Abstract
Cooperative behaviors of the hydrogen bonding networks in proteins have been discovered for a long time. The structural origin of this cooperativity, however, is still under debate. Here we report a new investigation combining excess infrared spectroscopy and density functional theory calculation on peptide analogs, represented by N-methylformamide (NMF) and N-methylacetamide (NMA). Interestingly, addition of the strong hydrogen bond acceptor, dimethyl sulfoxide, to the pure analogs caused opposite effects, namely red- and blue-shift of the N−H stretching infrared absorption in NMF and NMA, respectively. The contradiction can be reconciled by the marked lowering of the energy levels of the self-associates between NMA molecules due to a cooperative effect of the hydrogen bonds. On the contrary, NMF molecules cannot form long-chain cooperative hydrogen bonds because they tend to form dimers. Even more interestingly, we found excellent linear relationships between changes on bond orders of N−H/N−C/C = O and the hydrogen bond energy gains upon the formation of hydrogen bonding multimers in NMA, suggesting strongly that the cooperativity originates from resonance-assisted hydrogen bonds. Our findings provide insights on the structures of proteins and may also shed lights on the rational design of novel molecular recognition systems.
Highlights
The origin of the cooperativity has been investigated by calculation for years and different opinions were proposed[12]
In NMF-dimethyl sulphoxide (DMSO)-d6 system, for each concentration, the negative peak is on the high wavenumber side and the positive peak is on the low wavenumber side
By comparing the behavior of two similar molecules, N-methylformamide and N-methylacetamide, in forming hydrogen bonds with an aprotic molecule, dimethylsulphoxide, we have been fortunate to observe opposite shifting of the N−Hstretching absorption bands. This allows us to state that N-methylacetamide is a better molecular model in studying peptide bonds, when the issue of cooperative hydrogen bonds in proteins/peptides is under concern
Summary
The origin of the cooperativity has been investigated by calculation for years and different opinions were proposed[12]. The self-association structures of both NMF and NMA have common hydrogen bonds in the form of N−H···O =C, the same as in proteins[38,39].
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