Light interaction with biological molecules is commonly acknowledged to depend on
the optical properties of these compounds and may result in the activation of events
with various beneficial or harmful outcomes in living organisms. These issues are
encompassed in the photobiology science, together with unceasing and extensive studies
intended to advance the knowledge on light interaction with biological systems, and
to develop applications to improve environment and human life. The journal Molecules
and its Photochemistry Section are greatly contributing to a continuous comprehensive
update in the panorama of photobiology and naturally fluorescing biomolecules, as
demonstrated by the numerous papers recently published therein, some of which are
recalled in this Editorial.
The sun is the unique, natural energy source, which has directed the development of
biomolecules with absorption properties suitable to capture solar light and convert
it into chemical energy, essential for the growing and evolution of living organisms.
Chlorophylls and carotenoid are the pigments contained in plant photosynthetic antennae
engaged in the harvesting of light energy and its transfer, making algae and plants
the primary producers of biomass in the flow of maintenance of the whole biosphere.
The photophysical properties of chlorophylls have been widely investigated since a
long time, from the basic characterization of their absorption and fluorescence spectral
properties to the energy transfer and exciton events. Anyway, some intriguing aspects
are continuously newly revealed. This is the case of the near-infrared spectral region
of chlorophyll α, for which a very recent characterization by means of anti-Stokes
fluorescence excitation spectroscopy has shown the novelty of a weak, although reliably
detectable, band in the red absorption tail. Because of the spectral position and
weak absorption, this band relatable to intramolecular vibrational modes has been
suggested unlikely to contribute directly to light harvesting, while a possible role
could be played in energy transfer events in the network of photosynthetic antennae
[1]. Carotenoids, in addition to participating in light harvesting in photosystems,
interact with chlorophylls in the regulation of photosynthesis energy flow. A spectral
study on chlorophyll α and β-carotenoid as pure compounds in a solution confirmed
that the two pigments may interact with two mechanisms, electron transfer and energy
transfer events, the balance of which is affected by the polarity of the solvent.
As a consequence, caution is recommended in the choice and use of solvents when studying
photosystems in solutions [2]. Moreover, the derivatives of carotenoids containing
oxygen, namely xanthophylls, are essential in plant photoprotection strategies because
of their involvement in both non-photochemical quenching (NPhQ) and scavenging of
reactive oxygen species (ROS). An NPhQ photoprotection mechanism commonly operating
in plants and algae is based on the cycle conversion of de-epoxidized xanthophyll
molecules, as recalled in a review on 9’-cis-neoxanthin and its participation in photoprotection,
and its likely but still not completely elucidated role in photoisomerization and
ROS scavenging [3]. Antioxidant protection by carotenoids has evolved in plants and
has translated to animals and humans, in turn sharing with plants part of ROS generating
causes. In general, animals and humans cannot synthetize carotenoids, while they are
able to synthetize their retinoid derivatives. Antioxidants mechanisms in plants and
humans are based on Zeaxanthin and lutein, respectively, the derivative of β-carotene
involved the xantholphyll cycle and the structural isomer derived from α carotene,
which have been the subject of a review paper, which received great attention from
the scientific community, to the extent of becoming part of the Top Downloaded Articles
in July–October 2020 [4]. This review underscores the concept that in both plants
and animals the modulation of ROS production by antioxidants may result in both beneficial
and hazardous effects, in a balance dependent on the pre-existing biometabolic and
environmental context. In this view, a diet supplementation with carotenoid derivatives
and other antioxidants aiming to improve healthy life has been suggested to encompass
a careful analysis of the redox state of the subject and of the factors influencing
it, such as life style, metabolic disorders or pathologies.
For a long time, photoxidation events induced by the light irradiation of photosensitizing
molecules have been investigated for various beneficial applications. The massive
and unceasing efforts devoted to the advance of photodynamic diagnosis and therapy
strategies for the treatment of cancer and other pathologies, and for microbial inactivation,
based on the development of increasingly efficient photosensitizers from early natural
compounds [5,6,7], are beyond the aim of this Editorial. Nevertheless, it is worthwhile
to recall the particular case of the impact on water organic substances from photochemical
processes dependent on inorganic compounds, halide ions and halogens, such as chloride
and bromide. Non-radical and radical reactive species in natural waters may be derived
from these compounds following the UV-activated photolysis or reactions with oxidizing
species formerly produced by natural solar light, such as ozone, hydroxyl and sulfate
radicals, and in general ROS. Halogens and halide ions in photochemical processes
and photoinduced halogenation reactions may have complex effects on the natural environment,
the natural cycling of halogens and the decomposition of organic matter in terrestrial
and oceanic water, and on the treatment of pollution and wastewaters. Attention is
anyway to be paid also to undesired production of toxic derivatives [8].
