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Prominent in the resonance Raman spectra of the molybdenum center in oxidoreductive enzymes are the bands at $1527\pm 5cm-1$ (mono- and bis-pterin) and at $1575\pm 5cm-1$ (bis-pterin). First principles electronic structure calculations of the molybdopterin cofactor (UHF/DF/B3L YP methods at LanL2DZ+5D+SP level) suggest a predominance of two (out of 618) tautomers: the R-C6-protonated, highly twisted $(\theta \sim 65\pm 12\circ )$ and the C6-unprotonated, less twisted $(\theta \sim 19\pm 7\circ )$ form. At lower pH regime, the high degree of pterin protonation in the first tautomer class is accompanied by elongation of the Mo-dithiolene C=C bond (with the stretching fundamental vibration at $1527cm-1$). The opposite holds true for the unprotonated pro-R C6 tautomer (the $1575cm-1$ band). The presence of both tautomer forms in bis-pterin enzymes suggests the existence of a proton-gradient across the molybdenum center domain. These results are in excellent agreement with the x-ray structural data. (Supported in part by NIH grant AR 38917 to R.H.)