Journal of Physical Chemistry A, Vol.111, No.49, 12409-12416, 2007
Modeling competitive interactions in proteins: Vibrational spectroscopy of M+(n-methylacetamide)(1)(H2O)(n)(=0-3), M = Na and K, in the 3 mu m region
To properly understand the preferred structures and biological properties of proteins; it is important to understand how they are influenced by their immediate environment. Competitive intrapeptide, peptide center dot center dot center dot water, ion center dot center dot center dot water, and ion center dot center dot center dot peptide interactions, such as hydrogen bonding, play a key role in determining the structures, properties, and functionality of proteins. The primary types of hydrogen bonding involving proteins are intramolecular amide center dot center dot center dot amide (N-H center dot center dot center dot O=C) and intermolecular amide center dot center dot center dot water (O-H center dot center dot center dot O=C and H-O center dot center dot center dot H-N). n-Methylacetamide (NMA) is a convenient model for investigating these competitive interactions. An analysis of the IR photodissociation (IRPD) spectra of M+(n-methylacetamide)(1)(H2O)(n=0-3) (M = Na and K) in the O-H and N-H spectral regions is,presented. Ab initio calculations (MP2/cc-pVDZ) are used as a guide in identifying both the type and location of hydrogen bonds present. In larger clusters, where several structural isomers may be present in the molecular beam, ab initio calculations are also used to suggest assignments for the observed spectral features. The results presented offer insight to the nature of ion center dot center dot center dot NMA interactions in an aqueous environment and reveal how different ion center dot center dot center dot ligand pairwise interactions direct the extent of water center dot center dot center dot water and water center dot center dot center dot NMA hydrogen bonding observed.