The Analytical and Surface Chemistry (ASC) program of the Division of Chemistry will support the research program of Prof. Paul Cremer of Texas A&M University. Prof. Cremer and his students will study the fundamental mechanisms of the interactions of salts with model polypeptide systems. The study will reveal the profound effects that salts have on the physical properties of proteins. These interactions play crucial role in many processes including protein folding, enzyme turnover and supramolecular ordering. The study will involve the use of a wide variety of analysis tools including vibrational sum frequency spectroscopy (VSFS) and a novel temperature gradient microfluidic device which was developed by Prof. Cremer and his students. The study will provide an excellent educational opportunity for graduate students and postdoctoral trainees to carry out a fundamental protein dynamics study using state of the art analytical techniques. The study will be applicable to a wide variety of problems ranging from polymer physics and medicine to colloid chemistry and protein isolation.
The research conducted under NSF Chemistry Grant 0809854 was conducted by Prof. Paul S. Cremer and his students at Texas A&M University. This team of researchers conducted work in solutions consisting of water, salt, and protein molecules. The idea was to understand how ions in water interact with proteins. These interactions influence the ability of proteins to fold and function. Ions in the body can actually regulate protein folding, enzyme catalysis, ion channel activity and cell signaling. The interaction of proteins with improper ions or incorrect quantities of ions can lead to numerous diseases. These systems have been under investigation for over 120 years, yet it is still mystery to scientists exactly how ions interact with proteins. It was found at the end of the 19th century that ions influence the properties of proteins in a systematic way that depends upon the identification of the ions. The rank order of activities of these ions can be placed into a series call the Hofmeister series. The Cremer laboratory over the last three years was able to elucidate the molecular-level mechanisms of the Hofmeister series for the interactions of ions with the polypeptide chains that make up proteins. For uncharged polypeptides, it was found that anions interacted with the chains much more strongly than cations. The interaction sites for both the anions and cations were the amide backbone of the polypeptides rather than hydrophobic side chains. The introduction of charged side chains was found to modulate these interactions. As expected, cations interacted with carboxylate groups, which are negatively charged. Anions were able to interact with positively charged groups such as lysine and arginine residues. These ion pairing interactions were stronger than the interactions with the polypeptide backbone, but there were generally far fewer of them. Under the appropriate circumstances, such ion pairing interactions would lead to the reversal of the Hofmeister series. In particular, more hydrophobic and larger anions (called chaotropic anions) would cause hydrophobic collapse and protein aggregation via a reverse series if the protein was strongly positively charged. This occurred because binding of the anions worked to neutralize the protein, thus leading to aggregate formation. More hydrophilic and highly charged anions could also lead to partial Hofmeister series reversal. This occurred by interaction of the anions directly with the sites of positive charge. No series reversal was found for cations with the systems investigated. In addition to salts, two more co-solutes were studies: urea and trimethylamine N-oxide (TMAO). Urea is known to denature proteins while TMAO is a powerful protein stabilizer. Many questions have been raised over the past century as to whether these molecules, called osmolytes, affect protein folding through direct interactions or by changing water structure. It was found in the case of urea that direct interactions with amide groups by hydrogen bonding were very weak. If one urea bound to two amides, then it would actually stabilize protein structure rather than denature it. Therefore, any denaturation affects need to be with only one amide per urea or via interactions with the hydrophobic groups. On the other hand, TMAO was generally excluded from the region of proteins. When it did reach a hydrophobic surface, its’ methyl group were oriented away from the interface. This is surprising since methyl groups normally orient toward hydrophilic surfaces. It therefore must be a combination of exclusion from protein/water interfaces and perhaps the unusual orientation of TMAO that leads to protein stabilization.
