Automated 2D IR spectroscopy using a mid-IR pulse shaper and application of this technology to the human islet amyloid polypeptide

Abstract

The capability of 2D IR spectroscopy to elucidate time-evolving structures is enhanced by a programmable mid-IR pulse shaper that greatly improves the ease, speed, and accuracy of data collection. Traditional ways of collecting 2D IR spectra are difficult to implement, cause distorted peak shapes, and result in poor time resolution and/or phase problems. We report on several methods for collecting 2D IR spectra by using a computer-controlled germanium acoustooptic modulator that overcomes the above problems. The accuracy and resolution of each method is evaluated by using model metal carbonyl compounds that have well defined lineshapes. Furthermore, phase cycling can now be employed to largely alleviate background scatter from heterogeneous samples. With these methods in hand, we apply 2D IR spectroscopy to study the structural diversity in amyloid fibers of aggregated human islet amyloid polypeptide (hIAPP), which is involved with type 2 diabetes. The 2D IR spectra reveal that the β-sheet fibers have a large structural distribution, as evidenced by an inhomogeneously broadened β-sheet peak and strong coupling to random coil conformations. Structural diversity is an important characteristic of hIAPP because it may be that partly folded peptides cause the disease. This experiment on hIAPP is an example of how computer generation of 2D IR pulse sequences is a key step toward automating 2D IR spectroscopy, so that new pulse sequences can be implemented quickly and a diverse range of systems can be studied more easily.