Light-Activated Molecular Conductivity in the Photoreactions of Vitamin D3

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Introduction
The mainstreaming of personal computers and the surge in
use of handheld digital devices such as cellular phones have
been accompanied by a continued demand for faster, more
versatile electronic circuits. On the forefront of research in this
area is nanotechnologysdesigning and fabricating nanosized
electronic components that provide increased functionality
compared to current silicon-based technologies while occupying
a fraction of the size. One promising solution for scaling down
electronic devices below the present limits of photolithography
is the molecular circuit. Understanding and manipulating molecular
conductivity is one of the initial steps in the development of such
circuits,1-4 and this requires knowledge of a particular molecule’s
electrical character. Indeed, many organic and biological molecules
with conductive properties have been examined as potential circuit
elements.5-7 Of even more interest are light-sensitive molecules
with conductive character. A molecule whose electrical conductance
changes in response to light is an excellent candidate for
light-activated circuits.
One light-sensitive biological molecule of interest is 7-dehydrocholesterol
(provitamin D3), which, upon exposure to
ultraviolet (UV) light in the range of 240-300 nm,8 undergoes
photoconversions that lead to the production of cholecalciferol
(vitamin D3). These reactions occur in the skin, naturally
providing the body with a supply of this vitamin. Its physiological
importance has been established; vitamin D is known
to offer protective effects against the development of cancers
of the breast, colon, prostate, lung, and ovary9,10 and the
therapeutic intervention and prevention of many disorders
including diabetes and osteoporosis.11
Through extensive studies using various cellular and
liposomal models, the production of vitamin D3 in the
epidermis is currently well-understood.12-17 An equal effort
has been given in characterizing these photoreactions outside
the body through experimental18-22 and, more recently,
computational techniques.23-26 Through their analyses, many
have contributed to a more complete understanding of the
photochemical process at each stage and of the factorsssolvent
and wavelength, in particularsthat influence the progression
through subsequent stages.8,27-30
Neither the electrical properties of the molecules in these
photoreactions nor their potential usage in a biomolecular circuit
has yet to be considered. No one has explored the ability to
maximize any useful electrical properties by controlling provitamin
D conversion and eliminating the undesired side
products that result from overirradiation.31
Therefore, in this work, we use ab initio methods to evaluate
the practicability of using the conductivity characteristics of the
moieties formed during the photoreactions of vitamin D3 and
experimentally measure the current-voltage characteristics as
a proof-of-concept. In the next section we describe the theoretical
and experimental methodologies, and then the following one
explains the results. Finally the last section summarizes the
conclusions.
2. Methodology
Calculations. The energies and geometries of the molecules
participating in the photoreaction of vitamin D3 are calculated with
first-principles density functional theory using the program Gaussian
0332 by means of the B3PW91 hybrid functional which uses
the fully nonlocal Becke 3 (B3) exchange functional and a
combination of the generalized gradient approximation (GGA) and
the GGA Perdew-Wang 1991 (PW91) correlation functional with
the 6-31G* basis set.33-35 This level of theory has been successfully
tested to yield acceptable energetics.36 Electron transport is
calculated using our program GENIP-07,37-39 which calculates
current through molecules using an ab initio density functional
theory for the discrete and continuum components of the system,
to merge through a Greens function approach