The Sleeping Giant Awakes: Defining a New Era of MRR Spectroscopy

Originally posted by Technology Networks on April 4, 2024 | Justin Neill, BrightSpec

What does it take to transform a tried and tested, but somewhat forgotten, “old-school” spectroscopy technology into something that scientists see as the next big thing in molecular structural validation across a range of industries and applications?

Molecular rotational resonance (MRR) spectroscopy has found renewed impetus in the digital age. Improved instrument design, easier-to-use control and analysis software and advances in computational chemistry have transformed MRR into a rapid, scalable and routine analytical solution that delivers unambiguous molecular identification and quantitation.

The first modern commercial MRR instrument is now available, and in this article, we summarize the fundamental technical advantages of MRR instruments, outline recent work being done in pharma discovery and reaction optimization applications and look forward to how MRR looks set to develop in the years ahead.

The foundations of modern MRR

MRR, also known as rotational spectroscopy or microwave spectroscopy because of the frequency of light used, has long been acknowledged as a uniquely incisive structural probe, known for its ability to return unambiguous structural information on compounds, including within mixtures and without requiring chromatography, complex sample preparation or reference standards.

Until recently, if a researcher wanted an MRR instrument, it was custom-made by each laboratory with home-built software and typically for a specific set of research projects, thus limiting its potential for widespread application.

Once a molecule has been fingerprinted by MRR, it is “forever recognizable.”¹ The technology’s exceptional selectivity and definitive structural characterization capabilities are indisputable.

MRR resolves different compounds and, most prominently, isomers in a mixture without having to first separate the components. Molecules are identified and quantified by their unique three-dimensional (3D) structure by determining the characteristic pattern of each component in the spectrum.

Because of the technology’s high degree of redundancy (many spectral lines are used to determine a few parameters), there is an extraordinary degree of certainty in the accuracy of a spectral fit.

As these parameters are directly related to the molecule’s 3D structure, identifications are made by comparing experimental parameters to those derived from quantum chemistry, which determines the structure to a degree of accuracy such that a structure identification is binary – it is either a match or not. Furthermore, the MRR spectrum is highly sensitive to the smallest structural changes, making distinguishing between similar species straightforward.

This offers powerful new capabilities to analytical chemists. Nuclear magnetic resonance (NMR), considered the gold standard for structural identification, faces inherent sensitivity and performance challenges when working with mixtures and requires a high degree of expertise to interpret.

Mass spectrometry (MS) tends to be used in tandem with liquid or gas chromatography (LC/GC), leading to expensive consumables and significant method development time and, although highly sensitive, does not generally identify isomers unambiguously. Meanwhile, other forms of spectroscopy, such as Fourier transform infrared (FT-IR), are challenging to interpret for mixtures and when reference standards are not available.

MRR spectroscopy combines the specificity of hyphenated methods with the speed of MS and complex structural information of NMR to offer a highly compelling combination of specifications.

Early development and commercial breakthrough

Rotational spectroscopy has been used for years in basic research, including for characterizing molecules in interstellar space, and its potential as an analytical tool has long been anticipated because of its selectivity and sensitivity. Hewlett-Packard attempted to commercialize it in the early 1970s but couldn’t achieve the speed of analysis required, something that always raised a barrier to its commercial adoption.

Rapid improvements in the bandwidth of digital electronics have changed that. The recent successes of MRR spectroscopy in analytical chemistry are down to two major factors: firstly, instrument developments that have improved measurement speed and sensitivity and, secondly, major leaps forward in quantum chemistry methods for the accurate prediction of equilibrium geometry, from which rotational spectroscopy parameters are derived.

A technique called chirped pulse Fourier transform rotational spectroscopy, invented in 2006 by Brooks Pate, professor of chemistry at the University of Virginia, provided the breakthrough the field of microwave spectroscopy needed.² It enabled the acquisition of broadband, high-resolution microwave spectra several orders of magnitude faster than previously possible, essentially taking a process that took hours or even days down to seconds.

Modernizing chemical analysis

The techniques employed to do chemical analysis in the pharmaceutical, chemical and consumer goods industries have largely been unchanged over the last several decades. The most commonly used chromatography-based systems and their detectors are largely the same as what was employed 20 years ago.

