Peptide derivatization as a strategy to form fixed-charge peptide radicals

As a means of generating fixed-charge peptide radicals in the gas phase we have examined the collision-induced dissociation (CID) chemistry of ternary [Cu(II)(terpy)(TMPP-M)]2+ complexes, where terpy = 2,2':6'2'-terpyridine and TMPP-M represents a peptide (M) modified by conversion of the N-terminal amine to a [tris(2,4,6-trimethoxyphenyl)phosphonium]acetamide (TMPP-) fixed-charge derivative. The following modified peptides were examined: oligoglycines, (Gly)n (n = 1-5), alanylglycine, glycylalanine, dialanine, trialanine and leucine-enkephaline (YGGFL). The [Cu(II)(terpy)(TMPP-M)]2+ complexes are readily formed upon electrospray ionization (ESI) of a mixture of derivatized peptide and [Cu(II)(terpy)(NO3)2] and generally fragment to form transient peptide radical cations, TMPP-M+*, which undergo rapid decarboxylation for the simple aliphatic peptides. This is contrasted with the complexes containing the unmodified peptides, which predominantly undergo fragmentation of the coordinated peptide. These differences demonstrate the importance of proton mobility in directing fragmentation of ternary copper(II) peptide complexes. In the case of leucine-enkephaline, a sufficient yield of the radical cation was obtained to allow further CID. The TMPP-YGGFL+* ion showed a rich fragmentation chemistry, including CO2 loss, side-chain losses of an isopropyl radical, 2-methylpropene and p-quinomethide, and *a1 and *a4 sequence ion formation. In contrast, the even-electron TMPP-YGGFL+ ion fragments to form *a(n) and *b(n) sequence ions as well as the [*b4 + H2O]+ rearrangement ion.     

Dynamic wetting: hydrodynamic or molecular-kinetic?

The dynamic wetting behavior of simple liquids (water, glycerin, formamide, ethylene glycol, and a mixture of water and ethylene glycol) and polydimethylsiloxane (PDMS) oils with different viscosities has been investigated. The hydrodynamic, molecular-kinetic, and combined molecular-hydrodynamic models have been applied to the experimental results to evaluate the models' adequacy. Our work suggests that the molecular displacement, i.e., the adsorption and desorption process, seems to be dominant for the simple liquids investigated. For polydimethylsiloxanes, our work suggests that none of the evaluated models is sufficient to explain the experimentally observed dependence of the dynamic contact angle on contact velocity. This work, to the best of our knowledge, provides the first extensive comparison of the three models with experimental data over a wide range of viscosity. In addition, we have investigated the contact angle hysteresis and conclude that it is a strong function of the contact speed, the interactions between the fluids and the substrate, and the fluid viscosity.