Regioselective covalent modification of hemoglobin in search of antisickling agents.

Although the molecular defect in sickle hemoglobin that produces sickle cell disease has been known for decades, there is still no effective drug treatment that acts on hemoglobin itself. In this work, a series of diversely substituted isothiocyanates (R-NCS) were examined for their regioselective reaction with hemoglobin in an attempt to alter the solubility properties of sickle hemoglobin. Electrospray mass spectrometry, molecular modeling, X-ray crystallography, and conventional protein chemistry were used to study this regioselectivity and the resulting increase in solubility of the modified hemoglobin. Depending on the attached R-group, the isothiocyanates were found to react either with the Cysbeta93 or the N-terminal amine of the alpha-chain. One of the most effective compounds in the series, 2-(N,N-dimethylamino)ethyl isothiocyanate, selectively reacts with the thiol of Cysbeta93 which, in conjunction with the cationic group, was seen to perturb the local hemoglobin structure. This modified HbS shows an approximately 30% increase in solubility for the fully deoxygenated state, along with a significant increase in oxygen affinity. This compound and a related analogue appear to readily traverse the erythrocyte membrane. A discussion of the relation of these structural changes to inhibition of gelation is presented. The dual activities of increasing HbS oxygen affinity and directly inhibiting deoxy HbS polymerization, in conjunction with facile membrane traversal, suggest that these cationic isothiocyanates show substantial promise as lead compounds for development of therapeutic agents for sickle cell disease.

Rhenium(V) Oxo Complexes of Novel N(2)S(2) Dithiourea (DTU) Chelate Ligands: Synthesis and Structural Characterization.

The compounds RNHC(=S)NH(CH(2))(n)()NHC(=S)NHR were prepared in a search for new, relatively small N(2)S(2) ligands. These dithiourea (DTU) ligands are the first chelates containing two potentially bidentate thiourea moieties. A one-step reaction of 1,3-diaminopropane (1) with aryl or alkyl isothiocyanates or of 1,2-diaminoethane (2) with phenyl isothiocyanate afforded the target ligands in excellent yields (95-98%). The Re(V)=O complexes of RNHC(=S)NH(CH(2))(3)NHC(=S)NHR ligands were obtained through ligand exchange reactions with Re(V) precursors. The chemistry required neither protection of the sulfur atoms for ligand synthesis nor deprotection prior to metal complexation. The structure of (1-phenyl-3-(3-phenylthioureido)propyl]thioureato)oxorhenium(V) (7a), determined by X-ray diffraction methods, revealed the expected pseudo-square-pyramidal geometry with an N(2)S(2) basal and an apical oxo donor set. Both coordinated N's (N(c)) were deprotonated. One uncoordinated N (N(u)) was deprotonated, producing a neutral complex containing an unexpected new type of dianionic, four-membered N,S chelate. In the crystal, the N(u) atoms, N(3)H and N(4), of one complex each formed an H-bond with N(4) and N(3)H, respectively, of a symmetry-related complex. The N(c)-C-S bond angles (106.1(6) and 101.5(6) degrees ) were severely distorted from the 120 degrees expected for an sp(2)-hybridized C. However, these small bite angles and the large N-Re-N bond angle (86.1(3) degrees ) allowed for the formation of two four-membered chelate rings with normal Re-N and Re-S bond distances. Attempts to prepare complexes with the PhNHC(=S)NH(CH(2))(2)NHC(=S)NHPh ligand were unsuccessful. These results suggest that a central five-membered chelate ring is too small to accommodate bidentate coordination of both thiourea moieties. NMR studies in methanol established that the neutral complex with one uncoordinated N deprotonated was the favored form in neutral and basic solutions. However, under acidic conditions, a cationic form with both uncoordinated N's protonated was favored.