Protein Conformation and Supercharging with DMSO from Aqueous Solution

The efficacy of dimethyl sulfoxide (DMSO) as a supercharging reagent for protein ions formed by electrospray ionization from aqueous solution and the mechanism for supercharging were investigated. Addition of small amounts of DMSO to aqueous solutions containing hen egg white lysozyme or equine myoglobin results in a lowering of charge, whereas a significant increase in charge occurs at higher concentrations. Results from both near-UV circular dichroism spectroscopy and solution-phase hydrogen/deuterium exchange mass spectrometry indicate that DMSO causes a compaction of the native structure of these proteins at low concentration, but significant unfolding occurs at ~63% and ~43% DMSO for lysozyme and myoglobin, respectively. The DMSO concentrations required to denature these two proteins in bulk solution are ~3–5 times higher than the concentrations required for the onset of supercharging, consistent with a significantly increased concentration of this high boiling point supercharging reagent in the ESI droplet as preferential evaporation of water occurs. DMSO is slightly more basic than m-nitrobenzyl alcohol and sulfolane, two other supercharging reagents, based on calculated proton affinity and gas-phase basicity values both at the B3LYP and MP2 levels of theory, and all three of these supercharging reagents are significantly more basic than water. These results provide additional evidence that the origin of supercharging from aqueous solution is the result of chemical and/or thermal denaturation that occurs in the ESI droplet as the concentration of these supercharging reagents increases, and that proton transfer reactivity does not play a significant role in the charge enhancement observed

Deformability of poly(amidoamine) dendrimers

Experimental data indicates that poly(amidoamine) (PAMAM) dendrimers flatten when in contact with a substrate, i.e. they are no longer spherical, but resemble flat disks. In order to better understand the deformation behavior of these branched polymers, a series of atomistic molecular dynamics simulations is performed. The resulting flattened dendrimer conformations are compared to atomic force microscopy (AFM) images of individual dendrimers at air/mica and water/mica interfaces. The ability of the polymers to deform is investigated as a function of dendrimer generation (2-5) and the required energies are calculated. Our modeling results show good agreement with the experimental AFM images, namely that dendrimers are highly flexible and capable of forming multiple interaction sites between most of their branch ends and the substrate. The deformation energy scales with dendrimer generation and does not indicate an increase in stiffness between generations 2 and 5 due to steric effects.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45831/1/10189_2003_Article_10087.pd

Peek Oligomers - a Model for the Polymer Physical Behavior .2. Structure and Thermal-behavior of Linear Monodisperse Oligomers

The thermal and structural properties of monodisperse linear poly(oxy-1,4-phenyloxy-1,4-phenylenecarbonyl-1,4-phenylene) (PEEK) oligomers have been investigated by differential calorimetry, optical microscopy, and wide-angle and small-angle X-ray wattering. The complex melting, observed for the longest oligomers, is related to variations in the relative placement of the ether and ketone bridges of the chains in the lamellae (eclipsed and staggered lamellae). Chain packing is discussed in terms of chain staggering, and appropriate conclusions are drawn concerning the variations of the polymer unit cell dimensions with crystallization temperature. Two different types of spherulites have been detected for the longest oligomer; these are shown to result from differences in the preferred main orientation of the (100) planes of the lamellae