As for the diagnostic applications in biomedicine based in the direct detection of
endogenous fluorophores, we must remember that endogenous fluorophores in living organisms
are powerful intrinsic biomarkers, due to their ubiquitous presence, involvement in
metabolic and catabolic reactions and in tissue architecture, and their changes occurring
under physiological conditions or following the rising of disorders and pathologies.
An example is given by the intracellular accumulation of the heterogeneous products
of oxidation of lipids, nucleic acids and proteins, which have been proposed to contribute
to cell fluorescence in the red and near infrared regions. Despite this signal is
a possible cause of disturbance in imaging analysis based on exogenous fluorophores
as labeling agents, these red fluorescing oxidation products could play a role as
an additional diagnostic endogenous biomarker of oxidative stress or of particular
cell and tissue bio-metabolic conditions in autofluorescence-based investigations
[9]. In this respect, such a kind of fluorophores might at least partially explain
the near infrared fluorescence of parathyroid, currently exploited in the clinics
to support the intraoperative discrimination of these glands during the surgical operation
of thyroid or parathyroid [10]. These two latter reports have contributed to the Special
Issue “Autofluorescence Spectroscopy and Imaging”, aimed to promote the knowledge
on autofluorescence analysis for manifold diagnostic applications. Going back to chlorophyll,
its autofluorescence has been proposed as a diagnostic biomarker to monitor the growing
and quality of microalgae, with the perspective of applications on environmental surveillance,
aquaculture, production of biomass as well as of food and chemical compounds to be
used in the health and medical fields [11]. Plants, besides chlorophylls and its fluorescence
as an important biomarker for the surveillance of their physiological conditions also
via remote systems, several additional fluorophores can be exploited for very different
purposes. An example is lignin, useful for the phenotyping or assessing the chemical
modification of wood. Valuable results have also been obtained by fluorescence induction
following aldehyde fixation of pine needles for the imaging diagnosis of fungal infestation
[12]. As for the clinics, a systematic revision has recalled various application based
on autofluorescence analysis as a common diagnostic tool to detect neoplastic lesions
in the oral cavity, gastrointestinal tract, bronchial mucosa, hidden caries in dentistry,
or to assess different diseases such as skin damage, the skin accumulation of advanced
glycation products as biomarkers of diabetes or kidney and/or cardiovascular disfunction,
lung ischemia and the risk of rejection after transplantation, and again, as a help
in the preservation of parathyroid during surgical operation [13]. These reports have
been incisively considered, leading to the conclusion that autofluorescence can generally
help diagnosis, in some case more to inexperienced than experienced operators, and
that in any case a still lacking standardization of operative and analysis procedures
is recommended, to improve the real time, cost effective application of autofluorescence
in the clinical diagnosis. These suggestions can be valid also to advance the autofluorescence-based
diagnosis of liver metabolic disorders and disease progression, for which promising
results have been reported from animal models, to assess and monitor the risk of progression
to severe pathology [14,15,16]. Beyond the identification of endogenous fluorophores
as biomarkers of the metabolic and morphological condition of a biological substrate
in which they are directly involved, it is to underscore the conceptual advance given
by the promising attempt to put autofluorescence in relation with the status of genes,
such as p53 and p16, regulating cell proliferation and their consequent engagement
in cell metabolism [17].
The intrinsic spectral properties of peptides and proteins are also at the basis of
multipurpose investigations by means of spectroscopic techniques. A particular application
concerns the control of the freshness of seafood and of the effects of freezing and
thawing, which affect the quality and texture of the muscle by inducing the physical
damages, autolysis, degradation and oxidation of proteins and lipids. To detect these
changes in the absorption and fluorescence spectroscopy of the overall near UV–visible-red
and near infrared range are increasingly considered cost effective, sensitive and
real time techniques for the quality control of seafood [18].
In conclusion, beyond fields which have been overlooked in this editorial, such as
specific photochemical studies on chemically synthesized organic and inorganic molecules
and exogenous dyes used as fluorescing labeling agents and phototherapy, photobiology
and autofluorescence-based issues are extensively represented by the various effective
papers timely published in Molecules.