Observing picomolar protein unfolding using resonance light scattering

We here present a novel and sensitive methodology for determining the melting point (MP) of Bovine Serum Albumin (BSA) from micromolar to picomolar concentration levels under label free conditions. At 1 pM we could model the melting with a sharp gaussian. However, from the transient state observed during the melting process by using a simple exponential decay model we determined a time constant of 67 seconds. We applied this methodology by studying a 3.3 pM sample of a botulinum toxin A (BoNT-A) (stabilized with 2.8 nanomolar denatured Human Serum Albumin (HSA)). We were able to determine the Tm of BoNT-A in the presence of the approximately 1000-fold more concentrated HSA. Entry for the Table of Contents O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=153 SRC="FIGDIR/small/624557v1_ufig1.gif" ALT="Figure 1"> View larger version (45K): org.highwire.dtl.DTLVardef@12d93a7org.highwire.dtl.DTLVardef@138d01dorg.highwire.dtl.DTLVardef@e725acorg.highwire.dtl.DTLVardef@15a5f77_HPS_FORMAT_FIGEXP M_FIG C_FIG Protein label-free melting point (MP) determination at ultralow concentrations is a huge problem which concern to the biopharmaceutical industry. Here, we present a novel method to determine the MP of bovine serum albumin (BSA) from 1M to 1pM under label-free conditions. The benefits of this study match the purposes of stability studies in formulations, in which the protein active component is successful at very low concentrations, such as botulinum toxin A (BoNT-A). We used BOCOUTRE, a commercially available pharmaceutical product based on BoNT-A, and Human Serum Albumin (HSA) as a stabilizer. Our method can detect the MP of the stabilizer protein, even if its concentration is markedly different from that of the active component protein (1000-fold) in the case of BOCOUTRE.