Microfluidic Modulation Spectroscopy:

Yes!  In addition to particle sizes interrogated with DLS, absorption methods provide fundamental structural insights. In the case of proteins, this helps to understand underlying mechanisms that may stabilize or destabilize protein formulations, for example as a response to external stresses or in course of binding events.

Both circular dichroism (CD) and MMS are spectroscopic techniques that measure protein secondary structures. Both these techniques are known for their high sensitivities and the ability to provide accurate assessment of the secondary structures. These techniques differ by their principles. Because proteins are chiral molecules, they interact with circularly polarized light, and the different secondary structures in protein give rise to differential absorbance upon interactions with left-handed and right-handed polarized light. CD uses this principle to detect the different secondary structures in proteins. MMS on the other hand, probes the vibrationa lresponses of the backbone carbonyl groups of the protein. For CD, the signal quality depends on the total absorbance of a sample. Because of this limitation, the concentration range for CD is narrow (0.01-1 mg/mL) and the buffer compatibility is poor. In contrast, because MMS uses IR spectroscopy andallows for real-time buffer subtraction, it allows a wide range of absorbance and is not limited by buffer signals. MMS can measure samples at a wide concentration range (0.1-200 mg/mL) and is compatible with most buffers and excipients. A direct comparison between CD, FTIR and MMS is given in this application note:

MMS Measures Protein Misfolding and Protein Aggregation

Other than providing structural information about the proteins, spectroscopy allows a quick and easy way to quantify protein concentration using the Beer-Lambert Law, or Beer’s Law. For proteins in solution, Beer’s Law states that when light passes through the solution, the absorbance is proportional to the concentration of the protein, given that the optical path length and the absorptivity of the protein stay constant. This gives spectroscopic techniques advantages to not only analyze protein structures but also provide accurate quantification of protein concentrations.

Fourier Transform Infrared (FTIR) and MMS are both  IR-based spectroscopic techniques, meaning that the way they probe the  proteins (backbone carbonyl groups) is the same. Therefore, both techniques  measure the secondary structure of proteins. The main differences between  FTIR and MMS are the sensitivity and concentration range. With the high-power  quantum cascade and the microfluidic flow cell that allows for real-time  buffer subtraction, MMS largely improves the signal-to-noise ratio and  provides 30x sensitivity compared to FTIR (Kendrick, B. S.; Gabrielson, J. P.; Solsberg,  C. W.; Ma, E.; Wang, L. Determining Spectroscopic Quantitation Limits for Misfolded Structures. Journal of Pharmaceutical Sciences 2020, 109 (1),  933–936. https://doi.org/10.1016/j.xphs.2019.09.004).

Typically, FTIR requires  high concentration of proteins (10 mg/mL) for measurements, but MMS can  measure proteins at a range from 0.1 to >200 mg/mL. In addition, compared  to FTIR’s manual workflows, MMS is a fully automated technique. An entire  workflow from high throughput (up to 96-well plate) sample measurement to cleaning, baselining, and data analysis can be done with a single mouse click using MMS.