While stalwart, these techniques complicate the development process across industries by requiring complex method development, and delivering sometimes temperamental quantitative performance. MRR can deliver highly specific and robust results with simplifying methodologies and reduced analysis times.

These benefits, along with eliminating the need for consumables and solvents, are how MRR is bringing the analysis of volatile organic compounds into the modern conceptualization of advanced manufacturing and green technologies.

New potential in accelerating drug development

MRR is also set to have a substantial impact on drug development times and costs. According to Deloitte, the average development time for a new drug increased from 6.9 years in 2021 to 7.1 years in 2022, and these longer development times pushed the cost in the same timeframe from $298 million to a staggering $2.3 billion.³

One area where there is a clear unmet need is impurity profiling – initially in development, but also in subsequent production. This is currently done using a variety of hyphenated methods, including LC-MS, LC-NMR, LC-NMR-MS and GC-MS.

From a time and cost point of view, the industry would welcome a streamlined process. Developing reference standards is highly time-consuming. The required sample isolation, scale-up and subsequent validation – tasks that are often outsourced to a CRO and can take weeks – are no longer needed.

To use MRR as a commercial structural confirmation tool requires computational chemistry methods to predict the molecular mass distribution accurately – and it’s these new methods that remove the need for a reference standard in MRR analysis.

Considering the increasing use of AI to propose synthetic routes, impurities and degradation products, bringing this level of computation to the analytical arena offers the potential to close the loop in development faster, and significantly reduce costs and risk in the development process.

Improving synthesis and manufacturing

Optimization of synthetic routes is an integral part of the US process analytical technology (PAT) initiative that aims to improve pharmaceutical manufacturing processes through timely measurement of the critical process parameters (CPP) that affect drug quality.

Optical techniques such as Raman and IR are valued for their rapid in situ measurement capabilities, but the structural information they reveal and their ability to resolve compounds in mixtures is limited. Alternatives, such as MS, NMR and HPLC can directly quantify each component in a reaction mixture, but each comes with other performance trade-offs.

In contrast, MRR’s ability to directly identify and quantify individual components out of reaction mixtures – with no need for chromatography – makes it an attractive technique for automated real-time reaction monitoring.

New work, published early in 2024, explored MRR as a selective technology to resolve compounds directly. The authors concluded that MRR offers an “Emerging and extraordinarily selective spectroscopic technique to perform automated reaction monitoring measurements.”⁴

These capabilities are translating valuable advances in pharmaceutical and chemical process monitoring, where it offers the possibility of significant reductions in measurement time compared to other techniques.

For example, a recent study measured the enantiomeric excess (EE) of pantolactone, a chiral lactone used as a central intermediate in the synthesis of panthenol and pantothenic acid (vitamin B5), marketed globally in personal care products and over-the-counter medication.⁵ It was confirmed that MRR demonstrated “Significant reductions in measurement time.”

 

A brighter future

Having made the leap from classical research technique to routine applied analysis solution, the introduction of the first commercial MRR instrument looks set to stimulate further exciting developments.

As noted above, every molecule has a singular, unique MRR spectrum – a feature that makes it easy to imagine how data from MRR, once added to a data lake, could be used in combination with predictive AI or neural network approaches.

In addition, combining high-efficiency GC separations with arguably the most chemically specific form of spectroscopy offers exciting potential. A prototype GC-MRR spectrometer has been shown to far exceed the performance of high-resolution MS and NMR in terms of selectivity, resolution and compound identification of co-eluting compounds, including isotopes. Initial results demonstrate that GC-MRR enables straightforward qualitative and quantitative determinations with very little sample preparation.⁶

In conclusion, as researchers investigate the power of MRR, using the newly accessible commercial MRR instrument, we can look forward to a new era in spectroscopy. By removing the barriers to its widespread adoption, the vast applications potential of MRR across pharma, life science and materials sciences is set to deliver on its promise.

About the author:
Justin L. Neill is the chief technology officer and co-founder at BrightSpec, Inc. He holds a PhD in Chemistry from the University of Virginia. He has been an author on more than 60 peer-reviewed publications in the design and application of MRR spectroscopy, and is an inventor on several patents on modern MRR instrumentation.