Guanidinium

H a NH arginine guanidinium cation N,Nbis (adamantyl-oxycarbonyl) = Adoc pH< 10 iQA. H+... 

Gel filtration separations are performed in an aqueous environment and the requirement on chemical resistance is therefore modest. Most column tubes and accessories (e.g., support net, sealings) withstand the solvent mixtures sometimes used in aqueous SEC such as 20% ethanol, 3 M guanidinium hydrochloride, 6 M urea, or 0.1 M HCl and 1 M NaOH, the last two being used for cleaning the packed column (see Section V). 

Shifts in the SEC fractionation range are not new. It has been known for decades that adding chaotropes to mobile phases causes proteins to elute as if they were much larger molecules. Sodium dodecyl sulfate (SDS) (9) and guanidinium hydrochloride (Gd.HCl) (9-12) have been used for this purpose. It has not been clearly determined in every case if these shifts reflect effects of the chaotropes on the solutes or on the stationary phase. Proteins are denatured by chaotropes the loss of tertiary structure increases their hydrodynamic radius. However, a similar shift in elution times has been observed with SEC of peptides in 0.1% trifluoroacetic acid (TEA) (13-15) or 0.1 M formic acid (16), even if they were too small to have significant tertiary structure. Speculation as to the cause involved solvation effects that decreased the effective pore size of the... 

If the protein of interest is a heteromultimer (composed of more than one type of polypeptide chain), then the protein must be dissociated and its component polypeptide subunits must be separated from one another and sequenced individually. Subunit associations in multimeric proteins are typically maintained solely by noncovalent forces, and therefore most multimeric proteins can usually be dissociated by exposure to pEI extremes, 8 M urea, 6 M guanidinium hydrochloride, or high salt concentrations. (All of these treatments disrupt polar interactions such as hydrogen bonds both within the protein molecule and between the protein and the aqueous solvent.) Once dissociated, the individual polypeptides can be isolated from one another on the basis of differences in size and/or charge. Occasionally, heteromultimers are linked together by interchain S—S bridges. In such instances, these cross-links must be cleaved prior to dissociation and isolation of the individual chains. The methods described under step 2 are applicable for this purpose. 

HOAt, 7-aza-l-hydroxybenzotriazole HATU (CAS Registry No. 148893-10-1), A-[(dimethylamino) (3//-1,2,3-triazolo(4,5- )pyridin-3-yloxy)methylene]-A-methyl-methanaminium hexafluorophosphate, previously known as G-(7-azabenzotriazol-l-yl)-l,l,3,3-tetramethyluronium hexafluorophosphate. [Note Assignment of structure to HATU as a guanidinium species rather than as a uronium species, i.e., attachment of the (Mc2NC=NMe2) unit to N3 of 7-azabenzotriazole 1-A-oxide instead of to the O, is based on X-ray analysis (ref. 33b)]. 

Scheme 5.2-15 Synthesis of a guanidinium-modified triphenylphosphine ligand.

While unmodified xanthene ligands (compound a in Figure 5.2-4) show highly preferential solubility in the organic phase in the biphasic l-octene/[BMIM][PFg] mixture even at room temperature, the application of the guanidinium-modified xanthene ligand (compound b in Figure 5.2-4) resulted in excellent immobilization of the Rh-catalyst in the ionic liquid. 

After ten consecutive runs the overall turnover number reaches up to 3500 mol 1-octene converted per mol Rh-catalyst. In agreement with these recycling experiments, no Rh could be detected in the product layer by AAS or ICP, indicating leaching of less then 0.07 %. In all experiments, very good selectivities for the linear aldehyde were obtained, thus proving that the attachment of the guanidinium moiety onto the xanthene backbone had not influenced its known positive effect on... 

Figure 5.2-4 Unmodified (a) and guanidinium-modified (b) xanthene ligand as used in the...

BMIM][PFg] with a guanidinium-modified diphosphine ligand with xanthene backbone. 

The introduction of another organic cation function, guanidinium group, into macrocyclic structures such as (IV)-(VI) produces ligands which also display affinity for phosphate anions58). 

A novel guanidinium ylide-mediated procedure has recently been reported by Ishi-kawa [62]. Though not an imine transformation, it does employ an imine precursor in the fonn of an aldehyde. Guanidinium ylides react with aldehydes to form aziridines (Scheme 1.35). The mechanism for the formation of the aziridine is believed to involve [3+2] cycloaddition between the guanidinium ylide 112 and the aldehyde, followed by stereospecific extrusion of the urea with concomitant aziridine formation. 

---