An accelerated exposure and testing apparatus for building joint sealants.

The design, fabrication, and implementation of a computer-controlled exposure and testing apparatus for building joint sealants are described in this paper. This apparatus is unique in its ability to independently control and monitor temperature, relative humidity, ultraviolet (UV) radiation, and mechanical deformation. Each of these environmental factors can be controlled precisely over a wide range of conditions during periods of a month or more. Moreover, as controlled mechanical deformations can be generated, in situ mechanical characterization tests can be performed without removing specimens from the chamber. Temperature and humidity were controlled during our experiments via a precision temperature regulator and proportional mixing of dry and moisture-saturated air; while highly uniform UV radiation was attained by attaching the chamber to an integrating sphere-based radiation source. A computer-controlled stepper motor and a transmission system were used to provide precise movement control. The reliability and effectiveness of the apparatus were demonstrated on a model sealant material. The results clearly show that this apparatus provides an excellent platform to study the long-term durability of building joint sealants.

Design, fabrication, and implementation of thermally driven outdoor testing devices for building joint sealants.

The paper describes the development, implementation, and testing of two thermally driven outdoor exposure instruments. These devices are unique in their ability to impose field generated thermally induced strain on sealant specimens while monitoring their resulting load and displacement. The instruments combine a fixed wood and steel supporting frame with a moving polyvinyl chloride frame, and employ differences in the coefficients of thermal expansion between the supporting frame and moving frame to induce strain on the sealant specimens. Two different kinds of instruments have been fabricated, "winter/tension" and "winter/compression" designs. In the winter/tension design, the thermally induced dimensional change is directly transferred to the specimens; while in the winter/compression design, the samples are loaded in an opposite direction with the dimensional change. Both designs are instrumented to monitor load and displacement and are built so that the strain on the specimen does not exceed ±25% over the range of temperatures expected in Gaithersburg, MD. Additionally, a weather station is colocated with the device to record environmental conditions in 1 min intervals. This combination of weather information with mechanical property data enables a direct link between environmental conditions and the corresponding sealant response. The reliability and effectiveness of these instruments are demonstrated with a typical sealant material. The results show that the instruments work according to the design criteria and provide a meaningful quantitative platform to monitor the mechanical response of sealant exposed to outdoor weathering.

Mathematical models for ligand-receptor binding. Real sites, ghost sites.

In the basic life sciences the term "model" implies a physical, chemical, or molecular construct that provides a representation for the interpretation of experimental observations. To the statistician, however, a model is a mathematical expression for correlating data, which may or may not have roots in a molecular picture. With regard to ligand-receptor interactions, the mathematical model used plays a crucial role in extrapolations of binding measurements. Regardless of the statistical goodness of fit of data to an equation, the relationships of the parameters of a mathematical formalism to the molecular features of ligand-receptor complexes are generally very complex. Oversimplified interpretations of the molecular significance of the constants derived from binding measurements are unwarranted, unless one has independent information from molecular probes.

Analytical and graphical examination of strong binding by half-of-sites in proteins: illustration with aspartate transcarbamylase.

In multiple binding of ligands to a protein, the binding sites may seem to behave as if they are partitioned equally between two modalities. This paper analyzes three different molecular situations in which two actual assemblages appear: (i) two classes of sites exist at the outset in the ligand-free macromolecule; (ii) all sites are initially identical but after half are occupied, the affinity of the residual ones is altered; (iii) all sites are initially identical but they interact in a pairwise manner. The contours of affinity profiles-graphs of normalized stoichiometric binding constants (iK(i)) versus stoichiometric step number i-are examined for each situation to provide a basis for discriminating among them. Proper procedures for evaluating the site binding constants are then described. To illustrate these procedures, published experimental data for two real systems, binding of substrate or modifier by the enzyme aspartate transcarbamylase (carbamoylphosphate: L-aspartate carbamoyltransferase, EC 2.1.3.2), are scrutinized and the meaning of the calculated binding parameters is examined. The results demonstrate concretely that site binding constants cannot be specified without assuming a particular molecular model, but the stoichiometric constants can be assigned unambiguously without regard to the type of behavior at the individual sites.

Protein interactions with small molecules. Relationships between stoichiometric binding constants, site binding constants, and empirical binding parameters.

The multiple equilibria for the binding of a ligand A by a macromolecule P with n binding sites may be formulated in terms of a stoichiometric analysis or on the basis of a site-oriented scrutiny. The dependence of binding on ligand concentration can always be correlated in terms of n stoichiometric binding constants,Ki, even if there are interactions between sites that accentuate or attenuate binding affinities. A corresponding correlation in terms of site binding constants, kj, under the most general circumstances depends on the definition of n2n-1 different constants of which 2n-1 are independent. If experimental data are correlated in terms of n parameters kalpha, kbeta ... klambda in an equation of the site-binding form, (see article for formular) then there is no guarantee that the values of ka, kb, etc., have any unique relationships to site binging constants. Examples are given to illustrate this point. Equation are derived for relating stoichiometric binding constants to site binding constants, for the general case and for various special circumstances. These equations make it possible to define and analyze binding insystems with interactions and conformational accommodations. Accordingly, a graphical procedure is described (in which iKi is plotted against i, the stoichiometric binding step) that provides an affinity profile for concise representation of magnitudes of binding constants and for detecting interactions that accentuate or attenuate site binding affinities.