Estimation of the B-II conformer substate population in nonoriented hydrated B-DNA via curve resolution of infrared spectra

Hydrated films of the self-complementary B-type d(CGCGAATTCGCG)(2) dodecame r, with 13 or 14 water molecules per nucleotide, were studied by Fourier tr ansform infrared (FT-IR) spectroscopy either as equilibrated samples betwee n 290 and 260 K or, after quenching into the glassy state, as nonequilibrat ed films isothermally at 200 K, IR spectral changes on isothermal relaxatio n at 200 K revealed that difference spectra are caused by interconversion o f B-DNA's conformer substates, B-I converting into B-II. Curve resolution o f IR spectra in the spectral region containing the symmetric stretching vib ration of the ionic phosphate group, with a composite band centered at 1087 cm(-1) for B-I and at 1109 cm(-1) for B-II, gives the B-I/B-II conformer s ubstate population ratio and its dependence on temperature between 290 and 260 K, and on time for isothermal relaxation at 200 K. Curve resolution of the overlapping composite band profile was optimized by comparison of secon d-derivative and of difference curves, aiming thereby for optimal correspon dence between experimental and curve-fitted bands as a criterion for goodne ss-of-fit. The spectroscopic aspects relevant for reliable curve resolution of the overlapping composite band profile are reported and discussed in de tail.