller liquid fraction, so local concentrations of peptide and salt climb well above their nominal values. Buffer components may crystallise at different stages — sodium phosphate buffers are known to change pH by over one unit on freezing — subjecting the peptide to conditions it would never meet at room temperature. And every ice–water boundary provides a surface on which partly unfolded chains can stick together irreversibly.
What you may see is cloudiness, particles or reduced measured potency; what you will not see is a gradual loss of intact peptide that only a new HPLC analysis would uncover. The usual safeguard is to divide a reconstituted stock into single-use aliquots before it is frozen for the first time, using sterile empty vials. The storage guide and the aliquoting guide give practical detail.
Related terms
stability · degradation · cold chain