Conformational analysis of a secondary hydroxamic acid in aqueous solution by NOE spectroscopy

Abstract

Hydroxamic acids are metal-binding compounds used by micro-organisms and possess applications in medicine and industry. Hydroxamic acids favor two conformations, E and Z; metal binding is limited to the Z conformation. The Z conformation may be identifiable by NOE spectroscopy, but analysis is complicated by the potential for long-range coupling as well as for relayed NOEs due to conformational switching. In this report, we re-examine the reported conformational preference of N-methyl acetohydroxamic acid (NMHA) in D(2)O using NOE spectroscopy. We find that the favored conformation of NMHA in aqueous solution is the E conformation, contrary to an earlier report. NOE build-up curves are proposed as a valuable tool to probe conformational behavior in similar systems.

Kinetic and thermodynamic characterization of C–N bond rotation by N‐methylacetohydroxamic acid in aqueous media

“…The NMHA/D 2 O system met none of these requirements. Our prior work documented weak long‐range coupling between methyl groups in NMHA, and populations of species undergoing exchange exist in an approximately 3 : 1 ratio . Moreover, at the solvent‐limited lower end of the accessible temperature range in D 2 O, methyl peaks still appear to be narrowing.…”

“…N ‐methylacetohydroxamic acid was prepared as previously described and was additionally distilled under high vacuum in acid‐washed glassware before use . Trace amounts of the synthetic precursor to NMHA co‐distilled with the product and were removed after distillation by extraction from an aqueous solution of NMHA with hexanes (mixture of isomers).…”

“…NMHA was dissolved in low‐paramagnetic D 2 O (Cambridge Isotope Laboratories, Cambridge, MA, USA, used without further purification) to a concentration of 35 m m . The solute was previously confirmed to be monomeric at this concentration by dilution studies . The NMR probe temperature was controlled for all data collection.…”

“…The bond rotation was modeled as a zero order reaction, consistent with a system in which rate is independent of concentration. NMHA has been demonstrated to be at a non‐aggregating concentration at 0.035 m in D 2 O, so this treatment is believed valid . The rate of the process is therefore equivalent to the rate constant, enabling Arrhenius analysis to be performed using bond rotation rates at various temperatures.…”

“…Recently, we showed via NOE studies that the favored conformation of NMHA is E in D 2 O . This result suggests that, in water, secondary HAs favor a conformation that cannot function as a bidentate ligand to iron or other metals.…”