A nuclear magnetic resonance study of the helix–coil transition of poly(L‐glutamic acid)

Abstract

SynopsisThere have been many reports that the nuclear magnetic resonance (nrnr) spectra of a large number of polypeptides exhibit peak doubling of the a-carbon and the a-carbon proton in the helix-coil transition region. One apparent exception to this generalization has been polypeptides with ionizable side chains, where the helix-coil transition is induced by changes in pH in aqueous solution. Because it is important to establish the proper theoretical reason for the peak doubling and its relation to the rate of conformational change of amino acid residues, we have reexamined the proton and carbon-13 nrnr spectra, a t high field, for two polydisperse samples of poly(L-glutamic acid). Doubling of the a-carbon proton resonance as well as those of the a-and &carbon, and backbone carbonyl are observed for a low-molecular-weight sample (DP = 54), while a higher molecular weight sample (DP = 309), exhibits only single resonances. Thus, polydispersity by itself is not sufficient to observe peak doubling; low-molecular weight is also required.

Characterization of selectively carbon-13-labeled synthetic melittin and melittin analogs in isotropic solvents by circular dichroism, fluorescence, and NMR spectroscopy

“…i3C NMR spectra of the synthetic peptides in D20/CD30D solutions give evidence favoring the latter interpretation. In support of this, we note that doubling of a peptide -carbon resonance has been observed for a-helical poly(L-glutamic acid) (Lader et al, 1977). Specifically, these authors found that a downfield Ca peak at --56.7 ppm becomes more prominent as the solution pH is lowered and helix content increases.…”

15N NMR spectroscopy 28—solvent effects on the 15N13C coupling constants of amides, imides, ureas and polypeptides

“…Whereas poly-L-alanine and poly-L-valine are soluble in TFA, poly-L-leucine is only soluble in sulphonic acids or mixtures of sulphonic acids with TFA. Because of the line width of the 14N isotopomers (8)(9)(10)(11)(12)(13)(14)(15)(16) the "N-13C-a and 15N-13C-p coupling constants, which are expected to be only of the order of 3-9Hz, were not measurable. The line widths are mainly due to the short transverse relaxation time (T2) of the carbons in question.…”

“…The digital resolution must be better than 0.1 Hz and the expensive "N enrichment should be better than 50%. Thus, the measurements of 13C and "N NMR chemical shifts, which are good or moderate monitors of helical-coil transitions, [13][14][15][16][17][18] are more practical. Furthermore, a change of ' J ( N C 0 ) in the course of a solvent induced helix-coil transition is the result of at least two influences: first the net conformational effect and, second, the direct solvent effect.…”

Origin of multipeak behavior in the NMR spectra of poly(L‐glutamic acid)

“…We next wish to analyze the spectra for polydisperse ( G~u )~~, for which multiple peaks are expected based on our previous experimental results. 9 The molecular-weight distribution of this sample was determined by the methods described above. The elution curve of ( G~u )~~ is shown on Fig.…”

“…However, it has been previously noted that the presence or absence of multiple resonances is the same by either carbon-13 or proton nmr for a given sample. 9 The samples used to obtain the proton nmr spectra were dissolved in 99.9 at. % D20 (Wilmad) and allowed to exchange at room temperature for a few hours prior to lyophilization.…”