Biological Systems and Polymers

Article on the use of MARI with biomolecules and polymers

The peptide link is one of the fundamental structural units in biological systems. Understanding what holds these molecules together is vital to the understanding of many biological structures and processes.

N-methyl acetemide (CD3 - NHCO - CD3) is the canonical system in the study of the peptide chain. The peptide chain itself is at the heart of many biological systems and structures. One of the benefits of neutron spectroscopy is its ability to probe in momentum transfer as well as energy transfer. In the case of molecular systems this means that it is possible to measure the effective mass of the vibration, as well as its frequency.

From measurements on N-methyl acetemide it is clear that all the vibrations peak on the recoil line for a single hydrogen. That is, the hydrogen must be considered as an isolated atom sitting in a potential created by the surrounding heavier atoms. This is completely unexpected. It is not possible to treat the hydrogen as if it is part of the molecule, i.e. it is not tightly bound to one of the heavier atoms. This requires a radically different way of thinking about the structure and dynamics of the peptide chain. It also means that the conventional assignment of the modes cannot be correct and that the strength of the hydrogen is almost half that assumed in earlier models.

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