Head group modifications

However, the findings from modification of the aminoguanidine moiety, which resulted in decreased potencies, were consistent with the hypothesis of a dual-type binding mode for this head group. On the other hand, replacing one nitrogen of the guanidine with a carbon or a sulfur retained activity. 

Effect of salt type and concentration The ionic strength of the aqueous solution in eontaet with a reverse micelle phase affects protein partitioning in a number of ways [18,23]. The first is through modification of electrostatic interactions between the protein surface and the surfaetant head groups by modifieation of the eleetrieal double layers adjacent to both the eharged inner mieelle wall and the protein surface. The second effect is to salt out the protein from the mieelle phase because of the inereased propensity of the ionie speeies to migrate to the micelle water pool, reduee the size of the reverse mieelles, and thus displace the protein. 

PDMS in its native form does not possess reactive groups that can be used for the covalent attachment of NAs [51]. However, the PDMS surface can be plasma induced oxidized and then fimctionaUzed with organosilanes carrying the desired head group. For example, a PDMS surface has been modified with 3-mercaptotrimethoxysilane to yield a thiol-terminated surface, to which a 5 -acrylamide modified DNA has been covalently attached [52]. See Fig. 13 for a representation of the PDMS surface-modification process. 

Compounds 15-19 belong to the major class of moth pheromones that are biosynthesized by the action of a desaturase on a fatty acyl precursor followed by modification of the polar head group of the alkyl chain (Francke and Schulz,... 

Synthesis of most phospholipids starts from glycerol-3-phosphate, which is formed in one step from the central metabolic pathways, and acyl-CoA, which arises in one step from activation of a fatty acid. In two acylation steps the key compound phosphatidic acid is formed. This can be converted to many other lipid compounds as well as CDP-diacylglycerol, which is a key branchpoint intermediate that can be converted to other lipids. Distinct routes to phosphatidylethanolamine and phosphatidylcholine are found in prokaryotes and eukaryotes. The pathway found in eukaryotes starts with transport across the plasma membrane of ethanolamine and/or choline. The modified derivatives of these compounds are directly condensed with diacylglycerol to form the corresponding membrane lipids. Modification of the head-groups or tail-groups on preformed lipids is a common reaction. For example, the ethanolamine of the head-group in phosphatidylethanolamine can be replaced in one step by serine or modified in 3 steps to choline. 

More recently, Chen et al. described a surface modification whereby the polymer poly(Ar,Ar-dimethyl-Af-(ethoxycarbonylmethyl)-Ar-[2/-(methacryloyloxy)ethyl]-ammonium bromide) was grafted from a surface via ATRP [136], The cationic polymer effectively kills E. coli and is subsequently converted into a zwitterionic polymer by hydrolysis of the head group (Fig. 9). It then repels all attached cells dead or alive. This is the first example of a surface that can kill microbes on contact and repels them after that. The only downside of this elegant system is that it will eventually exhaust and turn into a more or less effective repelling surface. 

Palmitate is the starting point for other fatty acids that use a set of related reactions to generate the modified chains and head groups of the lipid classes. Microsomal enzymes primarily catalyze these chain modifications. Desaturation uses O2 as the ultimate electron acceptor to introduce double bonds at the nine, six, and five positions of an acyl-CoA. 

PEG 300 on paclitaxel transport was observed. It was concluded that the efflux pump inhibition mechanism of PEG 300 was mediated by changes in the microenvironment of Caco-2 cell membranes (Hugger et al. 2002). These changes are probably related to modifications in the fluidity of the polar head group regions of cell membranes. 

Satouchi, K., Pinckard, R. N., McManus, L. M., and Hanahan, D. J. (1981) Modification of the polar head group of acetyl glyceryl ether phosphorylcholine and subsequent effects upon platelet activation, J. Biol. Chem. 256, 4425-4432. 

---