The pyramidal inversion of the N(5)-centre of several reduced flavins was measured by NMR. The inversion barrier was found to be ∼10 kcal/mol in acetone solutions and to be independent of the size of the N(5) substituent. An increase of the inversion barrier of ∼5 kcal/mol was observed in the case where the N(5) substituent could only be in axial position, and an increase of ∼3.5 kcal/mol was observed for an acyl-like N(5) substituent. In aqueous solution the inversion barrier increases by ∼3 kcal/mol. The stereochemistry of reduced flavin and its potential relevance in flavin-dependent biological dehydrogenations is discussed.

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NMR.‐Study of Nitrogen Inversion and Conformation of 1,5‐Dihydro‐isoalloxazines (‘Reduced Flavin’). Studies in the flavin series, XIX. Communication

Restricted rotation in amides—VI : Configurations and conformations of unsymmetrical tertiary benzamides

A substrate study was undertaken in order to probe the scope of S2Cl2 coupling of alcohols to form dialkoxy disulfides. Compounds 1b and 1f are new; along with 1a, 1c, 1h, and 1j, all of the title ...

Generalized Synthesis and Physical Properties of Dialkoxy Disulfides

The 13C nmr equilibration method lends itself as a tool for study of conformational rate processes involving aqueous media in conjunction with high activation barriers. This method is applied for measurement of kinetic and thermodynamic parameters of isomerism in the dipeptide L-Pro-L-4Hyp. The activation barrier for cis ⇋ trans interconversion (ω 0° 180°) is determined, ΔG≠ = 22.3 kcal/mol. From low-temperature study, an upper limit ΔG≠ < 9.7 kcal/mol is evaluated for cis′ ⇋ trans′ rotation (ψ −40° 160°). These data are compared with computed values found in literature. The results are discussed in connection with the helix–coil transition of collagen involving Gly-L-Pro-L-4Hyp as nucleation sites.

Determination of the rates and barriers to conformational isomerization in the dipeptide L‐pro‐L‐4hyp by direct 13C nmr thermal equilibration

Rates of rotation about the C-9-C-ar bond of 9-(2-isopropylphenyl) fluorene were measured by 1H DNMR, 13C DNMR, double resonance, and a kinetic method. The kinetic data obtained by these methods were critically examined. The DNMR methods afforded satisfactory results if rate data obtained by total line shape analyses of complicated signals with couplings were used, and those derived from simple patterns were abandoned. Agreement among the data yielded by the four different methods was excellent, provided that the data which might contain dubiously large errors were excluded.

Restricted rotation involving the tetrahedral carbon: XXIII—comparison of barriers to rotation of 9‐(2‐isopropylphenyl) fluorene obtained by 1H DNMR, 13C DNMR, double resonance and kinetic measurements

Barrier to conformational isomerism in an N-acetylpyrrolidine by total nuclear magnetic resonance line shape analysis and direct thermal equilibration

Additional experimental evidence supporting total n.m.r. line shape analysis for obtaining activation parameters results from a comparison of rate data derived from the direct equilibration at low temperatures of a conformationally pure rotamer of (I) and rate data from a total n.m.r. line shape analysis at higher temperatures.

Activation parameters for conformational isomerism in an N-acetylpyrrolidine by total nuclear magnetic resonance line shape analysis and direct thermal stereomutation

Untersuchung der barriere der konform.-isomerie eines n-acetyl-pyrrolidins durch vollstaendige nmr-linienprofilanalyse und direkte thermische gleichgewichtseinstellung

Maurice H. Halford

Papers

Barrier to conformational isomerism in an N-acetylpyrrolidine by total nuclear magnetic resonance line shape analysis and direct thermal equilibration

Activation parameters for conformational isomerism in an N-acetylpyrrolidine by total nuclear magnetic resonance line shape analysis and direct thermal stereomutation

Untersuchung der barriere der konform.-isomerie eines n-acetyl-pyrrolidins durch vollstaendige nmr-linienprofilanalyse und direkte thermische gleichgewichtseinstellung