TY - JOUR

T1 - Lifetime distributions and anisotropy decays of indole fluorescence in cyclohexane/ethanol mixtures by frequency-domain fluorometry

AU - Gryczynski, Ignacy

AU - Wiczk, Wieslaw

AU - Johnson, Michael L.

AU - Lakowicz, Joseph R.

PY - 1988/12

Y1 - 1988/12

N2 - We used frequency-domain fluorometry to measure intensity and anisotropy decay of indole fluorescence in cyclohexane/ethanol mixtures at 20°C. In 100% cyclohexane or 100% ethanol the intensity decay of indole appears to be a single exponential with decay times of 7.66 and 4.10 ns, respectively. In cyclohexane containing a small percentage of ethanol (up to 10%), we observed increased heterogeneity in intensity decay, resulting in a 10-fold increase in χR2 for the single-exponential fit, as compared with the double-exponential model. We obtained comparable or better fits using unimodal Lorentzian and Gaussian lifetime distributions (two floating parameters) than for the two-exponential model (three floating parameters). We believe that the distribution of decay times reflects a range of indole salvation states in the dominately nonpolar solutions. This result suggests that a variety of hydrogen-bonding configurations could be one origin of the distributions of decay times observed for tryptophan emission from proteins. We also measured rotational diffusion of indole in cyclohexane, ethanol and its mixtures at 20°C. The picosecond correlation times required that the mean decay times be decreased by acrylamide quenching (in ethanol) or energy transfer (in cyclohexane). In ethanol we observed nearly isotropic rotation of indole; in cyclohexane we obtained two correlation times of 17 and 73 ps. The shorter correlation time in cyclohexane appears to be due to the slip boundary condition, which was found to be progressively eliminated by small percentages of ethanol. Hence, hydrogen-bonding interactions appear to have a substantial effect on the rotational dynamics of indole.

AB - We used frequency-domain fluorometry to measure intensity and anisotropy decay of indole fluorescence in cyclohexane/ethanol mixtures at 20°C. In 100% cyclohexane or 100% ethanol the intensity decay of indole appears to be a single exponential with decay times of 7.66 and 4.10 ns, respectively. In cyclohexane containing a small percentage of ethanol (up to 10%), we observed increased heterogeneity in intensity decay, resulting in a 10-fold increase in χR2 for the single-exponential fit, as compared with the double-exponential model. We obtained comparable or better fits using unimodal Lorentzian and Gaussian lifetime distributions (two floating parameters) than for the two-exponential model (three floating parameters). We believe that the distribution of decay times reflects a range of indole salvation states in the dominately nonpolar solutions. This result suggests that a variety of hydrogen-bonding configurations could be one origin of the distributions of decay times observed for tryptophan emission from proteins. We also measured rotational diffusion of indole in cyclohexane, ethanol and its mixtures at 20°C. The picosecond correlation times required that the mean decay times be decreased by acrylamide quenching (in ethanol) or energy transfer (in cyclohexane). In ethanol we observed nearly isotropic rotation of indole; in cyclohexane we obtained two correlation times of 17 and 73 ps. The shorter correlation time in cyclohexane appears to be due to the slip boundary condition, which was found to be progressively eliminated by small percentages of ethanol. Hence, hydrogen-bonding interactions appear to have a substantial effect on the rotational dynamics of indole.

KW - Fluorescence decay

KW - Frequency-domain fluorometry

KW - Indole

KW - Lifetime distribution

UR - http://www.scopus.com/inward/record.url?scp=0024202514&partnerID=8YFLogxK

U2 - 10.1016/0301-4622(88)87005-4

DO - 10.1016/0301-4622(88)87005-4

M3 - Article

C2 - 3251567

AN - SCOPUS:0024202514

VL - 32

SP - 173

EP - 185

JO - Biophysical Chemistry

JF - Biophysical Chemistry

SN - 0301-4622

IS - 2-3

ER -