Segmental motion in pseudomonic acid A: A carbon-13 spin-lattice relaxation time study at two field strengths.

Abstract

The mobility of pseudomonic acid A in dimethyl sulfoxide-d3 solution has been determined from 13C spin-lattice relaxation time measurements at two field strengths. The central tetrahydropyran ring reorientates isotropically with a rotational correlation time of -280 picoseconds/radian.Both side-chains exhibit marked segmental motion with correlation times decreasing to only -50 picoseconds/radian at the end of the nonanoic acid side-chain.Pseudomonic acid A is a naturally occurring antibiotic produced by fermentation of a strain of Pseudomonas flourescens.l" The gross structure was elucidated on the basis of spectroscopic and degradative data.'' This structure was confirmed and the absolute stereochemistry shown to be I by further spectroscopic and X-ray work.3)We have now measured the 13C spin-lattice relaxation times (T1's) of all the carbon atoms in I in order to gain information about the mobility of I in solution. This information is potentially useful in the understanding of both the biological activity of I and also the interaction of I with other molecules and ions in solution. The T1 data has also served as a check on the 13C spectral assignments.Results Table 1 gives the 20 MHz 13C T1 and nuclear overhauser effect (NOE) results, the 63 MHz 13C T1and NOE results and the effective 13C correlation times for a dimethyl sulfoxide-de solution of I. Fig. 1 shows three partially relaxed 63 MHz 13C spectra of I in dimethyl sulfoxide-de. ExperimentalAll experiments were conducted at ambient temperature on -0.6 M solutions of pseudomonic acid A in DMSO-de. The 20 MHz 13C NMR data was acquired on a Bruker WP 80 using 10 mm tubes. The sweep width was 3787.9 Hz over 8 K data points.The 90° pulse for 13C was -17 pseconds. The free induction decays (FIDs) were zero-filled to 16 K points and a line broadening of 1 to 1.5 Hz was applied prior to Fourier transformation.The 63 MHz 13C data was acquired on a Bruker WM250 using 5 mm tubes in a 13C/1H dual probe.The sweep width was 11627.9 Hz over 16 K data points. The 90° pulse for 13C was -10 lrseconds.