Selectivity in the modification of the α-amino groups of hemoglobin on reductive alkylation with aliphatic carbonyl compounds. Influence of derivatization on the polymerization of hemoglobin S

The reactivity of the α-amino groups of the α- and β-chains of hemoglobin toward reductive alkylation using limiting concentrations of the aliphatic carbonyl compounds, acetaldehyde (ethylation), glyoxylic acid (carboxymethylation), glycolaldehyde (hydroxyethylation), glyceraldehyde (dihydroxypropylation), and dihydroxyacetone (dihydroxyisopropylation) has been investigated. Hemoglobin A reductively ethylated at the α-amino groups eluted on CM-52 ahead of unmodified hemoglobin A, and hemoglobin A reductively ethylated at the ε-amino groups. This observation is similar to that seen on hydroxyethylation and dihydroxypropylation of the α-amino group of hemoglobin A. The presence of the α-hydroxyl or the carboxyl group in the carboxyl component used in the reductive alkylation influences considerably the selectivity pattern during the derivatization. The α-amino groups of the α- and β-chains are modified to nearly the same degree during reductive hydroxyethylation as well as during reductive dihydroxypropylation. Reductive ethylation (aldehyde lacking the α-hydroxyl group) exhibited a slight preferential reaction at Val-1(β). The presence of a negatively charged carboxyl group in the carbonyl component, i.e., glyoxylic acid, made this preferential reaction at Val-1(β) even more pronounced. When the reductive alkylation is carried out with dihydroxyacetone (a ketone instead of an aldehyde), the dihydroxyisopropylation occurred at a slower rate and exclusively at Val-1(β). The ethylation, hydroxyethylation, carboxymethylation, and dihydroxypropylation of the α-amino groups of hemoglobin S increased its solubility from the value of 16 g/dl for the unmodified protein to about 25 g/dl for the modified protein. Thus, the alkyl chains on the α-amino groups on the polymerization have a strong inhibitory influence. In order to determine the influence of the alkyl chains at the α-amino groups of α- and β-chains on polymerization, hybrid hemoglobin S tetramers with hydroxyethylation either at Val-1(α) or at Val-1(β) have been prepared. The solubility of each hybrid is about 26 g/dl. Thus, the hydroxyethyl group either on the α- or the β-chain appears to interfere with the polymerization of deoxygenated HbS to the same degree. The inhibitory influence of the hydroxyethyl chain at Val-1(α) on the polymerization, compared with the lack of such an influence when this α-amino group is modified by cyanate, suggests that a carbamoyl group on Val-1(α) can be accommodated in the intermolecular contact region involving the segment of the molecule without seriously perturbing the molecular fit of this contact region, whereas the hydroxyethyl group cannot be accommodated easily at this site, and hence the inhibition of polymerization.